path: root/doc/rfc/rfc2328.txt

Network Working Group                                             J. Moy
Request for Comments: 2328                   Ascend Communications, Inc.
STD: 54                                                       April 1998
Obsoletes: 2178
Category: Standards Track


                             OSPF Version 2


Status of this Memo

    This document specifies an Internet standards track protocol for the
    Internet community, and requests discussion and suggestions for
    improvements.  Please refer to the current edition of the "Internet
    Official Protocol Standards" (STD 1) for the standardization state
    and status of this protocol.  Distribution of this memo is
    unlimited.

Copyright Notice

    Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

    This memo documents version 2 of the OSPF protocol.  OSPF is a
    link-state routing protocol.  It is designed to be run internal to a
    single Autonomous System.  Each OSPF router maintains an identical
    database describing the Autonomous System's topology.  From this
    database, a routing table is calculated by constructing a shortest-
    path tree.

    OSPF recalculates routes quickly in the face of topological changes,
    utilizing a minimum of routing protocol traffic.  OSPF provides
    support for equal-cost multipath.  An area routing capability is
    provided, enabling an additional level of routing protection and a
    reduction in routing protocol traffic.  In addition, all OSPF
    routing protocol exchanges are authenticated.

    The differences between this memo and RFC 2178 are explained in
    Appendix G. All differences are backward-compatible in nature.




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    Implementations of this memo and of RFCs 2178, 1583, and 1247 will
    interoperate.

    Please send comments to ospf@gated.cornell.edu.

Table of Contents

    1        Introduction ........................................... 6
    1.1      Protocol Overview ...................................... 6
    1.2      Definitions of commonly used terms ..................... 8
    1.3      Brief history of link-state routing technology ........ 11
    1.4      Organization of this document ......................... 12
    1.5      Acknowledgments ....................................... 12
    2        The link-state database: organization and calculations  13
    2.1      Representation of routers and networks ................ 13
    2.1.1    Representation of non-broadcast networks .............. 15
    2.1.2    An example link-state database ........................ 18
    2.2      The shortest-path tree ................................ 21
    2.3      Use of external routing information ................... 23
    2.4      Equal-cost multipath .................................. 26
    3        Splitting the AS into Areas ........................... 26
    3.1      The backbone of the Autonomous System ................. 27
    3.2      Inter-area routing .................................... 27
    3.3      Classification of routers ............................. 28
    3.4      A sample area configuration ........................... 29
    3.5      IP subnetting support ................................. 35
    3.6      Supporting stub areas ................................. 37
    3.7      Partitions of areas ................................... 38
    4        Functional Summary .................................... 40
    4.1      Inter-area routing .................................... 41
    4.2      AS external routes .................................... 41
    4.3      Routing protocol packets .............................. 42
    4.4      Basic implementation requirements ..................... 43
    4.5      Optional OSPF capabilities ............................ 46
    5        Protocol data structures .............................. 47
    6        The Area Data Structure ............................... 49
    7        Bringing Up Adjacencies ............................... 52
    7.1      The Hello Protocol .................................... 52
    7.2      The Synchronization of Databases ...................... 53
    7.3      The Designated Router ................................. 54
    7.4      The Backup Designated Router .......................... 56
    7.5      The graph of adjacencies .............................. 56



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    8        Protocol Packet Processing ............................ 58
    8.1      Sending protocol packets .............................. 58
    8.2      Receiving protocol packets ............................ 61
    9        The Interface Data Structure .......................... 63
    9.1      Interface states ...................................... 67
    9.2      Events causing interface state changes ................ 70
    9.3      The Interface state machine ........................... 72
    9.4      Electing the Designated Router ........................ 75
    9.5      Sending Hello packets ................................. 77
    9.5.1    Sending Hello packets on NBMA networks ................ 79
    10       The Neighbor Data Structure ........................... 80
    10.1     Neighbor states ....................................... 83
    10.2     Events causing neighbor state changes ................. 87
    10.3     The Neighbor state machine ............................ 89
    10.4     Whether to become adjacent ............................ 95
    10.5     Receiving Hello Packets ............................... 96
    10.6     Receiving Database Description Packets ................ 99
    10.7     Receiving Link State Request Packets ................. 102
    10.8     Sending Database Description Packets ................. 103
    10.9     Sending Link State Request Packets ................... 104
    10.10    An Example ........................................... 105
    11       The Routing Table Structure .......................... 107
    11.1     Routing table lookup ................................. 111
    11.2     Sample routing table, without areas .................. 111
    11.3     Sample routing table, with areas ..................... 112
    12       Link State Advertisements (LSAs) ..................... 115
    12.1     The LSA Header ....................................... 116
    12.1.1   LS age ............................................... 116
    12.1.2   Options .............................................. 117
    12.1.3   LS type .............................................. 117
    12.1.4   Link State ID ........................................ 117
    12.1.5   Advertising Router ................................... 119
    12.1.6   LS sequence number ................................... 120
    12.1.7   LS checksum .......................................... 121
    12.2     The link state database .............................. 121
    12.3     Representation of TOS ................................ 122
    12.4     Originating LSAs ..................................... 123
    12.4.1   Router-LSAs .......................................... 126
    12.4.1.1 Describing point-to-point interfaces ................. 130
    12.4.1.2 Describing broadcast and NBMA interfaces ............. 130
    12.4.1.3 Describing virtual links ............................. 131
    12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 131



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    12.4.1.5 Examples of router-LSAs .............................. 132
    12.4.2   Network-LSAs ......................................... 133
    12.4.2.1 Examples of network-LSAs ............................. 134
    12.4.3   Summary-LSAs ......................................... 135
    12.4.3.1 Originating summary-LSAs into stub areas ............. 137
    12.4.3.2 Examples of summary-LSAs ............................. 138
    12.4.4   AS-external-LSAs ..................................... 139
    12.4.4.1 Examples of AS-external-LSAs ......................... 140
    13       The Flooding Procedure ............................... 143
    13.1     Determining which LSA is newer ....................... 146
    13.2     Installing LSAs in the database ...................... 147
    13.3     Next step in the flooding procedure .................. 148
    13.4     Receiving self-originated LSAs ....................... 151
    13.5     Sending Link State Acknowledgment packets ............ 152
    13.6     Retransmitting LSAs .................................. 154
    13.7     Receiving link state acknowledgments ................. 155
    14       Aging The Link State Database ........................ 156
    14.1     Premature aging of LSAs .............................. 157
    15       Virtual Links ........................................ 158
    16       Calculation of the routing table ..................... 160
    16.1     Calculating the shortest-path tree for an area ....... 161
    16.1.1   The next hop calculation ............................. 167
    16.2     Calculating the inter-area routes .................... 178
    16.3     Examining transit areas' summary-LSAs ................ 170
    16.4     Calculating AS external routes ....................... 173
    16.4.1   External path preferences ............................ 175
    16.5     Incremental updates -- summary-LSAs .................. 175
    16.6     Incremental updates -- AS-external-LSAs .............. 177
    16.7     Events generated as a result of routing table changes  177
    16.8     Equal-cost multipath ................................. 178
             Footnotes ............................................ 179
             References ........................................... 183
    A        OSPF data formats .................................... 185
    A.1      Encapsulation of OSPF packets ........................ 185
    A.2      The Options field .................................... 187
    A.3      OSPF Packet Formats .................................. 189
    A.3.1    The OSPF packet header ............................... 190
    A.3.2    The Hello packet ..................................... 193
    A.3.3    The Database Description packet ...................... 195
    A.3.4    The Link State Request packet ........................ 197
    A.3.5    The Link State Update packet ......................... 199
    A.3.6    The Link State Acknowledgment packet ................. 201



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    A.4      LSA formats .......................................... 203
    A.4.1    The LSA header ....................................... 204
    A.4.2    Router-LSAs .......................................... 206
    A.4.3    Network-LSAs ......................................... 210
    A.4.4    Summary-LSAs ......................................... 212
    A.4.5    AS-external-LSAs ..................................... 214
    B        Architectural Constants .............................. 217
    C        Configurable Constants ............................... 219
    C.1      Global parameters .................................... 219
    C.2      Area parameters ...................................... 220
    C.3      Router interface parameters .......................... 221
    C.4      Virtual link parameters .............................. 224
    C.5      NBMA network parameters .............................. 224
    C.6      Point-to-MultiPoint network parameters ............... 225
    C.7      Host route parameters ................................ 226
    D        Authentication ....................................... 227
    D.1      Null authentication .................................. 227
    D.2      Simple password authentication ....................... 228
    D.3      Cryptographic authentication ......................... 228
    D.4      Message generation ................................... 231
    D.4.1    Generating Null authentication ....................... 231
    D.4.2    Generating Simple password authentication ............ 232
    D.4.3    Generating Cryptographic authentication .............. 232
    D.5      Message verification ................................. 234
    D.5.1    Verifying Null authentication ........................ 234
    D.5.2    Verifying Simple password authentication ............. 234
    D.5.3    Verifying Cryptographic authentication ............... 235
    E        An algorithm for assigning Link State IDs ............ 236
    F        Multiple interfaces to the same network/subnet ....... 239
    G        Differences from RFC 2178 ............................ 240
    G.1      Flooding modifications ............................... 240
    G.2      Changes to external path preferences ................. 241
    G.3      Incomplete resolution of virtual next hops ........... 241
    G.4      Routing table lookup ................................. 241
             Security Considerations .............................. 243
             Author's Address ..................................... 243
             Full Copyright Statement ............................. 244








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1.  Introduction

    This document is a specification of the Open Shortest Path First
    (OSPF) TCP/IP internet routing protocol.  OSPF is classified as an
    Interior Gateway Protocol (IGP).  This means that it distributes
    routing information between routers belonging to a single Autonomous
    System.  The OSPF protocol is based on link-state or SPF technology.
    This is a departure from the Bellman-Ford base used by traditional
    TCP/IP internet routing protocols.

    The OSPF protocol was developed by the OSPF working group of the
    Internet Engineering Task Force.  It has been designed expressly for
    the TCP/IP internet environment, including explicit support for CIDR
    and the tagging of externally-derived routing information.  OSPF
    also provides for the authentication of routing updates, and
    utilizes IP multicast when sending/receiving the updates.  In
    addition, much work has been done to produce a protocol that
    responds quickly to topology changes, yet involves small amounts of
    routing protocol traffic.

    1.1.  Protocol overview

        OSPF routes IP packets based solely on the destination IP
        address found in the IP packet header.  IP packets are routed
        "as is" -- they are not encapsulated in any further protocol
        headers as they transit the Autonomous System.  OSPF is a
        dynamic routing protocol.  It quickly detects topological
        changes in the AS (such as router interface failures) and
        calculates new loop-free routes after a period of convergence.
        This period of convergence is short and involves a minimum of
        routing traffic.

        In a link-state routing protocol, each router maintains a
        database describing the Autonomous System's topology.  This
        database is referred to as the link-state database. Each
        participating router has an identical database.  Each individual
        piece of this database is a particular router's local state
        (e.g., the router's usable interfaces and reachable neighbors).
        The router distributes its local state throughout the Autonomous
        System by flooding.





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        All routers run the exact same algorithm, in parallel.  From the
        link-state database, each router constructs a tree of shortest
        paths with itself as root.  This shortest-path tree gives the
        route to each destination in the Autonomous System.  Externally
        derived routing information appears on the tree as leaves.

        When several equal-cost routes to a destination exist, traffic
        is distributed equally among them.  The cost of a route is
        described by a single dimensionless metric.

        OSPF allows sets of networks to be grouped together.  Such a
        grouping is called an area.  The topology of an area is hidden
        from the rest of the Autonomous System.  This information hiding
        enables a significant reduction in routing traffic.  Also,
        routing within the area is determined only by the area's own
        topology, lending the area protection from bad routing data.  An
        area is a generalization of an IP subnetted network.

        OSPF enables the flexible configuration of IP subnets.  Each
        route distributed by OSPF has a destination and mask.  Two
        different subnets of the same IP network number may have
        different sizes (i.e., different masks).  This is commonly
        referred to as variable length subnetting.  A packet is routed
        to the best (i.e., longest or most specific) match.  Host routes
        are considered to be subnets whose masks are "all ones"
        (0xffffffff).

        All OSPF protocol exchanges are authenticated.  This means that
        only trusted routers can participate in the Autonomous System's
        routing.  A variety of authentication schemes can be used; in
        fact, separate authentication schemes can be configured for each
        IP subnet.

        Externally derived routing data (e.g., routes learned from an
        Exterior Gateway Protocol such as BGP; see [Ref23]) is
        advertised throughout the Autonomous System.  This externally
        derived data is kept separate from the OSPF protocol's link
        state data.  Each external route can also be tagged by the
        advertising router, enabling the passing of additional
        information between routers on the boundary of the Autonomous
        System.




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    1.2.  Definitions of commonly used terms

        This section provides definitions for terms that have a specific
        meaning to the OSPF protocol and that are used throughout the
        text.  The reader unfamiliar with the Internet Protocol Suite is
        referred to [Ref13] for an introduction to IP.


        Router
            A level three Internet Protocol packet switch.  Formerly
            called a gateway in much of the IP literature.

        Autonomous System
            A group of routers exchanging routing information via a
            common routing protocol.  Abbreviated as AS.

        Interior Gateway Protocol
            The routing protocol spoken by the routers belonging to an
            Autonomous system.  Abbreviated as IGP.  Each Autonomous
            System has a single IGP.  Separate Autonomous Systems may be
            running different IGPs.

        Router ID
            A 32-bit number assigned to each router running the OSPF
            protocol.  This number uniquely identifies the router within
            an Autonomous System.

        Network
            In this memo, an IP network/subnet/supernet.  It is possible
            for one physical network to be assigned multiple IP
            network/subnet numbers.  We consider these to be separate
            networks.  Point-to-point physical networks are an exception
            - they are considered a single network no matter how many
            (if any at all) IP network/subnet numbers are assigned to
            them.

        Network mask
            A 32-bit number indicating the range of IP addresses
            residing on a single IP network/subnet/supernet.  This
            specification displays network masks as hexadecimal numbers.





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            For example, the network mask for a class C IP network is
            displayed as 0xffffff00.  Such a mask is often displayed
            elsewhere in the literature as 255.255.255.0.

        Point-to-point networks
            A network that joins a single pair of routers.  A 56Kb
            serial line is an example of a point-to-point network.

        Broadcast networks
            Networks supporting many (more than two) attached routers,
            together with the capability to address a single physical
            message to all of the attached routers (broadcast).
            Neighboring routers are discovered dynamically on these nets
            using OSPF's Hello Protocol.  The Hello Protocol itself
            takes advantage of the broadcast capability.  The OSPF
            protocol makes further use of multicast capabilities, if
            they exist.  Each pair of routers on a broadcast network is
            assumed to be able to communicate directly. An ethernet is
            an example of a broadcast network.

        Non-broadcast networks
            Networks supporting many (more than two) routers, but having
            no broadcast capability.  Neighboring routers are maintained
            on these nets using OSPF's Hello Protocol.  However, due to
            the lack of broadcast capability, some configuration
            information may be necessary to aid in the discovery of
            neighbors.  On non-broadcast networks, OSPF protocol packets
            that are normally multicast need to be sent to each
            neighboring router, in turn. An X.25 Public Data Network
            (PDN) is an example of a non-broadcast network.

            OSPF runs in one of two modes over non-broadcast networks.
            The first mode, called non-broadcast multi-access or NBMA,
            simulates the operation of OSPF on a broadcast network. The
            second mode, called Point-to-MultiPoint, treats the non-
            broadcast network as a collection of point-to-point links.
            Non-broadcast networks are referred to as NBMA networks or
            Point-to-MultiPoint networks, depending on OSPF's mode of
            operation over the network.






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        Interface
            The connection between a router and one of its attached
            networks.  An interface has state information associated
            with it, which is obtained from the underlying lower level
            protocols and the routing protocol itself.  An interface to
            a network has associated with it a single IP address and
            mask (unless the network is an unnumbered point-to-point
            network).  An interface is sometimes also referred to as a
            link.

        Neighboring routers
            Two routers that have interfaces to a common network.
            Neighbor relationships are maintained by, and usually
            dynamically discovered by, OSPF's Hello Protocol.

        Adjacency
            A relationship formed between selected neighboring routers
            for the purpose of exchanging routing information.  Not
            every pair of neighboring routers become adjacent.

        Link state advertisement
            Unit of data describing the local state of a router or
            network. For a router, this includes the state of the
            router's interfaces and adjacencies.  Each link state
            advertisement is flooded throughout the routing domain. The
            collected link state advertisements of all routers and
            networks forms the protocol's link state database.
            Throughout this memo, link state advertisement is
            abbreviated as LSA.

        Hello Protocol
            The part of the OSPF protocol used to establish and maintain
            neighbor relationships.  On broadcast networks the Hello
            Protocol can also dynamically discover neighboring routers.

        Flooding
            The part of the OSPF protocol that distributes and
            synchronizes the link-state database between OSPF routers.

        Designated Router
            Each broadcast and NBMA network that has at least two
            attached routers has a Designated Router.  The Designated



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            Router generates an LSA for the network and has other
            special responsibilities in the running of the protocol.
            The Designated Router is elected by the Hello Protocol.

            The Designated Router concept enables a reduction in the
            number of adjacencies required on a broadcast or NBMA
            network.  This in turn reduces the amount of routing
            protocol traffic and the size of the link-state database.

        Lower-level protocols
            The underlying network access protocols that provide
            services to the Internet Protocol and in turn the OSPF
            protocol.  Examples of these are the X.25 packet and frame
            levels for X.25 PDNs, and the ethernet data link layer for
            ethernets.


    1.3.  Brief history of link-state routing technology

        OSPF is a link state routing protocol.  Such protocols are also
        referred to in the literature as SPF-based or distributed-
        database protocols.  This section gives a brief description of
        the developments in link-state technology that have influenced
        the OSPF protocol.

        The first link-state routing protocol was developed for use in
        the ARPANET packet switching network.  This protocol is
        described in [Ref3].  It has formed the starting point for all
        other link-state protocols.  The homogeneous ARPANET
        environment, i.e., single-vendor packet switches connected by
        synchronous serial lines, simplified the design and
        implementation of the original protocol.

        Modifications to this protocol were proposed in [Ref4].  These
        modifications dealt with increasing the fault tolerance of the
        routing protocol through, among other things, adding a checksum
        to the LSAs (thereby detecting database corruption).  The paper
        also included means for reducing the routing traffic overhead in
        a link-state protocol.  This was accomplished by introducing
        mechanisms which enabled the interval between LSA originations
        to be increased by an order of magnitude.




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        A link-state algorithm has also been proposed for use as an ISO
        IS-IS routing protocol.  This protocol is described in [Ref2].
        The protocol includes methods for data and routing traffic
        reduction when operating over broadcast networks.  This is
        accomplished by election of a Designated Router for each
        broadcast network, which then originates an LSA for the network.

        The OSPF Working Group of the IETF has extended this work in
        developing the OSPF protocol.  The Designated Router concept has
        been greatly enhanced to further reduce the amount of routing
        traffic required.  Multicast capabilities are utilized for
        additional routing bandwidth reduction.  An area routing scheme
        has been developed enabling information
        hiding/protection/reduction.  Finally, the algorithms have been
        tailored for efficient operation in TCP/IP internets.


    1.4.  Organization of this document

        The first three sections of this specification give a general
        overview of the protocol's capabilities and functions.  Sections
        4-16 explain the protocol's mechanisms in detail.  Packet
        formats, protocol constants and configuration items are
        specified in the appendices.

        Labels such as HelloInterval encountered in the text refer to
        protocol constants.  They may or may not be configurable.
        Architectural constants are summarized in Appendix B.
        Configurable constants are summarized in Appendix C.

        The detailed specification of the protocol is presented in terms
        of data structures.  This is done in order to make the
        explanation more precise.  Implementations of the protocol are
        required to support the functionality described, but need not
        use the precise data structures that appear in this memo.


    1.5.  Acknowledgments

        The author would like to thank Ran Atkinson, Fred Baker, Jeffrey
        Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra
        Jujjavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui



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        Zhang and the rest of the OSPF Working Group for the ideas and
        support they have given to this project.

        The OSPF Point-to-MultiPoint interface is based on work done by
        Fred Baker.

        The OSPF Cryptographic Authentication option was developed by
        Fred Baker and Ran Atkinson.


2.  The Link-state Database: organization and calculations

    The following subsections describe the organization of OSPF's link-
    state database, and the routing calculations that are performed on
    the database in order to produce a router's routing table.


    2.1.  Representation of routers and networks

        The Autonomous System's link-state database describes a directed
        graph.  The vertices of the graph consist of routers and
        networks.  A graph edge connects two routers when they are
        attached via a physical point-to-point network.  An edge
        connecting a router to a network indicates that the router has
        an interface on the network. Networks can be either transit or
        stub networks. Transit networks are those capable of carrying
        data traffic that is neither locally originated nor locally
        destined. A transit network is represented by a graph vertex
        having both incoming and outgoing edges. A stub network's vertex
        has only incoming edges.

        The neighborhood of each network node in the graph depends on
        the network's type (point-to-point, broadcast, NBMA or Point-
        to-MultiPoint) and the number of routers having an interface to
        the network.  Three cases are depicted in Figure 1a.  Rectangles
        indicate routers.  Circles and oblongs indicate networks.
        Router names are prefixed with the letters RT and network names
        with the letter N.  Router interface names are prefixed by the
        letter I.  Lines between routers indicate point-to-point
        networks.  The left side of the figure shows networks with their
        connected routers, with the resulting graphs shown on the right.




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                                                  **FROM**

                                           *      |RT1|RT2|
                +---+Ia    +---+           *   ------------
                |RT1|------|RT2|           T   RT1|   | X |
                +---+    Ib+---+           O   RT2| X |   |
                                           *    Ia|   | X |
                                           *    Ib| X |   |

                     Physical point-to-point networks


                                                  **FROM**
                      +---+                *
                      |RT7|                *      |RT7| N3|
                      +---+                T   ------------
                        |                  O   RT7|   |   |
            +----------------------+       *    N3| X |   |
                       N3                  *

                              Stub networks

                                                  **FROM**
                +---+      +---+
                |RT3|      |RT4|              |RT3|RT4|RT5|RT6|N2 |
                +---+      +---+        *  ------------------------
                  |    N2    |          *  RT3|   |   |   |   | X |
            +----------------------+    T  RT4|   |   |   |   | X |
                  |          |          O  RT5|   |   |   |   | X |
                +---+      +---+        *  RT6|   |   |   |   | X |
                |RT5|      |RT6|        *   N2| X | X | X | X |   |
                +---+      +---+

                          Broadcast or NBMA networks



                    Figure 1a: Network map components




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             Networks and routers are represented by vertices.
             An edge connects Vertex A to Vertex B iff the
             intersection of Column A and Row B is marked with
                                  an X.



        The top of Figure 1a shows two routers connected by a point-to-
        point link. In the resulting link-state database graph, the two
        router vertices are directly connected by a pair of edges, one
        in each direction. Interfaces to point-to-point networks need
        not be assigned IP addresses.  When interface addresses are
        assigned, they are modelled as stub links, with each router
        advertising a stub connection to the other router's interface
        address. Optionally, an IP subnet can be assigned to the point-
        to-point network. In this case, both routers advertise a stub
        link to the IP subnet, instead of advertising each others' IP
        interface addresses.

        The middle of Figure 1a shows a network with only one attached
        router (i.e., a stub network). In this case, the network appears
        on the end of a stub connection in the link-state database's
        graph.

        When multiple routers are attached to a broadcast network, the
        link-state database graph shows all routers bidirectionally
        connected to the network vertex. This is pictured at the bottom
        of Figure 1a.

        Each network (stub or transit) in the graph has an IP address
        and associated network mask.  The mask indicates the number of
        nodes on the network.  Hosts attached directly to routers
        (referred to as host routes) appear on the graph as stub
        networks.  The network mask for a host route is always
        0xffffffff, which indicates the presence of a single node.


        2.1.1.  Representation of non-broadcast networks

            As mentioned previously, OSPF can run over non-broadcast
            networks in one of two modes: NBMA or Point-to-MultiPoint.
            The choice of mode determines the way that the Hello



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            protocol and flooding work over the non-broadcast network,
            and the way that the network is represented in the link-
            state database.

            In NBMA mode, OSPF emulates operation over a broadcast
            network: a Designated Router is elected for the NBMA
            network, and the Designated Router originates an LSA for the
            network. The graph representation for broadcast networks and
            NBMA networks is identical. This representation is pictured
            in the middle of Figure 1a.

            NBMA mode is the most efficient way to run OSPF over non-
            broadcast networks, both in terms of link-state database
            size and in terms of the amount of routing protocol traffic.
            However, it has one significant restriction: it requires all
            routers attached to the NBMA network to be able to
            communicate directly. This restriction may be met on some
            non-broadcast networks, such as an ATM subnet utilizing
            SVCs. But it is often not met on other non-broadcast
            networks, such as PVC-only Frame Relay networks. On non-
            broadcast networks where not all routers can communicate
            directly you can break the non-broadcast network into
            logical subnets, with the routers on each subnet being able
            to communicate directly, and then run each separate subnet
            as an NBMA network (see [Ref15]). This however requires
            quite a bit of administrative overhead, and is prone to
            misconfiguration. It is probably better to run such a non-
            broadcast network in Point-to-Multipoint mode.

            In Point-to-MultiPoint mode, OSPF treats all router-to-
            router connections over the non-broadcast network as if they
            were point-to-point links. No Designated Router is elected
            for the network, nor is there an LSA generated for the
            network. In fact, a vertex for the Point-to-MultiPoint
            network does not appear in the graph of the link-state
            database.

            Figure 1b illustrates the link-state database representation
            of a Point-to-MultiPoint network. On the left side of the
            figure, a Point-to-MultiPoint network is pictured. It is
            assumed that all routers can communicate directly, except
            for routers RT4 and RT5. I3 though I6 indicate the routers'



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            IP interface addresses on the Point-to-MultiPoint network.
            In the graphical representation of the link-state database,
            routers that can communicate directly over the Point-to-
            MultiPoint network are joined by bidirectional edges, and
            each router also has a stub connection to its own IP
            interface address (which is in contrast to the
            representation of real point-to-point links; see Figure 1a).

            On some non-broadcast networks, use of Point-to-MultiPoint
            mode and data-link protocols such as Inverse ARP (see
            [Ref14]) will allow autodiscovery of OSPF neighbors even
            though broadcast support is not available.






                                                  **FROM**
                +---+      +---+
                |RT3|      |RT4|              |RT3|RT4|RT5|RT6|
                +---+      +---+        *  --------------------
                I3|    N2    |I4        *  RT3|   | X | X | X |
            +----------------------+    T  RT4| X |   |   | X |
                I5|          |I6        O  RT5| X |   |   | X |
                +---+      +---+        *  RT6| X | X | X |   |
                |RT5|      |RT6|        *   I3| X |   |   |   |
                +---+      +---+            I4|   | X |   |   |
                                            I5|   |   | X |   |
                                            I6|   |   |   | X |



                    Figure 1b: Network map components
                       Point-to-MultiPoint networks

             All routers can communicate directly over N2, except
                routers RT4 and RT5. I3 through I6 indicate IP
                           interface addresses






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        2.1.2.  An example link-state database

            Figure 2 shows a sample map of an Autonomous System.  The
            rectangle labelled H1 indicates a host, which has a SLIP
            connection to Router RT12.  Router RT12 is therefore
            advertising a host route.  Lines between routers indicate
            physical point-to-point networks.  The only point-to-point
            network that has been assigned interface addresses is the
            one joining Routers RT6 and RT10.  Routers RT5 and RT7 have
            BGP connections to other Autonomous Systems.  A set of BGP-
            learned routes have been displayed for both of these
            routers.

            A cost is associated with the output side of each router
            interface.  This cost is configurable by the system
            administrator.  The lower the cost, the more likely the
            interface is to be used to forward data traffic.  Costs are
            also associated with the externally derived routing data
            (e.g., the BGP-learned routes).

            The directed graph resulting from the map in Figure 2 is
            depicted in Figure 3.  Arcs are labelled with the cost of
            the corresponding router output interface.  Arcs having no
            labelled cost have a cost of 0.  Note that arcs leading from
            networks to routers always have cost 0; they are significant
            nonetheless.  Note also that the externally derived routing
            data appears on the graph as stubs.

            The link-state database is pieced together from LSAs
            generated by the routers.  In the associated graphical
            representation, the neighborhood of each router or transit
            network is represented in a single, separate LSA.  Figure 4
            shows these LSAs graphically. Router RT12 has an interface
            to two broadcast networks and a SLIP line to a host.
            Network N6 is a broadcast network with three attached
            routers.  The cost of all links from Network N6 to its
            attached routers is 0.  Note that the LSA for Network N6 is
            actually generated by one of the network's attached routers:
            the router that has been elected Designated Router for the
            network.





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                 +
                 | 3+---+                     N12      N14
               N1|--|RT1|\ 1                    \ N13 /
                 |  +---+ \                     8\ |8/8
                 +         \ ____                 \|/
                            /    \   1+---+8    8+---+6
                           *  N3  *---|RT4|------|RT5|--------+
                            \____/    +---+      +---+        |
                  +         /   |                  |7         |
                  | 3+---+ /    |                  |          |
                N2|--|RT2|/1    |1                 |6         |
                  |  +---+    +---+8            6+---+        |
                  +           |RT3|--------------|RT6|        |
                              +---+              +---+        |
                                |2               Ia|7         |
                                |                  |          |
                           +---------+             |          |
                               N4                  |          |
                                                   |          |
                                                   |          |
                       N11                         |          |
                   +---------+                     |          |
                        |                          |          |    N12
                        |3                         |          |6 2/
                      +---+                        |        +---+/
                      |RT9|                        |        |RT7|---N15
                      +---+                        |        +---+ 9
                        |1                   +     |          |1
                       _|__                  |   Ib|5       __|_
                      /    \      1+----+2   |  3+----+1   /    \
                     *  N9  *------|RT11|----|---|RT10|---*  N6  *
                      \____/       +----+    |   +----+    \____/
                        |                    |                |
                        |1                   +                |1
             +--+   10+----+                N8              +---+
             |H1|-----|RT12|                                |RT8|
             +--+SLIP +----+                                +---+
                        |2                                    |4
                        |                                     |
                   +---------+                            +--------+
                       N10                                    N7



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                    Figure 2: A sample Autonomous System

                                **FROM**

                 |RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
                 |1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
              ----- ---------------------------------------------
              RT1|  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |  |
              RT2|  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |  |
              RT3|  |  |  |  |  |6 |  |  |  |  |  |  |0 |  |  |  |
              RT4|  |  |  |  |8 |  |  |  |  |  |  |  |0 |  |  |  |
              RT5|  |  |  |8 |  |6 |6 |  |  |  |  |  |  |  |  |  |
              RT6|  |  |8 |  |7 |  |  |  |  |5 |  |  |  |  |  |  |
              RT7|  |  |  |  |6 |  |  |  |  |  |  |  |  |0 |  |  |
          *   RT8|  |  |  |  |  |  |  |  |  |  |  |  |  |0 |  |  |
          *   RT9|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |
          T  RT10|  |  |  |  |  |7 |  |  |  |  |  |  |  |0 |0 |  |
          O  RT11|  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |0 |
          *  RT12|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |0 |
          *    N1|3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N2|  |3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N3|1 |1 |1 |1 |  |  |  |  |  |  |  |  |  |  |  |  |
               N4|  |  |2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
               N6|  |  |  |  |  |  |1 |1 |  |1 |  |  |  |  |  |  |
               N7|  |  |  |  |  |  |  |4 |  |  |  |  |  |  |  |  |
               N8|  |  |  |  |  |  |  |  |  |3 |2 |  |  |  |  |  |
               N9|  |  |  |  |  |  |  |  |1 |  |1 |1 |  |  |  |  |
              N10|  |  |  |  |  |  |  |  |  |  |  |2 |  |  |  |  |
              N11|  |  |  |  |  |  |  |  |3 |  |  |  |  |  |  |  |
              N12|  |  |  |  |8 |  |2 |  |  |  |  |  |  |  |  |  |
              N13|  |  |  |  |8 |  |  |  |  |  |  |  |  |  |  |  |
              N14|  |  |  |  |8 |  |  |  |  |  |  |  |  |  |  |  |
              N15|  |  |  |  |  |  |9 |  |  |  |  |  |  |  |  |  |
               H1|  |  |  |  |  |  |  |  |  |  |  |10|  |  |  |  |


                     Figure 3: The resulting directed graph

                 Networks and routers are represented by vertices.
                 An edge of cost X connects Vertex A to Vertex B iff
                 the intersection of Column A and Row B is marked
                                     with an X.



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                     **FROM**                       **FROM**

                  |RT12|N9|N10|H1|                 |RT9|RT11|RT12|N9|
           *  --------------------          *  ----------------------
           *  RT12|    |  |   |  |          *   RT9|   |    |    |0 |
           T    N9|1   |  |   |  |          T  RT11|   |    |    |0 |
           O   N10|2   |  |   |  |          O  RT12|   |    |    |0 |
           *    H1|10  |  |   |  |          *    N9|   |    |    |  |
           *                                *
                RT12's router-LSA              N9's network-LSA

                  Figure 4: Individual link state components

              Networks and routers are represented by vertices.
              An edge of cost X connects Vertex A to Vertex B iff
              the intersection of Column A and Row B is marked
                                  with an X.

    2.2.  The shortest-path tree

        When no OSPF areas are configured, each router in the Autonomous
        System has an identical link-state database, leading to an
        identical graphical representation.  A router generates its
        routing table from this graph by calculating a tree of shortest
        paths with the router itself as root.  Obviously, the shortest-
        path tree depends on the router doing the calculation.  The
        shortest-path tree for Router RT6 in our example is depicted in
        Figure 5.

        The tree gives the entire path to any destination network or
        host.  However, only the next hop to the destination is used in
        the forwarding process.  Note also that the best route to any
        router has also been calculated.  For the processing of external
        data, we note the next hop and distance to any router
        advertising external routes.  The resulting routing table for
        Router RT6 is pictured in Table 2.  Note that there is a
        separate route for each end of a numbered point-to-point network
        (in this case, the serial line between Routers RT6 and RT10).


        Routes to networks belonging to other AS'es (such as N12) appear
        as dashed lines on the shortest path tree in Figure 5.  Use of



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                                RT6(origin)
                    RT5 o------------o-----------o Ib
                       /|\    6      |\     7
                     8/8|8\          | \
                     /  |  \        6|  \
                    o   |   o        |   \7
                   N12  o  N14       |    \
                       N13        2  |     \
                            N4 o-----o RT3  \
                                    /        \    5
                                  1/     RT10 o-------o Ia
                                  /           |\
                       RT4 o-----o N3        3| \1
                                /|            |  \ N6     RT7
                               / |         N8 o   o---------o
                              /  |            |   |        /|
                         RT2 o   o RT1        |   |      2/ |9
                            /    |            |   |RT8   /  |
                           /3    |3      RT11 o   o     o   o
                          /      |            |   |    N12 N15
                      N2 o       o N1        1|   |4
                                              |   |
                                           N9 o   o N7
                                             /|
                                            / |
                        N11      RT9       /  |RT12
                         o--------o-------o   o--------o H1
                             3                |   10
                                              |2
                                              |
                                              o N10


                     Figure 5: The SPF tree for Router RT6

              Edges that are not marked with a cost have a cost of
              of zero (these are network-to-router links). Routes
              to networks N12-N15 are external information that is
                         considered in Section 2.3





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                   Destination   Next  Hop   Distance
                   __________________________________
                   N1            RT3         10
                   N2            RT3         10
                   N3            RT3         7
                   N4            RT3         8
                   Ib            *           7
                   Ia            RT10        12
                   N6            RT10        8
                   N7            RT10        12
                   N8            RT10        10
                   N9            RT10        11
                   N10           RT10        13
                   N11           RT10        14
                   H1            RT10        21
                   __________________________________
                   RT5           RT5         6
                   RT7           RT10        8


    Table 2: The portion of Router RT6's routing table listing local
                             destinations.

        this externally derived routing information is considered in the
        next section.


    2.3.  Use of external routing information

        After the tree is created the external routing information is
        examined.  This external routing information may originate from
        another routing protocol such as BGP, or be statically
        configured (static routes).  Default routes can also be included
        as part of the Autonomous System's external routing information.

        External routing information is flooded unaltered throughout the
        AS.  In our example, all the routers in the Autonomous System
        know that Router RT7 has two external routes, with metrics 2 and
        9.

        OSPF supports two types of external metrics.  Type 1 external
        metrics are expressed in the same units as OSPF interface cost



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        (i.e., in terms of the link state metric).  Type 2 external
        metrics are an order of magnitude larger; any Type 2 metric is
        considered greater than the cost of any path internal to the AS.
        Use of Type 2 external metrics assumes that routing between
        AS'es is the major cost of routing a packet, and eliminates the
        need for conversion of external costs to internal link state
        metrics.

        As an example of Type 1 external metric processing, suppose that
        the Routers RT7 and RT5 in Figure 2 are advertising Type 1
        external metrics.  For each advertised external route, the total
        cost from Router RT6 is calculated as the sum of the external
        route's advertised cost and the distance from Router RT6 to the
        advertising router.  When two routers are advertising the same
        external destination, RT6 picks the advertising router providing
        the minimum total cost. RT6 then sets the next hop to the
        external destination equal to the next hop that would be used
        when routing packets to the chosen advertising router.

        In Figure 2, both Router RT5 and RT7 are advertising an external
        route to destination Network N12.  Router RT7 is preferred since
        it is advertising N12 at a distance of 10 (8+2) to Router RT6,
        which is better than Router RT5's 14 (6+8).  Table 3 shows the
        entries that are added to the routing table when external routes
        are examined:



                         Destination   Next  Hop   Distance
                         __________________________________
                         N12           RT10        10
                         N13           RT5         14
                         N14           RT5         14
                         N15           RT10        17


                 Table 3: The portion of Router RT6's routing table
                           listing external destinations.


        Processing of Type 2 external metrics is simpler.  The AS
        boundary router advertising the smallest external metric is



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        chosen, regardless of the internal distance to the AS boundary
        router.  Suppose in our example both Router RT5 and Router RT7
        were advertising Type 2 external routes.  Then all traffic
        destined for Network N12 would be forwarded to Router RT7, since
        2 < 8.  When several equal-cost Type 2 routes exist, the
        internal distance to the advertising routers is used to break
        the tie.

        Both Type 1 and Type 2 external metrics can be present in the AS
        at the same time.  In that event, Type 1 external metrics always
        take precedence.

        This section has assumed that packets destined for external
        destinations are always routed through the advertising AS
        boundary router.  This is not always desirable.  For example,
        suppose in Figure 2 there is an additional router attached to
        Network N6, called Router RTX.  Suppose further that RTX does
        not participate in OSPF routing, but does exchange BGP
        information with the AS boundary router RT7.  Then, Router RT7
        would end up advertising OSPF external routes for all
        destinations that should be routed to RTX.  An extra hop will
        sometimes be introduced if packets for these destinations need
        always be routed first to Router RT7 (the advertising router).

        To deal with this situation, the OSPF protocol allows an AS
        boundary router to specify a "forwarding address" in its AS-
        external-LSAs.  In the above example, Router RT7 would specify
        RTX's IP address as the "forwarding address" for all those
        destinations whose packets should be routed directly to RTX.

        The "forwarding address" has one other application.  It enables
        routers in the Autonomous System's interior to function as
        "route servers".  For example, in Figure 2 the router RT6 could
        become a route server, gaining external routing information
        through a combination of static configuration and external
        routing protocols.  RT6 would then start advertising itself as
        an AS boundary router, and would originate a collection of OSPF
        AS-external-LSAs.  In each AS-external-LSA, Router RT6 would
        specify the correct Autonomous System exit point to use for the
        destination through appropriate setting of the LSA's "forwarding
        address" field.




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    2.4.  Equal-cost multipath

        The above discussion has been simplified by considering only a
        single route to any destination.  In reality, if multiple
        equal-cost routes to a destination exist, they are all
        discovered and used.  This requires no conceptual changes to the
        algorithm, and its discussion is postponed until we consider the
        tree-building process in more detail.

        With equal cost multipath, a router potentially has several
        available next hops towards any given destination.


3.  Splitting the AS into Areas

    OSPF allows collections of contiguous networks and hosts to be
    grouped together.  Such a group, together with the routers having
    interfaces to any one of the included networks, is called an area.
    Each area runs a separate copy of the basic link-state routing
    algorithm.  This means that each area has its own link-state
    database and corresponding graph, as explained in the previous
    section.

    The topology of an area is invisible from the outside of the area.
    Conversely, routers internal to a given area know nothing of the
    detailed topology external to the area.  This isolation of knowledge
    enables the protocol to effect a marked reduction in routing traffic
    as compared to treating the entire Autonomous System as a single
    link-state domain.

    With the introduction of areas, it is no longer true that all
    routers in the AS have an identical link-state database.  A router
    actually has a separate link-state database for each area it is
    connected to.  (Routers connected to multiple areas are called area
    border routers).  Two routers belonging to the same area have, for
    that area, identical area link-state databases.

    Routing in the Autonomous System takes place on two levels,
    depending on whether the source and destination of a packet reside
    in the same area (intra-area routing is used) or different areas
    (inter-area routing is used).  In intra-area routing, the packet is
    routed solely on information obtained within the area; no routing



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    information obtained from outside the area can be used.  This
    protects intra-area routing from the injection of bad routing
    information.  We discuss inter-area routing in Section 3.2.


    3.1.  The backbone of the Autonomous System

        The OSPF backbone is the special OSPF Area 0 (often written as
        Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP
        addresses). The OSPF backbone always contains all area border
        routers. The backbone is responsible for distributing routing
        information between non-backbone areas. The backbone must be
        contiguous. However, it need not be physically contiguous;
        backbone connectivity can be established/maintained through the
        configuration of virtual links.

        Virtual links can be configured between any two backbone routers
        that have an interface to a common non-backbone area.  Virtual
        links belong to the backbone.  The protocol treats two routers
        joined by a virtual link as if they were connected by an
        unnumbered point-to-point backbone network.  On the graph of the
        backbone, two such routers are joined by arcs whose costs are
        the intra-area distances between the two routers.  The routing
        protocol traffic that flows along the virtual link uses intra-
        area routing only.


    3.2.  Inter-area routing

        When routing a packet between two non-backbone areas the
        backbone is used.  The path that the packet will travel can be
        broken up into three contiguous pieces: an intra-area path from
        the source to an area border router, a backbone path between the
        source and destination areas, and then another intra-area path
        to the destination.  The algorithm finds the set of such paths
        that have the smallest cost.

        Looking at this another way, inter-area routing can be pictured
        as forcing a star configuration on the Autonomous System, with
        the backbone as hub and each of the non-backbone areas as
        spokes.




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        The topology of the backbone dictates the backbone paths used
        between areas.  The topology of the backbone can be enhanced by
        adding virtual links.  This gives the system administrator some
        control over the routes taken by inter-area traffic.

        The correct area border router to use as the packet exits the
        source area is chosen in exactly the same way routers
        advertising external routes are chosen.  Each area border router
        in an area summarizes for the area its cost to all networks
        external to the area.  After the SPF tree is calculated for the
        area, routes to all inter-area destinations are calculated by
        examining the summaries of the area border routers.


    3.3.  Classification of routers

        Before the introduction of areas, the only OSPF routers having a
        specialized function were those advertising external routing
        information, such as Router RT5 in Figure 2.  When the AS is
        split into OSPF areas, the routers are further divided according
        to function into the following four overlapping categories:


        Internal routers
            A router with all directly connected networks belonging to
            the same area. These routers run a single copy of the basic
            routing algorithm.

        Area border routers
            A router that attaches to multiple areas.  Area border
            routers run multiple copies of the basic algorithm, one copy
            for each attached area. Area border routers condense the
            topological information of their attached areas for
            distribution to the backbone.  The backbone in turn
            distributes the information to the other areas.

        Backbone routers
            A router that has an interface to the backbone area.  This
            includes all routers that interface to more than one area
            (i.e., area border routers).  However, backbone routers do
            not have to be area border routers.  Routers with all
            interfaces connecting to the backbone area are supported.



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        AS boundary routers
            A router that exchanges routing information with routers
            belonging to other Autonomous Systems.  Such a router
            advertises AS external routing information throughout the
            Autonomous System.  The paths to each AS boundary router are
            known by every router in the AS.  This classification is
            completely independent of the previous classifications: AS
            boundary routers may be internal or area border routers, and
            may or may not participate in the backbone.


    3.4.  A sample area configuration

        Figure 6 shows a sample area configuration.  The first area
        consists of networks N1-N4, along with their attached routers
        RT1-RT4.  The second area consists of networks N6-N8, along with
        their attached routers RT7, RT8, RT10 and RT11.  The third area
        consists of networks N9-N11 and Host H1, along with their
        attached routers RT9, RT11 and RT12.  The third area has been
        configured so that networks N9-N11 and Host H1 will all be
        grouped into a single route, when advertised external to the
        area (see Section 3.5 for more details).

        In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
        internal routers.  Routers RT3, RT4, RT7, RT10 and RT11 are area
        border routers.  Finally, as before, Routers RT5 and RT7 are AS
        boundary routers.

        Figure 7 shows the resulting link-state database for the Area 1.
        The figure completely describes that area's intra-area routing.
        It also shows the complete view of the internet for the two
        internal routers RT1 and RT2.  It is the job of the area border
        routers, RT3 and RT4, to advertise into Area 1 the distances to
        all destinations external to the area.  These are indicated in
        Figure 7 by the dashed stub routes.  Also, RT3 and RT4 must
        advertise into Area 1 the location of the AS boundary routers
        RT5 and RT7.  Finally, AS-external-LSAs from RT5 and RT7 are
        flooded throughout the entire AS, and in particular throughout
        Area 1.  These LSAs are included in Area 1's database, and yield
        routes to Networks N12-N15.

        Routers RT3 and RT4 must also summarize Area 1's topology for



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             ...........................
             .   +                     .
             .   | 3+---+              .      N12      N14
             . N1|--|RT1|\ 1           .        \ N13 /
             .   |  +---+ \            .        8\ |8/8
             .   +         \ ____      .          \|/
             .              /    \   1+---+8    8+---+6
             .             *  N3  *---|RT4|------|RT5|--------+
             .              \____/    +---+      +---+        |
             .    +         /      \   .           |7         |
             .    | 3+---+ /        \  .           |          |
             .  N2|--|RT2|/1        1\ .           |6         |
             .    |  +---+            +---+8    6+---+        |
             .    +                   |RT3|------|RT6|        |
             .                        +---+      +---+        |
             .                      2/ .         Ia|7         |
             .                      /  .           |          |
             .             +---------+ .           |          |
             .Area 1           N4      .           |          |
             ...........................           |          |
          ..........................               |          |
          .            N11         .               |          |
          .        +---------+     .               |          |
          .             |          .               |          |    N12
          .             |3         .             Ib|5         |6 2/
          .           +---+        .             +----+     +---+/
          .           |RT9|        .    .........|RT10|.....|RT7|---N15.
          .           +---+        .    .        +----+     +---+ 9    .
          .             |1         .    .    +  /3    1\      |1       .
          .            _|__        .    .    | /        \   __|_       .
          .           /    \      1+----+2   |/          \ /    \      .
          .          *  N9  *------|RT11|----|            *  N6  *     .
          .           \____/       +----+    |             \____/      .
          .             |          .    .    |                |        .
          .             |1         .    .    +                |1       .
          .  +--+   10+----+       .    .   N8              +---+      .
          .  |H1|-----|RT12|       .    .                   |RT8|      .
          .  +--+SLIP +----+       .    .                   +---+      .
          .             |2         .    .                     |4       .
          .             |          .    .                     |        .
          .        +---------+     .    .                 +--------+   .



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          .            N10         .    .                     N7       .
          .                        .    .Area 2                        .
          .Area 3                  .    ................................
          ..........................

                    Figure 6: A sample OSPF area configuration

        distribution to the backbone.  Their backbone LSAs are shown in
        Table 4.  These summaries show which networks are contained in
        Area 1 (i.e., Networks N1-N4), and the distance to these
        networks from the routers RT3 and RT4 respectively.


        The link-state database for the backbone is shown in Figure 8.
        The set of routers pictured are the backbone routers.  Router
        RT11 is a backbone router because it belongs to two areas.  In
        order to make the backbone connected, a virtual link has been
        configured between Routers R10 and R11.

        The area border routers RT3, RT4, RT7, RT10 and RT11 condense
        the routing information of their attached non-backbone areas for
        distribution via the backbone; these are the dashed stubs that
        appear in Figure 8.  Remember that the third area has been
        configured to condense Networks N9-N11 and Host H1 into a single
        route.  This yields a single dashed line for networks N9-N11 and
        Host H1 in Figure 8.  Routers RT5 and RT7 are AS boundary
        routers; their externally derived information also appears on
        the graph in Figure 8 as stubs.



                     Network   RT3 adv.   RT4 adv.
                     _____________________________
                     N1        4          4
                     N2        4          4
                     N3        1          1
                     N4        2          3

              Table 4: Networks advertised to the backbone
                        by Routers RT3 and RT4.





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                               **FROM**

                          |RT|RT|RT|RT|RT|RT|
                          |1 |2 |3 |4 |5 |7 |N3|
                       ----- -------------------
                       RT1|  |  |  |  |  |  |0 |
                       RT2|  |  |  |  |  |  |0 |
                       RT3|  |  |  |  |  |  |0 |
                   *   RT4|  |  |  |  |  |  |0 |
                   *   RT5|  |  |14|8 |  |  |  |
                   T   RT7|  |  |20|14|  |  |  |
                   O    N1|3 |  |  |  |  |  |  |
                   *    N2|  |3 |  |  |  |  |  |
                   *    N3|1 |1 |1 |1 |  |  |  |
                        N4|  |  |2 |  |  |  |  |
                     Ia,Ib|  |  |20|27|  |  |  |
                        N6|  |  |16|15|  |  |  |
                        N7|  |  |20|19|  |  |  |
                        N8|  |  |18|18|  |  |  |
                 N9-N11,H1|  |  |29|36|  |  |  |
                       N12|  |  |  |  |8 |2 |  |
                       N13|  |  |  |  |8 |  |  |
                       N14|  |  |  |  |8 |  |  |
                       N15|  |  |  |  |  |9 |  |

                      Figure 7: Area 1's Database.

              Networks and routers are represented by vertices.
              An edge of cost X connects Vertex A to Vertex B iff
              the intersection of Column A and Row B is marked
                               with an X.













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                                  **FROM**

                            |RT|RT|RT|RT|RT|RT|RT
                            |3 |4 |5 |6 |7 |10|11|
                         ------------------------
                         RT3|  |  |  |6 |  |  |  |
                         RT4|  |  |8 |  |  |  |  |
                         RT5|  |8 |  |6 |6 |  |  |
                         RT6|8 |  |7 |  |  |5 |  |
                         RT7|  |  |6 |  |  |  |  |
                     *  RT10|  |  |  |7 |  |  |2 |
                     *  RT11|  |  |  |  |  |3 |  |
                     T    N1|4 |4 |  |  |  |  |  |
                     O    N2|4 |4 |  |  |  |  |  |
                     *    N3|1 |1 |  |  |  |  |  |
                     *    N4|2 |3 |  |  |  |  |  |
                          Ia|  |  |  |  |  |5 |  |
                          Ib|  |  |  |7 |  |  |  |
                          N6|  |  |  |  |1 |1 |3 |
                          N7|  |  |  |  |5 |5 |7 |
                          N8|  |  |  |  |4 |3 |2 |
                   N9-N11,H1|  |  |  |  |  |  |11|
                         N12|  |  |8 |  |2 |  |  |
                         N13|  |  |8 |  |  |  |  |
                         N14|  |  |8 |  |  |  |  |
                         N15|  |  |  |  |9 |  |  |


                     Figure 8: The backbone's database.

              Networks and routers are represented by vertices.
              An edge of cost X connects Vertex A to Vertex B iff
              the intersection of Column A and Row B is marked
                                 with an X.

        The backbone enables the exchange of summary information between
        area border routers.  Every area border router hears the area
        summaries from all other area border routers.  It then forms a
        picture of the distance to all networks outside of its area by
        examining the collected LSAs, and adding in the backbone
        distance to each advertising router.




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        Again using Routers RT3 and RT4 as an example, the procedure
        goes as follows: They first calculate the SPF tree for the
        backbone.  This gives the distances to all other area border
        routers.  Also noted are the distances to networks (Ia and Ib)
        and AS boundary routers (RT5 and RT7) that belong to the
        backbone.  This calculation is shown in Table 5.


        Next, by looking at the area summaries from these area border
        routers, RT3 and RT4 can determine the distance to all networks
        outside their area.  These distances are then advertised
        internally to the area by RT3 and RT4.  The advertisements that
        Router RT3 and RT4 will make into Area 1 are shown in Table 6.
        Note that Table 6 assumes that an area range has been configured
        for the backbone which groups Ia and Ib into a single LSA.


        The information imported into Area 1 by Routers RT3 and RT4
        enables an internal router, such as RT1, to choose an area
        border router intelligently.  Router RT1 would use RT4 for
        traffic to Network N6, RT3 for traffic to Network N10, and would


                              dist  from   dist  from
                              RT3          RT4
                   __________________________________
                   to  RT3    *            21
                   to  RT4    22           *
                   to  RT7    20           14
                   to  RT10   15           22
                   to  RT11   18           25
                   __________________________________
                   to  Ia     20           27
                   to  Ib     15           22
                   __________________________________
                   to  RT5    14           8
                   to  RT7    20           14

                 Table 5: Backbone distances calculated
                        by Routers RT3 and RT4.





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                   Destination   RT3 adv.   RT4 adv.
                   _________________________________
                   Ia,Ib         20         27
                   N6            16         15
                   N7            20         19
                   N8            18         18
                   N9-N11,H1     29         36
                   _________________________________
                   RT5           14         8
                   RT7           20         14

              Table 6: Destinations advertised into Area 1
                        by Routers RT3 and RT4.

        load share between the two for traffic to Network N8.

        Router RT1 can also determine in this manner the shortest path
        to the AS boundary routers RT5 and RT7.  Then, by looking at RT5
        and RT7's AS-external-LSAs, Router RT1 can decide between RT5 or
        RT7 when sending to a destination in another Autonomous System
        (one of the networks N12-N15).

        Note that a failure of the line between Routers RT6 and RT10
        will cause the backbone to become disconnected.  Configuring a
        virtual link between Routers RT7 and RT10 will give the backbone
        more connectivity and more resistance to such failures.


    3.5.  IP subnetting support

        OSPF attaches an IP address mask to each advertised route.  The
        mask indicates the range of addresses being described by the
        particular route.  For example, a summary-LSA for the
        destination 128.185.0.0 with a mask of 0xffff0000 actually is
        describing a single route to the collection of destinations
        128.185.0.0 - 128.185.255.255.  Similarly, host routes are
        always advertised with a mask of 0xffffffff, indicating the
        presence of only a single destination.





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        Including the mask with each advertised destination enables the
        implementation of what is commonly referred to as variable-
        length subnetting.  This means that a single IP class A, B, or C
        network number can be broken up into many subnets of various
        sizes.  For example, the network 128.185.0.0 could be broken up
        into 62 variable-sized subnets: 15 subnets of size 4K, 15
        subnets of size 256, and 32 subnets of size 8.  Table 7 shows
        some of the resulting network addresses together with their
        masks.



                  Network address   IP address mask   Subnet size
                  _______________________________________________
                  128.185.16.0      0xfffff000        4K
                  128.185.1.0       0xffffff00        256
                  128.185.0.8       0xfffffff8        8


                         Table 7: Some sample subnet sizes.


        There are many possible ways of dividing up a class A, B, and C
        network into variable sized subnets.  The precise procedure for
        doing so is beyond the scope of this specification.  This
        specification however establishes the following guideline: When
        an IP packet is forwarded, it is always forwarded to the network
        that is the best match for the packet's destination.  Here best
        match is synonymous with the longest or most specific match.
        For example, the default route with destination of 0.0.0.0 and
        mask 0x00000000 is always a match for every IP destination.  Yet
        it is always less specific than any other match.  Subnet masks
        must be assigned so that the best match for any IP destination
        is unambiguous.

        Attaching an address mask to each route also enables the support
        of IP supernetting. For example, a single physical network
        segment could be assigned the [address,mask] pair
        [192.9.4.0,0xfffffc00]. The segment would then be single IP
        network, containing addresses from the four consecutive class C
        network numbers 192.9.4.0 through 192.9.7.0. Such addressing is
        now becoming commonplace with the advent of CIDR (see [Ref10]).



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        In order to get better aggregation at area boundaries, area
        address ranges can be employed (see Section C.2 for more
        details).  Each address range is defined as an [address,mask]
        pair.  Many separate networks may then be contained in a single
        address range, just as a subnetted network is composed of many
        separate subnets.  Area border routers then summarize the area
        contents (for distribution to the backbone) by advertising a
        single route for each address range.  The cost of the route is
        the maximum cost to any of the networks falling in the specified
        range.

        For example, an IP subnetted network might be configured as a
        single OSPF area.  In that case, a single address range could be
        configured:  a class A, B, or C network number along with its
        natural IP mask.  Inside the area, any number of variable sized
        subnets could be defined.  However, external to the area a
        single route for the entire subnetted network would be
        distributed, hiding even the fact that the network is subnetted
        at all.  The cost of this route is the maximum of the set of
        costs to the component subnets.


    3.6.  Supporting stub areas

        In some Autonomous Systems, the majority of the link-state
        database may consist of AS-external-LSAs.  An OSPF AS-external-
        LSA is usually flooded throughout the entire AS.  However, OSPF
        allows certain areas to be configured as "stub areas".  AS-
        external-LSAs are not flooded into/throughout stub areas;
        routing to AS external destinations in these areas is based on a
        (per-area) default only.  This reduces the link-state database
        size, and therefore the memory requirements, for a stub area's
        internal routers.

        In order to take advantage of the OSPF stub area support,
        default routing must be used in the stub area.  This is
        accomplished as follows.  One or more of the stub area's area
        border routers must advertise a default route into the stub area
        via summary-LSAs.  These summary defaults are flooded throughout
        the stub area, but no further.  (For this reason these defaults
        pertain only to the particular stub area).  These summary
        default routes will be used for any destination that is not



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        explicitly reachable by an intra-area or inter-area path (i.e.,
        AS external destinations).

        An area can be configured as a stub when there is a single exit
        point from the area, or when the choice of exit point need not
        be made on a per-external-destination basis.  For example, Area
        3 in Figure 6 could be configured as a stub area, because all
        external traffic must travel though its single area border
        router RT11.  If Area 3 were configured as a stub, Router RT11
        would advertise a default route for distribution inside Area 3
        (in a summary-LSA), instead of flooding the AS-external-LSAs for
        Networks N12-N15 into/throughout the area.

        The OSPF protocol ensures that all routers belonging to an area
        agree on whether the area has been configured as a stub.  This
        guarantees that no confusion will arise in the flooding of AS-
        external-LSAs.

        There are a couple of restrictions on the use of stub areas.
        Virtual links cannot be configured through stub areas.  In
        addition, AS boundary routers cannot be placed internal to stub
        areas.


    3.7.  Partitions of areas

        OSPF does not actively attempt to repair area partitions.  When
        an area becomes partitioned, each component simply becomes a
        separate area.  The backbone then performs routing between the
        new areas.  Some destinations reachable via intra-area routing
        before the partition will now require inter-area routing.

        However, in order to maintain full routing after the partition,
        an address range must not be split across multiple components of
        the area partition. Also, the backbone itself must not
        partition.  If it does, parts of the Autonomous System will
        become unreachable.  Backbone partitions can be repaired by
        configuring virtual links (see Section 15).

        Another way to think about area partitions is to look at the
        Autonomous System graph that was introduced in Section 2.  Area
        IDs can be viewed as colors for the graph's edges.[1] Each edge



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        of the graph connects to a network, or is itself a point-to-
        point network.  In either case, the edge is colored with the
        network's Area ID.

        A group of edges, all having the same color, and interconnected
        by vertices, represents an area.  If the topology of the
        Autonomous System is intact, the graph will have several regions
        of color, each color being a distinct Area ID.

        When the AS topology changes, one of the areas may become
        partitioned.  The graph of the AS will then have multiple
        regions of the same color (Area ID).  The routing in the
        Autonomous System will continue to function as long as these
        regions of same color are connected by the single backbone
        region.






























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4.  Functional Summary

    A separate copy of OSPF's basic routing algorithm runs in each area.
    Routers having interfaces to multiple areas run multiple copies of
    the algorithm.  A brief summary of the routing algorithm follows.

    When a router starts, it first initializes the routing protocol data
    structures.  The router then waits for indications from the lower-
    level protocols that its interfaces are functional.

    A router then uses the OSPF's Hello Protocol to acquire neighbors.
    The router sends Hello packets to its neighbors, and in turn
    receives their Hello packets.  On broadcast and point-to-point
    networks, the router dynamically detects its neighboring routers by
    sending its Hello packets to the multicast address AllSPFRouters.
    On non-broadcast networks, some configuration information may be
    necessary in order to discover neighbors.  On broadcast and NBMA
    networks the Hello Protocol also elects a Designated router for the
    network.

    The router will attempt to form adjacencies with some of its newly
    acquired neighbors.  Link-state databases are synchronized between
    pairs of adjacent routers.  On broadcast and NBMA networks, the
    Designated Router determines which routers should become adjacent.

    Adjacencies control the distribution of routing information.
    Routing updates are sent and received only on adjacencies.

    A router periodically advertises its state, which is also called
    link state.  Link state is also advertised when a router's state
    changes.  A router's adjacencies are reflected in the contents of
    its LSAs.  This relationship between adjacencies and link state
    allows the protocol to detect dead routers in a timely fashion.

    LSAs are flooded throughout the area.  The flooding algorithm is
    reliable, ensuring that all routers in an area have exactly the same
    link-state database.  This database consists of the collection of
    LSAs originated by each router belonging to the area.  From this
    database each router calculates a shortest-path tree, with itself as
    root.  This shortest-path tree in turn yields a routing table for
    the protocol.




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    4.1.  Inter-area routing

        The previous section described the operation of the protocol
        within a single area.  For intra-area routing, no other routing
        information is pertinent.  In order to be able to route to
        destinations outside of the area, the area border routers inject
        additional routing information into the area.  This additional
        information is a distillation of the rest of the Autonomous
        System's topology.

        This distillation is accomplished as follows: Each area border
        router is by definition connected to the backbone.  Each area
        border router summarizes the topology of its attached non-
        backbone areas for transmission on the backbone, and hence to
        all other area border routers.  An area border router then has
        complete topological information concerning the backbone, and
        the area summaries from each of the other area border routers.
        From this information, the router calculates paths to all
        inter-area destinations.  The router then advertises these paths
        into its attached areas.  This enables the area's internal
        routers to pick the best exit router when forwarding traffic
        inter-area destinations.


    4.2.  AS external routes

        Routers that have information regarding other Autonomous Systems
        can flood this information throughout the AS.  This external
        routing information is distributed verbatim to every
        participating router.  There is one exception: external routing
        information is not flooded into "stub" areas (see Section 3.6).

        To utilize external routing information, the path to all routers
        advertising external information must be known throughout the AS
        (excepting the stub areas).  For that reason, the locations of
        these AS boundary routers are summarized by the (non-stub) area
        border routers.








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    4.3.  Routing protocol packets

        The OSPF protocol runs directly over IP, using IP protocol 89.
        OSPF does not provide any explicit fragmentation/reassembly
        support.  When fragmentation is necessary, IP
        fragmentation/reassembly is used.  OSPF protocol packets have
        been designed so that large protocol packets can generally be
        split into several smaller protocol packets.  This practice is
        recommended; IP fragmentation should be avoided whenever
        possible.

        Routing protocol packets should always be sent with the IP TOS
        field set to 0.  If at all possible, routing protocol packets
        should be given preference over regular IP data traffic, both
        when being sent and received.  As an aid to accomplishing this,
        OSPF protocol packets should have their IP precedence field set
        to the value Internetwork Control (see [Ref5]).

        All OSPF protocol packets share a common protocol header that is
        described in Appendix A.  The OSPF packet types are listed below
        in Table 8.  Their formats are also described in Appendix A.



             Type   Packet  name           Protocol  function
             __________________________________________________________
             1      Hello                  Discover/maintain  neighbors
             2      Database Description   Summarize database contents
             3      Link State Request     Database download
             4      Link State Update      Database update
             5      Link State Ack         Flooding acknowledgment


                            Table 8: OSPF packet types.


        OSPF's Hello protocol uses Hello packets to discover and
        maintain neighbor relationships.  The Database Description and
        Link State Request packets are used in the forming of
        adjacencies.  OSPF's reliable update mechanism is implemented by
        the Link State Update and Link State Acknowledgment packets.




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        Each Link State Update packet carries a set of new link state
        advertisements (LSAs) one hop further away from their point of
        origination.  A single Link State Update packet may contain the
        LSAs of several routers.  Each LSA is tagged with the ID of the
        originating router and a checksum of its link state contents.
        Each LSA also has a type field; the different types of OSPF LSAs
        are listed below in Table 9.

        OSPF routing packets (with the exception of Hellos) are sent
        only over adjacencies.  This means that all OSPF protocol
        packets travel a single IP hop, except those that are sent over
        virtual adjacencies.  The IP source address of an OSPF protocol
        packet is one end of a router adjacency, and the IP destination
        address is either the other end of the adjacency or an IP
        multicast address.


    4.4.  Basic implementation requirements

        An implementation of OSPF requires the following pieces of
        system support:


        Timers
            Two different kind of timers are required.  The first kind,
            called "single shot timers", fire once and cause a protocol
            event to be processed.  The second kind, called "interval
            timers", fire at continuous intervals.  These are used for
            the sending of packets at regular intervals.  A good example
            of this is the regular broadcast of Hello packets. The
            granularity of both kinds of timers is one second.

            Interval timers should be implemented to avoid drift.  In
            some router implementations, packet processing can affect
            timer execution.  When multiple routers are attached to a
            single network, all doing broadcasts, this can lead to the
            synchronization of routing packets (which should be
            avoided).  If timers cannot be implemented to avoid drift,
            small random amounts should be added to/subtracted from the
            interval timer at each firing.





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        LS     LSA                LSA description
        type   name
        ________________________________________________________
        1      Router-LSAs        Originated by all routers.
                                  This LSA describes
                                  the collected states of the
                                  router's interfaces to an
                                  area. Flooded throughout a
                                  single area only.
        ________________________________________________________
        2      Network-LSAs       Originated for broadcast
                                  and NBMA networks by
                                  the Designated Router. This
                                  LSA contains the
                                  list of routers connected
                                  to the network. Flooded
                                  throughout a single area only.
        ________________________________________________________
        3,4    Summary-LSAs       Originated by area border
                                  routers, and flooded through-
                                  out the LSA's associated
                                  area. Each summary-LSA
                                  describes a route to a
                                  destination outside the area,
                                  yet still inside the AS
                                  (i.e., an inter-area route).
                                  Type 3 summary-LSAs describe
                                  routes to networks. Type 4
                                  summary-LSAs describe
                                  routes to AS boundary routers.
        ________________________________________________________
        5      AS-external-LSAs   Originated by AS boundary
                                  routers, and flooded through-
                                  out the AS. Each
                                  AS-external-LSA describes
                                  a route to a destination in
                                  another Autonomous System.
                                  Default routes for the AS can
                                  also be described by
                                  AS-external-LSAs.



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RFC 2328                     OSPF Version 2                   April 1998


            Table 9: OSPF link state advertisements (LSAs).



        IP multicast
            Certain OSPF packets take the form of IP multicast
            datagrams.  Support for receiving and sending IP multicast
            datagrams, along with the appropriate lower-level protocol
            support, is required.  The IP multicast datagrams used by
            OSPF never travel more than one hop. For this reason, the
            ability to forward IP multicast datagrams is not required.
            For information on IP multicast, see [Ref7].

        Variable-length subnet support
            The router's IP protocol support must include the ability to
            divide a single IP class A, B, or C network number into many
            subnets of various sizes.  This is commonly called
            variable-length subnetting; see Section 3.5 for details.

        IP supernetting support
            The router's IP protocol support must include the ability to
            aggregate contiguous collections of IP class A, B, and C
            networks into larger quantities called supernets.
            Supernetting has been proposed as one way to improve the
            scaling of IP routing in the worldwide Internet. For more
            information on IP supernetting, see [Ref10].

        Lower-level protocol support
            The lower level protocols referred to here are the network
            access protocols, such as the Ethernet data link layer.
            Indications must be passed from these protocols to OSPF as
            the network interface goes up and down.  For example, on an
            ethernet it would be valuable to know when the ethernet
            transceiver cable becomes unplugged.

        Non-broadcast lower-level protocol support
            On non-broadcast networks, the OSPF Hello Protocol can be
            aided by providing an indication when an attempt is made to
            send a packet to a dead or non-existent router.  For
            example, on an X.25 PDN a dead neighboring router may be





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            indicated by the reception of a X.25 clear with an
            appropriate cause and diagnostic, and this information would
            be passed to OSPF.

        List manipulation primitives
            Much of the OSPF functionality is described in terms of its
            operation on lists of LSAs.  For example, the collection of
            LSAs that will be retransmitted to an adjacent router until
            acknowledged are described as a list.  Any particular LSA
            may be on many such lists.  An OSPF implementation needs to
            be able to manipulate these lists, adding and deleting
            constituent LSAs as necessary.

        Tasking support
            Certain procedures described in this specification invoke
            other procedures.  At times, these other procedures should
            be executed in-line, that is, before the current procedure
            is finished.  This is indicated in the text by instructions
            to execute a procedure.  At other times, the other
            procedures are to be executed only when the current
            procedure has finished.  This is indicated by instructions
            to schedule a task.


    4.5.  Optional OSPF capabilities

        The OSPF protocol defines several optional capabilities.  A
        router indicates the optional capabilities that it supports in
        its OSPF Hello packets, Database Description packets and in its
        LSAs.  This enables routers supporting a mix of optional
        capabilities to coexist in a single Autonomous System.

        Some capabilities must be supported by all routers attached to a
        specific area.  In this case, a router will not accept a
        neighbor's Hello Packet unless there is a match in reported
        capabilities (i.e., a capability mismatch prevents a neighbor
        relationship from forming).  An example of this is the
        ExternalRoutingCapability (see below).

        Other capabilities can be negotiated during the Database
        Exchange process.  This is accomplished by specifying the
        optional capabilities in Database Description packets.  A



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        capability mismatch with a neighbor in this case will result in
        only a subset of the link state database being exchanged between
        the two neighbors.

        The routing table build process can also be affected by the
        presence/absence of optional capabilities.  For example, since
        the optional capabilities are reported in LSAs, routers
        incapable of certain functions can be avoided when building the
        shortest path tree.

        The OSPF optional capabilities defined in this memo are listed
        below.  See Section A.2 for more information.


        ExternalRoutingCapability
            Entire OSPF areas can be configured as "stubs" (see Section
            3.6).  AS-external-LSAs will not be flooded into stub areas.
            This capability is represented by the E-bit in the OSPF
            Options field (see Section A.2).  In order to ensure
            consistent configuration of stub areas, all routers
            interfacing to such an area must have the E-bit clear in
            their Hello packets (see Sections 9.5 and 10.5).


5.  Protocol Data Structures

    The OSPF protocol is described herein in terms of its operation on
    various protocol data structures.  The following list comprises the
    top-level OSPF data structures.  Any initialization that needs to be
    done is noted.  OSPF areas, interfaces and neighbors also have
    associated data structures that are described later in this
    specification.

    Router ID
        A 32-bit number that uniquely identifies this router in the AS.
        One possible implementation strategy would be to use the
        smallest IP interface address belonging to the router. If a
        router's OSPF Router ID is changed, the router's OSPF software
        should be restarted before the new Router ID takes effect.  In
        this case the router should flush its self-originated LSAs from
        the routing domain (see Section 14.1) before restarting, or they
        will persist for up to MaxAge minutes.



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    Area structures
        Each one of the areas to which the router is connected has its
        own data structure.  This data structure describes the working
        of the basic OSPF algorithm.  Remember that each area runs a
        separate copy of the basic OSPF algorithm.

    Backbone (area) structure
        The OSPF backbone area is responsible for the dissemination of
        inter-area routing information.

    Virtual links configured
        The virtual links configured with this router as one endpoint.
        In order to have configured virtual links, the router itself
        must be an area border router.  Virtual links are identified by
        the Router ID of the other endpoint -- which is another area
        border router.  These two endpoint routers must be attached to a
        common area, called the virtual link's Transit area.  Virtual
        links are part of the backbone, and behave as if they were
        unnumbered point-to-point networks between the two routers.  A
        virtual link uses the intra-area routing of its Transit area to
        forward packets.  Virtual links are brought up and down through
        the building of the shortest-path trees for the Transit area.

    List of external routes
        These are routes to destinations external to the Autonomous
        System, that have been gained either through direct experience
        with another routing protocol (such as BGP), or through
        configuration information, or through a combination of the two
        (e.g., dynamic external information to be advertised by OSPF
        with configured metric). Any router having these external routes
        is called an AS boundary router.  These routes are advertised by
        the router into the OSPF routing domain via AS-external-LSAs.

    List of AS-external-LSAs
        Part of the link-state database.  These have originated from the
        AS boundary routers.  They comprise routes to destinations
        external to the Autonomous System.  Note that, if the router is
        itself an AS boundary router, some of these AS-external-LSAs
        have been self-originated.






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    The routing table
        Derived from the link-state database.  Each entry in the routing
        table is indexed by a destination, and contains the
        destination's cost and a set of paths to use in forwarding
        packets to the destination. A path is described by its type and
        next hop.  For more information, see Section 11.

    Figure 9 shows the collection of data structures present in a
    typical router.  The router pictured is RT10, from the map in Figure
    6.  Note that Router RT10 has a virtual link configured to Router
    RT11, with Area 2 as the link's Transit area.  This is indicated by
    the dashed line in Figure 9.  When the virtual link becomes active,
    through the building of the shortest path tree for Area 2, it
    becomes an interface to the backbone (see the two backbone
    interfaces depicted in Figure 9).

6.  The Area Data Structure

    The area data structure contains all the information used to run the
    basic OSPF routing algorithm. Each area maintains its own link-state
    database. A network belongs to a single area, and a router interface
    connects to a single area. Each router adjacency also belongs to a
    single area.

    The OSPF backbone is the special OSPF area responsible for
    disseminating inter-area routing information.

    The area link-state database consists of the collection of router-
    LSAs, network-LSAs and summary-LSAs that have originated from the
    area's routers.  This information is flooded throughout a single
    area only.  The list of AS-external-LSAs (see Section 5) is also
    considered to be part of each area's link-state database.

    Area ID
        A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
        reserved for the backbone.

    List of area address ranges
        In order to aggregate routing information at area boundaries,
        area address ranges can be employed. Each address range is
        specified by an [address,mask] pair and a status indication of
        either Advertise or DoNotAdvertise (see Section 12.4.3).



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                              +----+
                              |RT10|------+
                              +----+       \+-------------+
                             /      \       |Routing Table|
                            /        \      +-------------+
                           /          \
              +------+    /            \    +--------+
              |Area 2|---+              +---|Backbone|
              +------+***********+          +--------+
             /        \           *        /          \
            /          \           *      /            \
       +---------+  +---------+    +------------+       +------------+
       |Interface|  |Interface|    |Virtual Link|       |Interface Ib|
       |  to N6  |  |  to N8  |    |   to RT11  |       +------------+
       +---------+  +---------+    +------------+             |
           /  \           |               |                   |
          /    \          |               |                   |
   +--------+ +--------+  |        +-------------+      +------------+
   |Neighbor| |Neighbor|  |        |Neighbor RT11|      |Neighbor RT6|
   |  RT8   | |  RT7   |  |        +-------------+      +------------+
   +--------+ +--------+  |
                          |
                     +-------------+
                     |Neighbor RT11|
                     +-------------+


                Figure 9: Router RT10's Data structures

    Associated router interfaces
        This router's interfaces connecting to the area.  A router
        interface belongs to one and only one area (or the backbone).
        For the backbone area this list includes all the virtual links.
        A virtual link is identified by the Router ID of its other
        endpoint; its cost is the cost of the shortest intra-area path
        through the Transit area that exists between the two routers.






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    List of router-LSAs
        A router-LSA is generated by each router in the area.  It
        describes the state of the router's interfaces to the area.

    List of network-LSAs
        One network-LSA is generated for each transit broadcast and NBMA
        network in the area.  A network-LSA describes the set of routers
        currently connected to the network.

    List of summary-LSAs
        Summary-LSAs originate from the area's area border routers.
        They describe routes to destinations internal to the Autonomous
        System, yet external to the area (i.e., inter-area
        destinations).

    Shortest-path tree
        The shortest-path tree for the area, with this router itself as
        root.  Derived from the collected router-LSAs and network-LSAs
        by the Dijkstra algorithm (see Section 16.1).

    TransitCapability
        This parameter indicates whether the area can carry data traffic
        that neither originates nor terminates in the area itself. This
        parameter is calculated when the area's shortest-path tree is
        built (see Section 16.1, where TransitCapability is set to TRUE
        if and only if there are one or more fully adjacent virtual
        links using the area as Transit area), and is used as an input
        to a subsequent step of the routing table build process (see
        Section 16.3). When an area's TransitCapability is set to TRUE,
        the area is said to be a "transit area".

    ExternalRoutingCapability
        Whether AS-external-LSAs will be flooded into/throughout the
        area.  This is a configurable parameter.  If AS-external-LSAs
        are excluded from the area, the area is called a "stub". Within
        stub areas, routing to AS external destinations will be based
        solely on a default summary route.  The backbone cannot be
        configured as a stub area.  Also, virtual links cannot be
        configured through stub areas.  For more information, see
        Section 3.6.





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    StubDefaultCost
        If the area has been configured as a stub area, and the router
        itself is an area border router, then the StubDefaultCost
        indicates the cost of the default summary-LSA that the router
        should advertise into the area. See Section 12.4.3 for more
        information.


    Unless otherwise specified, the remaining sections of this document
    refer to the operation of the OSPF protocol within a single area.


7.  Bringing Up Adjacencies

    OSPF creates adjacencies between neighboring routers for the purpose
    of exchanging routing information.  Not every two neighboring
    routers will become adjacent.  This section covers the generalities
    involved in creating adjacencies.  For further details consult
    Section 10.


    7.1.  The Hello Protocol

        The Hello Protocol is responsible for establishing and
        maintaining neighbor relationships.  It also ensures that
        communication between neighbors is bidirectional.  Hello packets
        are sent periodically out all router interfaces.  Bidirectional
        communication is indicated when the router sees itself listed in
        the neighbor's Hello Packet.  On broadcast and NBMA networks,
        the Hello Protocol elects a Designated Router for the network.

        The Hello Protocol works differently on broadcast networks, NBMA
        networks and Point-to-MultiPoint networks.  On broadcast
        networks, each router advertises itself by periodically
        multicasting Hello Packets.  This allows neighbors to be
        discovered dynamically.  These Hello Packets contain the
        router's view of the Designated Router's identity, and the list
        of routers whose Hello Packets have been seen recently.

        On NBMA networks some configuration information may be necessary
        for the operation of the Hello Protocol.  Each router that may
        potentially become Designated Router has a list of all other



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        routers attached to the network.  A router, having Designated
        Router potential, sends Hello Packets to all other potential
        Designated Routers when its interface to the NBMA network first
        becomes operational.  This is an attempt to find the Designated
        Router for the network.  If the router itself is elected
        Designated Router, it begins sending Hello Packets to all other
        routers attached to the network.

        On Point-to-MultiPoint networks, a router sends Hello Packets to
        all neighbors with which it can communicate directly. These
        neighbors may be discovered dynamically through a protocol such
        as Inverse ARP (see [Ref14]), or they may be configured.

        After a neighbor has been discovered, bidirectional
        communication ensured, and (if on a broadcast or NBMA network) a
        Designated Router elected, a decision is made regarding whether
        or not an adjacency should be formed with the neighbor (see
        Section 10.4). If an adjacency is to be formed, the first step
        is to synchronize the neighbors' link-state databases.  This is
        covered in the next section.


    7.2.  The Synchronization of Databases

        In a link-state routing algorithm, it is very important for all
        routers' link-state databases to stay synchronized.  OSPF
        simplifies this by requiring only adjacent routers to remain
        synchronized.  The synchronization process begins as soon as the
        routers attempt to bring up the adjacency.  Each router
        describes its database by sending a sequence of Database
        Description packets to its neighbor.  Each Database Description
        Packet describes a set of LSAs belonging to the router's
        database.  When the neighbor sees an LSA that is more recent
        than its own database copy, it makes a note that this newer LSA
        should be requested.

        This sending and receiving of Database Description packets is
        called the "Database Exchange Process".  During this process,
        the two routers form a master/slave relationship.  Each Database
        Description Packet has a sequence number.  Database Description
        Packets sent by the master (polls) are acknowledged by the slave
        through echoing of the sequence number.  Both polls and their



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        responses contain summaries of link state data.  The master is
        the only one allowed to retransmit Database Description Packets.
        It does so only at fixed intervals, the length of which is the
        configured per-interface constant RxmtInterval.

        Each Database Description contains an indication that there are
        more packets to follow --- the M-bit.  The Database Exchange
        Process is over when a router has received and sent Database
        Description Packets with the M-bit off.

        During and after the Database Exchange Process, each router has
        a list of those LSAs for which the neighbor has more up-to-date
        instances.  These LSAs are requested in Link State Request
        Packets.  Link State Request packets that are not satisfied are
        retransmitted at fixed intervals of time RxmtInterval.  When the
        Database Description Process has completed and all Link State
        Requests have been satisfied, the databases are deemed
        synchronized and the routers are marked fully adjacent.  At this
        time the adjacency is fully functional and is advertised in the
        two routers' router-LSAs.

        The adjacency is used by the flooding procedure as soon as the
        Database Exchange Process begins.  This simplifies database
        synchronization, and guarantees that it finishes in a
        predictable period of time.


    7.3.  The Designated Router

        Every broadcast and NBMA network has a Designated Router.  The
        Designated Router performs two main functions for the routing
        protocol:

        o   The Designated Router originates a network-LSA on behalf of
            the network.  This LSA lists the set of routers (including
            the Designated Router itself) currently attached to the
            network.  The Link State ID for this LSA (see Section
            12.1.4) is the IP interface address of the Designated
            Router.  The IP network number can then be obtained by using
            the network's subnet/network mask.





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        o   The Designated Router becomes adjacent to all other routers
            on the network.  Since the link state databases are
            synchronized across adjacencies (through adjacency bring-up
            and then the flooding procedure), the Designated Router
            plays a central part in the synchronization process.


        The Designated Router is elected by the Hello Protocol.  A
        router's Hello Packet contains its Router Priority, which is
        configurable on a per-interface basis.  In general, when a
        router's interface to a network first becomes functional, it
        checks to see whether there is currently a Designated Router for
        the network.  If there is, it accepts that Designated Router,
        regardless of its Router Priority.  (This makes it harder to
        predict the identity of the Designated Router, but ensures that
        the Designated Router changes less often.  See below.)
        Otherwise, the router itself becomes Designated Router if it has
        the highest Router Priority on the network.  A more detailed
        (and more accurate) description of Designated Router election is
        presented in Section 9.4.

        The Designated Router is the endpoint of many adjacencies.  In
        order to optimize the flooding procedure on broadcast networks,
        the Designated Router multicasts its Link State Update Packets
        to the address AllSPFRouters, rather than sending separate
        packets over each adjacency.

        Section 2 of this document discusses the directed graph
        representation of an area.  Router nodes are labelled with their
        Router ID.  Transit network nodes are actually labelled with the
        IP address of their Designated Router.  It follows that when the
        Designated Router changes, it appears as if the network node on
        the graph is replaced by an entirely new node.  This will cause
        the network and all its attached routers to originate new LSAs.
        Until the link-state databases again converge, some temporary
        loss of connectivity may result.  This may result in ICMP
        unreachable messages being sent in response to data traffic.
        For that reason, the Designated Router should change only
        infrequently.  Router Priorities should be configured so that
        the most dependable router on a network eventually becomes
        Designated Router.




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    7.4.  The Backup Designated Router

        In order to make the transition to a new Designated Router
        smoother, there is a Backup Designated Router for each broadcast
        and NBMA network.  The Backup Designated Router is also adjacent
        to all routers on the network, and becomes Designated Router
        when the previous Designated Router fails.  If there were no
        Backup Designated Router, when a new Designated Router became
        necessary, new adjacencies would have to be formed between the
        new Designated Router and all other routers attached to the
        network.  Part of the adjacency forming process is the
        synchronizing of link-state databases, which can potentially
        take quite a long time.  During this time, the network would not
        be available for transit data traffic.  The Backup Designated
        obviates the need to form these adjacencies, since they already
        exist.  This means the period of disruption in transit traffic
        lasts only as long as it takes to flood the new LSAs (which
        announce the new Designated Router).

        The Backup Designated Router does not generate a network-LSA for
        the network.  (If it did, the transition to a new Designated
        Router would be even faster.  However, this is a tradeoff
        between database size and speed of convergence when the
        Designated Router disappears.)

        The Backup Designated Router is also elected by the Hello
        Protocol.  Each Hello Packet has a field that specifies the
        Backup Designated Router for the network.

        In some steps of the flooding procedure, the Backup Designated
        Router plays a passive role, letting the Designated Router do
        more of the work.  This cuts down on the amount of local routing
        traffic.  See Section 13.3 for more information.


    7.5.  The graph of adjacencies

        An adjacency is bound to the network that the two routers have
        in common.  If two routers have multiple networks in common,
        they may have multiple adjacencies between them.





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        One can picture the collection of adjacencies on a network as
        forming an undirected graph.  The vertices consist of routers,
        with an edge joining two routers if they are adjacent.  The
        graph of adjacencies describes the flow of routing protocol
        packets, and in particular Link State Update Packets, through
        the Autonomous System.

        Two graphs are possible, depending on whether a Designated
        Router is elected for the network.  On physical point-to-point
        networks, Point-to-MultiPoint networks and virtual links,
        neighboring routers become adjacent whenever they can
        communicate directly.  In contrast, on broadcast and NBMA
        networks only the Designated Router and the Backup Designated
        Router become adjacent to all other routers attached to the
        network.



          +---+            +---+
          |RT1|------------|RT2|            o---------------o
          +---+    N1      +---+           RT1             RT2



                                                 RT7
                                                  o---------+
            +---+   +---+   +---+                /|\        |
            |RT7|   |RT3|   |RT4|               / | \       |
            +---+   +---+   +---+              /  |  \      |
              |       |       |               /   |   \     |
         +-----------------------+        RT5o RT6o    oRT4 |
                  |       |     N2            *   *   *     |
                +---+   +---+                  *  *  *      |
                |RT5|   |RT6|                   * * *       |
                +---+   +---+                    ***        |
                                                  o---------+
                                                 RT3


                  Figure 10: The graph of adjacencies





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        These graphs are shown in Figure 10.  It is assumed that Router
        RT7 has become the Designated Router, and Router RT3 the Backup
        Designated Router, for the Network N2.  The Backup Designated
        Router performs a lesser function during the flooding procedure
        than the Designated Router (see Section 13.3).  This is the
        reason for the dashed lines connecting the Backup Designated
        Router RT3.


8.  Protocol Packet Processing

    This section discusses the general processing of OSPF routing
    protocol packets.  It is very important that the router link-state
    databases remain synchronized.  For this reason, routing protocol
    packets should get preferential treatment over ordinary data
    packets, both in sending and receiving.

    Routing protocol packets are sent along adjacencies only (with the
    exception of Hello packets, which are used to discover the
    adjacencies).  This means that all routing protocol packets travel a
    single IP hop, except those sent over virtual links.

    All routing protocol packets begin with a standard header.  The
    sections below provide details on how to fill in and verify this
    standard header.  Then, for each packet type, the section giving
    more details on that particular packet type's processing is listed.

    8.1.  Sending protocol packets

        When a router sends a routing protocol packet, it fills in the
        fields of the standard OSPF packet header as follows.  For more
        details on the header format consult Section A.3.1:

        Version #
            Set to 2, the version number of the protocol as documented
            in this specification.

        Packet type
            The type of OSPF packet, such as Link state Update or Hello
            Packet.





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        Packet length
            The length of the entire OSPF packet in bytes, including the
            standard OSPF packet header.

        Router ID
            The identity of the router itself (who is originating the
            packet).

        Area ID
            The OSPF area that the packet is being sent into.

        Checksum
            The standard IP 16-bit one's complement checksum of the
            entire OSPF packet, excluding the 64-bit authentication
            field.  This checksum is calculated as part of the
            appropriate authentication procedure; for some OSPF
            authentication types, the checksum calculation is omitted.
            See Section D.4 for details.

        AuType and Authentication
            Each OSPF packet exchange is authenticated.  Authentication
            types are assigned by the protocol and are documented in
            Appendix D.  A different authentication procedure can be
            used for each IP network/subnet.  Autype indicates the type
            of authentication procedure in use. The 64-bit
            authentication field is then for use by the chosen
            authentication procedure.  This procedure should be the last
            called when forming the packet to be sent. See Section D.4
            for details.


        The IP destination address for the packet is selected as
        follows.  On physical point-to-point networks, the IP
        destination is always set to the address AllSPFRouters.  On all
        other network types (including virtual links), the majority of
        OSPF packets are sent as unicasts, i.e., sent directly to the
        other end of the adjacency.  In this case, the IP destination is
        just the Neighbor IP address associated with the other end of
        the adjacency (see Section 10).  The only packets not sent as
        unicasts are on broadcast networks; on these networks Hello
        packets are sent to the multicast destination AllSPFRouters, the
        Designated Router and its Backup send both Link State Update



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        Packets and Link State Acknowledgment Packets to the multicast
        address AllSPFRouters, while all other routers send both their
        Link State Update and Link State Acknowledgment Packets to the
        multicast address AllDRouters.

        Retransmissions of Link State Update packets are ALWAYS sent
        directly to the neighbor. On multi-access networks, this means
        that retransmissions should be sent to the neighbor's IP
        address.

        The IP source address should be set to the IP address of the
        sending interface.  Interfaces to unnumbered point-to-point
        networks have no associated IP address.  On these interfaces,
        the IP source should be set to any of the other IP addresses
        belonging to the router.  For this reason, there must be at
        least one IP address assigned to the router.[2] Note that, for
        most purposes, virtual links act precisely the same as
        unnumbered point-to-point networks.  However, each virtual link
        does have an IP interface address (discovered during the routing
        table build process) which is used as the IP source when sending
        packets over the virtual link.

        For more information on the format of specific OSPF packet
        types, consult the sections listed in Table 10.



             Type   Packet name            detailed section (transmit)
             _________________________________________________________
             1      Hello                  Section  9.5
             2      Database description   Section 10.8
             3      Link state request     Section 10.9
             4      Link state update      Section 13.3
             5      Link state ack         Section 13.5


      Table 10: Sections describing OSPF protocol packet transmission.








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    8.2.  Receiving protocol packets

        Whenever a protocol packet is received by the router it is
        marked with the interface it was received on.  For routers that
        have virtual links configured, it may not be immediately obvious
        which interface to associate the packet with.  For example,
        consider the Router RT11 depicted in Figure 6.  If RT11 receives
        an OSPF protocol packet on its interface to Network N8, it may
        want to associate the packet with the interface to Area 2, or
        with the virtual link to Router RT10 (which is part of the
        backbone).  In the following, we assume that the packet is
        initially associated with the non-virtual  link.[3]

        In order for the packet to be accepted at the IP level, it must
        pass a number of tests, even before the packet is passed to OSPF
        for processing:


        o   The IP checksum must be correct.

        o   The packet's IP destination address must be the IP address
            of the receiving interface, or one of the IP multicast
            addresses AllSPFRouters or AllDRouters.

        o   The IP protocol specified must be OSPF (89).

        o   Locally originated packets should not be passed on to OSPF.
            That is, the source IP address should be examined to make
            sure this is not a multicast packet that the router itself
            generated.


        Next, the OSPF packet header is verified.  The fields specified
        in the header must match those configured for the receiving
        interface.  If they do not, the packet should be discarded:


        o   The version number field must specify protocol version 2.

        o   The Area ID found in the OSPF header must be verified.  If
            both of the following cases fail, the packet should be
            discarded.  The Area ID specified in the header must either:



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            (1) Match the Area ID of the receiving interface.  In this
                case, the packet has been sent over a single hop.
                Therefore, the packet's IP source address is required to
                be on the same network as the receiving interface.  This
                can be verified by comparing the packet's IP source
                address to the interface's IP address, after masking
                both addresses with the interface mask.  This comparison
                should not be performed on point-to-point networks. On
                point-to-point networks, the interface addresses of each
                end of the link are assigned independently, if they are
                assigned at all.

            (2) Indicate the backbone.  In this case, the packet has
                been sent over a virtual link.  The receiving router
                must be an area border router, and the Router ID
                specified in the packet (the source router) must be the
                other end of a configured virtual link.  The receiving
                interface must also attach to the virtual link's
                configured Transit area.  If all of these checks
                succeed, the packet is accepted and is from now on
                associated with the virtual link (and the backbone
                area).

        o   Packets whose IP destination is AllDRouters should only be
            accepted if the state of the receiving interface is DR or
            Backup (see Section 9.1).

        o   The AuType specified in the packet must match the AuType
            specified for the associated area.

        o   The packet must be authenticated.  The authentication
            procedure is indicated by the setting of AuType (see
            Appendix D).  The authentication procedure may use one or
            more Authentication keys, which can be configured on a per-
            interface basis.  The authentication procedure may also
            verify the checksum field in the OSPF packet header (which,
            when used, is set to the standard IP 16-bit one's complement
            checksum of the OSPF packet's contents after excluding the
            64-bit authentication field).  If the authentication
            procedure fails, the packet should be discarded.





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        If the packet type is Hello, it should then be further processed
        by the Hello Protocol (see Section 10.5).  All other packet
        types are sent/received only on adjacencies.  This means that
        the packet must have been sent by one of the router's active
        neighbors.  If the receiving interface connects to a broadcast
        network, Point-to-MultiPoint network or NBMA network the sender
        is identified by the IP source address found in the packet's IP
        header.  If the receiving interface connects to a point-to-point
        network or a virtual link, the sender is identified by the
        Router ID (source router) found in the packet's OSPF header.
        The data structure associated with the receiving interface
        contains the list of active neighbors.  Packets not matching any
        active neighbor are discarded.

        At this point all received protocol packets are associated with
        an active neighbor.  For the further input processing of
        specific packet types, consult the sections listed in Table 11.



              Type   Packet name            detailed section (receive)
              ________________________________________________________
              1      Hello                  Section 10.5
              2      Database description   Section 10.6
              3      Link state request     Section 10.7
              4      Link state update      Section 13
              5      Link state ack         Section 13.7


      Table 11: Sections describing OSPF protocol packet reception.



9.  The Interface Data Structure

    An OSPF interface is the connection between a router and a network.
    We assume a single OSPF interface to each attached network/subnet,
    although supporting multiple interfaces on a single network is
    considered in Appendix F. Each interface structure has at most one
    IP interface address.





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    An OSPF interface can be considered to belong to the area that
    contains the attached network.  All routing protocol packets
    originated by the router over this interface are labelled with the
    interface's Area ID.  One or more router adjacencies may develop
    over an interface.  A router's LSAs reflect the state of its
    interfaces and their associated adjacencies.

    The following data items are associated with an interface.  Note
    that a number of these items are actually configuration for the
    attached network; such items must be the same for all routers
    connected to the network.

    Type
        The OSPF interface type is either point-to-point, broadcast,
        NBMA, Point-to-MultiPoint or virtual link.

    State
        The functional level of an interface.  State determines whether
        or not full adjacencies are allowed to form over the interface.
        State is also reflected in the router's LSAs.

    IP interface address
        The IP address associated with the interface.  This appears as
        the IP source address in all routing protocol packets originated
        over this interface.  Interfaces to unnumbered point-to-point
        networks do not have an associated IP address.

    IP interface mask
        Also referred to as the subnet mask, this indicates the portion
        of the IP interface address that identifies the attached
        network.  Masking the IP interface address with the IP interface
        mask yields the IP network number of the attached network.  On
        point-to-point networks and virtual links, the IP interface mask
        is not defined. On these networks, the link itself is not
        assigned an IP network number, and so the addresses of each side
        of the link are assigned independently, if they are assigned at
        all.

    Area ID
        The Area ID of the area to which the attached network belongs.
        All routing protocol packets originating from the interface are
        labelled with this Area ID.



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    HelloInterval
        The length of time, in seconds, between the Hello packets that
        the router sends on the interface.  Advertised in Hello packets
        sent out this interface.

    RouterDeadInterval
        The number of seconds before the router's neighbors will declare
        it down, when they stop hearing the router's Hello Packets.
        Advertised in Hello packets sent out this interface.

    InfTransDelay
        The estimated number of seconds it takes to transmit a Link
        State Update Packet over this interface.  LSAs contained in the
        Link State Update packet will have their age incremented by this
        amount before transmission.  This value should take into account
        transmission and propagation delays; it must be greater than
        zero.

    Router Priority
        An 8-bit unsigned integer.  When two routers attached to a
        network both attempt to become Designated Router, the one with
        the highest Router Priority takes precedence.  A router whose
        Router Priority is set to 0 is ineligible to become Designated
        Router on the attached network.  Advertised in Hello packets
        sent out this interface.

    Hello Timer
        An interval timer that causes the interface to send a Hello
        packet.  This timer fires every HelloInterval seconds.  Note
        that on non-broadcast networks a separate Hello packet is sent
        to each qualified neighbor.

    Wait Timer
        A single shot timer that causes the interface to exit the
        Waiting state, and as a consequence select a Designated Router
        on the network.  The length of the timer is RouterDeadInterval
        seconds.

    List of neighboring routers
        The other routers attached to this network.  This list is formed
        by the Hello Protocol.  Adjacencies will be formed to some of




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        these neighbors.  The set of adjacent neighbors can be
        determined by an examination of all of the neighbors' states.

    Designated Router
        The Designated Router selected for the attached network.  The
        Designated Router is selected on all broadcast and NBMA networks
        by the Hello Protocol.  Two pieces of identification are kept
        for the Designated Router: its Router ID and its IP interface
        address on the network.  The Designated Router advertises link
        state for the network; this network-LSA is labelled with the
        Designated Router's IP address.  The Designated Router is
        initialized to 0.0.0.0, which indicates the lack of a Designated
        Router.

    Backup Designated Router
        The Backup Designated Router is also selected on all broadcast
        and NBMA networks by the Hello Protocol.  All routers on the
        attached network become adjacent to both the Designated Router
        and the Backup Designated Router.  The Backup Designated Router
        becomes Designated Router when the current Designated Router
        fails.  The Backup Designated Router is initialized to 0.0.0.0,
        indicating the lack of a Backup Designated Router.

    Interface output cost(s)
        The cost of sending a data packet on the interface, expressed in
        the link state metric.  This is advertised as the link cost for
        this interface in the router-LSA. The cost of an interface must
        be greater than zero.

    RxmtInterval
        The number of seconds between LSA retransmissions, for
        adjacencies belonging to this interface.  Also used when
        retransmitting Database Description and Link State Request
        Packets.

    AuType
        The type of authentication used on the attached network/subnet.
        Authentication types are defined in Appendix D.  All OSPF packet
        exchanges are authenticated.  Different authentication schemes
        may be used on different networks/subnets.





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    Authentication key
        This configured data allows the authentication procedure to
        generate and/or verify OSPF protocol packets.  The
        Authentication key can be configured on a per-interface basis.
        For example, if the AuType indicates simple password, the
        Authentication key would be a 64-bit clear password which is
        inserted into the OSPF packet header. If instead Autype
        indicates Cryptographic authentication, then the Authentication
        key is a shared secret which enables the generation/verification
        of message digests which are appended to the OSPF protocol
        packets. When Cryptographic authentication is used, multiple
        simultaneous keys are supported in order to achieve smooth key
        transition (see Section D.3).


    9.1.  Interface states

        The various states that router interfaces may attain is
        documented in this section.  The states are listed in order of
        progressing functionality.  For example, the inoperative state
        is listed first, followed by a list of intermediate states
        before the final, fully functional state is achieved.  The
        specification makes use of this ordering by sometimes making
        references such as "those interfaces in state greater than X".
        Figure 11 shows the graph of interface state changes.  The arcs
        of the graph are labelled with the event causing the state
        change.  These events are documented in Section 9.2.  The
        interface state machine is described in more detail in Section
        9.3.


        Down
            This is the initial interface state.  In this state, the
            lower-level protocols have indicated that the interface is
            unusable.  No protocol traffic at all will be sent or
            received on such a interface.  In this state, interface
            parameters should be set to their initial values.  All
            interface timers should be disabled, and there should be no
            adjacencies associated with the interface.

        Loopback
            In this state, the router's interface to the network is



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                                  +----+   UnloopInd   +--------+
                                  |Down|<--------------|Loopback|
                                  +----+               +--------+
                                     |
                                     |InterfaceUp
                          +-------+  |               +--------------+
                          |Waiting|<-+-------------->|Point-to-point|
                          +-------+                  +--------------+
                              |
                     WaitTimer|BackupSeen
                              |
                              |
                              |   NeighborChange
          +------+           +-+<---------------- +-------+
          |Backup|<----------|?|----------------->|DROther|
          +------+---------->+-+<-----+           +-------+
                    Neighbor  |       |
                    Change    |       |Neighbor
                              |       |Change
                              |     +--+
                              +---->|DR|
                                    +--+

                      Figure 11: Interface State changes

                 In addition to the state transitions pictured,
                 Event InterfaceDown always forces Down State, and
                 Event LoopInd always forces Loopback State


            looped back.  The interface may be looped back in hardware
            or software.  The interface will be unavailable for regular
            data traffic.  However, it may still be desirable to gain
            information on the quality of this interface, either through
            sending ICMP pings to the interface or through something
            like a bit error test.  For this reason, IP packets may
            still be addressed to an interface in Loopback state.  To







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            facilitate this, such interfaces are advertised in router-
            LSAs as single host routes, whose destination is the IP
            interface address.[4]

        Waiting
            In this state, the router is trying to determine the
            identity of the (Backup) Designated Router for the network.
            To do this, the router monitors the Hello Packets it
            receives.  The router is not allowed to elect a Backup
            Designated Router nor a Designated Router until it
            transitions out of Waiting state.  This prevents unnecessary
            changes of (Backup) Designated Router.

        Point-to-point
            In this state, the interface is operational, and connects
            either to a physical point-to-point network or to a virtual
            link.  Upon entering this state, the router attempts to form
            an adjacency with the neighboring router.  Hello Packets are
            sent to the neighbor every HelloInterval seconds.

        DR Other
            The interface is to a broadcast or NBMA network on which
            another router has been selected to be the Designated
            Router.  In this state, the router itself has not been
            selected Backup Designated Router either.  The router forms
            adjacencies to both the Designated Router and the Backup
            Designated Router (if they exist).

        Backup
            In this state, the router itself is the Backup Designated
            Router on the attached network.  It will be promoted to
            Designated Router when the present Designated Router fails.
            The router establishes adjacencies to all other routers
            attached to the network.  The Backup Designated Router
            performs slightly different functions during the Flooding
            Procedure, as compared to the Designated Router (see Section
            13.3).  See Section 7.4 for more details on the functions
            performed by the Backup Designated Router.

        DR  In this state, this router itself is the Designated Router
            on the attached network.  Adjacencies are established to all
            other routers attached to the network.  The router must also



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            originate a network-LSA for the network node.  The network-
            LSA will contain links to all routers (including the
            Designated Router itself) attached to the network.  See
            Section 7.3 for more details on the functions performed by
            the Designated Router.


    9.2.  Events causing interface state changes

        State changes can be effected by a number of events.  These
        events are pictured as the labelled arcs in Figure 11.  The
        label definitions are listed below.  For a detailed explanation
        of the effect of these events on OSPF protocol operation,
        consult Section 9.3.


        InterfaceUp
            Lower-level protocols have indicated that the network
            interface is operational.  This enables the interface to
            transition out of Down state.  On virtual links, the
            interface operational indication is actually a result of the
            shortest path calculation (see Section 16.7).

        WaitTimer
            The Wait Timer has fired, indicating the end of the waiting
            period that is required before electing a (Backup)
            Designated Router.

        BackupSeen
            The router has detected the existence or non-existence of a
            Backup Designated Router for the network.  This is done in
            one of two ways.  First, an Hello Packet may be received
            from a neighbor claiming to be itself the Backup Designated
            Router.  Alternatively, an Hello Packet may be received from
            a neighbor claiming to be itself the Designated Router, and
            indicating that there is no Backup Designated Router.  In
            either case there must be bidirectional communication with
            the neighbor, i.e., the router must also appear in the
            neighbor's Hello Packet.  This event signals an end to the
            Waiting state.





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        NeighborChange
            There has been a change in the set of bidirectional
            neighbors associated with the interface.  The (Backup)
            Designated Router needs to be recalculated.  The following
            neighbor changes lead to the NeighborChange event.  For an
            explanation of neighbor states, see Section 10.1.

            o   Bidirectional communication has been established to a
                neighbor.  In other words, the state of the neighbor has
                transitioned to 2-Way or higher.

            o   There is no longer bidirectional communication with a
                neighbor.  In other words, the state of the neighbor has
                transitioned to Init or lower.

            o   One of the bidirectional neighbors is newly declaring
                itself as either Designated Router or Backup Designated
                Router.  This is detected through examination of that
                neighbor's Hello Packets.

            o   One of the bidirectional neighbors is no longer
                declaring itself as Designated Router, or is no longer
                declaring itself as Backup Designated Router.  This is
                again detected through examination of that neighbor's
                Hello Packets.

            o   The advertised Router Priority for a bidirectional
                neighbor has changed.  This is again detected through
                examination of that neighbor's Hello Packets.

        LoopInd
            An indication has been received that the interface is now
            looped back to itself.  This indication can be received
            either from network management or from the lower level
            protocols.

        UnloopInd
            An indication has been received that the interface is no
            longer looped back.  As with the LoopInd event, this






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            indication can be received either from network management or
            from the lower level protocols.

        InterfaceDown
            Lower-level protocols indicate that this interface is no
            longer functional.  No matter what the current interface
            state is, the new interface state will be Down.

    9.3.  The Interface state machine

        A detailed description of the interface state changes follows.
        Each state change is invoked by an event (Section 9.2).  This
        event may produce different effects, depending on the current
        state of the interface.  For this reason, the state machine
        below is organized by current interface state and received
        event.  Each entry in the state machine describes the resulting
        new interface state and the required set of additional actions.

        When an interface's state changes, it may be necessary to
        originate a new router-LSA.  See Section 12.4 for more details.

        Some of the required actions below involve generating events for
        the neighbor state machine.  For example, when an interface
        becomes inoperative, all neighbor connections associated with
        the interface must be destroyed.  For more information on the
        neighbor state machine, see Section 10.3.


         State(s):  Down

            Event:  InterfaceUp

        New state:  Depends upon action routine

           Action:  Start the interval Hello Timer, enabling the
                    periodic sending of Hello packets out the interface.
                    If the attached network is a physical point-to-point
                    network, Point-to-MultiPoint network or virtual
                    link, the interface state transitions to Point-to-
                    Point.  Else, if the router is not eligible to
                    become Designated Router the interface state
                    transitions to DR Other.



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                    Otherwise, the attached network is a broadcast or
                    NBMA network and the router is eligible to become
                    Designated Router.  In this case, in an attempt to
                    discover the attached network's Designated Router
                    the interface state is set to Waiting and the single
                    shot Wait Timer is started.  Additionally, if the
                    network is an NBMA network examine the configured
                    list of neighbors for this interface and generate
                    the neighbor event Start for each neighbor that is
                    also eligible to become Designated Router.


         State(s):  Waiting

            Event:  BackupSeen

        New state:  Depends upon action routine.

           Action:  Calculate the attached network's Backup Designated
                    Router and Designated Router, as shown in Section
                    9.4.  As a result of this calculation, the new state
                    of the interface will be either DR Other, Backup or
                    DR.


         State(s):  Waiting

            Event:  WaitTimer

        New state:  Depends upon action routine.

           Action:  Calculate the attached network's Backup Designated
                    Router and Designated Router, as shown in Section
                    9.4.  As a result of this calculation, the new state
                    of the interface will be either DR Other, Backup or
                    DR.


         State(s):  DR Other, Backup or DR

            Event:  NeighborChange




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        New state:  Depends upon action routine.

           Action:  Recalculate the attached network's Backup Designated
                    Router and Designated Router, as shown in Section
                    9.4.  As a result of this calculation, the new state
                    of the interface will be either DR Other, Backup or
                    DR.


         State(s):  Any State

            Event:  InterfaceDown

        New state:  Down

           Action:  All interface variables are reset, and interface
                    timers disabled.  Also, all neighbor connections
                    associated with the interface are destroyed.  This
                    is done by generating the event KillNbr on all
                    associated neighbors (see Section 10.2).


         State(s):  Any State

            Event:  LoopInd

        New state:  Loopback

           Action:  Since this interface is no longer connected to the
                    attached network the actions associated with the
                    above InterfaceDown event are executed.


         State(s):  Loopback

            Event:  UnloopInd

        New state:  Down

           Action:  No actions are necessary.  For example, the
                    interface variables have already been reset upon
                    entering the Loopback state.  Note that reception of



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                    an InterfaceUp event is necessary before the
                    interface again becomes fully functional.


    9.4.  Electing the Designated Router

        This section describes the algorithm used for calculating a
        network's Designated Router and Backup Designated Router.  This
        algorithm is invoked by the Interface state machine.  The
        initial time a router runs the election algorithm for a network,
        the network's Designated Router and Backup Designated Router are
        initialized to 0.0.0.0.  This indicates the lack of both a
        Designated Router and a Backup Designated Router.

        The Designated Router election algorithm proceeds as follows:
        Call the router doing the calculation Router X.  The list of
        neighbors attached to the network and having established
        bidirectional communication with Router X is examined.  This
        list is precisely the collection of Router X's neighbors (on
        this network) whose state is greater than or equal to 2-Way (see
        Section 10.1).  Router X itself is also considered to be on the
        list.  Discard all routers from the list that are ineligible to
        become Designated Router.  (Routers having Router Priority of 0
        are ineligible to become Designated Router.)  The following
        steps are then executed, considering only those routers that
        remain on the list:

        (1) Note the current values for the network's Designated Router
            and Backup Designated Router.  This is used later for
            comparison purposes.

        (2) Calculate the new Backup Designated Router for the network
            as follows.  Only those routers on the list that have not
            declared themselves to be Designated Router are eligible to
            become Backup Designated Router.  If one or more of these
            routers have declared themselves Backup Designated Router
            (i.e., they are currently listing themselves as Backup
            Designated Router, but not as Designated Router, in their
            Hello Packets) the one having highest Router Priority is
            declared to be Backup Designated Router.  In case of a tie,
            the one having the highest Router ID is chosen.  If no
            routers have declared themselves Backup Designated Router,



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            choose the router having highest Router Priority, (again
            excluding those routers who have declared themselves
            Designated Router), and again use the Router ID to break
            ties.

        (3) Calculate the new Designated Router for the network as
            follows.  If one or more of the routers have declared
            themselves Designated Router (i.e., they are currently
            listing themselves as Designated Router in their Hello
            Packets) the one having highest Router Priority is declared
            to be Designated Router.  In case of a tie, the one having
            the highest Router ID is chosen.  If no routers have
            declared themselves Designated Router, assign the Designated
            Router to be the same as the newly elected Backup Designated
            Router.

        (4) If Router X is now newly the Designated Router or newly the
            Backup Designated Router, or is now no longer the Designated
            Router or no longer the Backup Designated Router, repeat
            steps 2 and 3, and then proceed to step 5.  For example, if
            Router X is now the Designated Router, when step 2 is
            repeated X will no longer be eligible for Backup Designated
            Router election.  Among other things, this will ensure that
            no router will declare itself both Backup Designated Router
            and Designated Router.[5]

        (5) As a result of these calculations, the router itself may now
            be Designated Router or Backup Designated Router.  See
            Sections 7.3 and 7.4 for the additional duties this would
            entail.  The router's interface state should be set
            accordingly.  If the router itself is now Designated Router,
            the new interface state is DR.  If the router itself is now
            Backup Designated Router, the new interface state is Backup.
            Otherwise, the new interface state is DR Other.

        (6) If the attached network is an NBMA network, and the router
            itself has just become either Designated Router or Backup
            Designated Router, it must start sending Hello Packets to
            those neighbors that are not eligible to become Designated
            Router (see Section 9.5.1).  This is done by invoking the
            neighbor event Start for each neighbor having a Router
            Priority of 0.



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        (7) If the above calculations have caused the identity of either
            the Designated Router or Backup Designated Router to change,
            the set of adjacencies associated with this interface will
            need to be modified.  Some adjacencies may need to be
            formed, and others may need to be broken.  To accomplish
            this, invoke the event AdjOK?  on all neighbors whose state
            is at least 2-Way.  This will cause their eligibility for
            adjacency to be reexamined (see Sections 10.3 and 10.4).


        The reason behind the election algorithm's complexity is the
        desire for an orderly transition from Backup Designated Router
        to Designated Router, when the current Designated Router fails.
        This orderly transition is ensured through the introduction of
        hysteresis: no new Backup Designated Router can be chosen until
        the old Backup accepts its new Designated Router
        responsibilities.

        The above procedure may elect the same router to be both
        Designated Router and Backup Designated Router, although that
        router will never be the calculating router (Router X) itself.
        The elected Designated Router may not be the router having the
        highest Router Priority, nor will the Backup Designated Router
        necessarily have the second highest Router Priority.  If Router
        X is not itself eligible to become Designated Router, it is
        possible that neither a Backup Designated Router nor a
        Designated Router will be selected in the above procedure.  Note
        also that if Router X is the only attached router that is
        eligible to become Designated Router, it will select itself as
        Designated Router and there will be no Backup Designated Router
        for the network.


    9.5.  Sending Hello packets

        Hello packets are sent out each functioning router interface.
        They are used to discover and maintain neighbor
        relationships.[6] On broadcast and NBMA networks, Hello Packets
        are also used to elect the Designated Router and Backup
        Designated Router.





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        The format of an Hello packet is detailed in Section A.3.2.  The
        Hello Packet contains the router's Router Priority (used in
        choosing the Designated Router), and the interval between Hello
        Packets sent out the interface (HelloInterval).  The Hello
        Packet also indicates how often a neighbor must be heard from to
        remain active (RouterDeadInterval).  Both HelloInterval and
        RouterDeadInterval must be the same for all routers attached to
        a common network.  The Hello packet also contains the IP address
        mask of the attached network (Network Mask).  On unnumbered
        point-to-point networks and on virtual links this field should
        be set to 0.0.0.0.

        The Hello packet's Options field describes the router's optional
        OSPF capabilities.  One optional capability is defined in this
        specification (see Sections 4.5 and A.2).  The E-bit of the
        Options field should be set if and only if the attached area is
        capable of processing AS-external-LSAs (i.e., it is not a stub
        area).  If the E-bit is set incorrectly the neighboring routers
        will refuse to accept the Hello Packet (see Section 10.5).
        Unrecognized bits in the Hello Packet's Options field should be
        set to zero.

        In order to ensure two-way communication between adjacent
        routers, the Hello packet contains the list of all routers on
        the network from which Hello Packets have been seen recently.
        The Hello packet also contains the router's current choice for
        Designated Router and Backup Designated Router.  A value of
        0.0.0.0 in these fields means that one has not yet been
        selected.

        On broadcast networks and physical point-to-point networks,
        Hello packets are sent every HelloInterval seconds to the IP
        multicast address AllSPFRouters.  On virtual links, Hello
        packets are sent as unicasts (addressed directly to the other
        end of the virtual link) every HelloInterval seconds. On Point-
        to-MultiPoint networks, separate Hello packets are sent to each
        attached neighbor every HelloInterval seconds. Sending of Hello
        packets on NBMA networks is covered in the next section.







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        9.5.1.  Sending Hello packets on NBMA networks

            Static configuration information may be necessary in order
            for the Hello Protocol to function on non-broadcast networks
            (see Sections C.5 and C.6).  On NBMA networks, every
            attached router which is eligible to become Designated
            Router becomes aware of all of its neighbors on the network
            (either through configuration or by some unspecified
            mechanism).  Each neighbor is labelled with the neighbor's
            Designated Router eligibility.

            The interface state must be at least Waiting for any Hello
            Packets to be sent out the NBMA interface.  Hello Packets
            are then sent directly (as unicasts) to some subset of a
            router's neighbors.  Sometimes an Hello Packet is sent
            periodically on a timer; at other times it is sent as a
            response to a received Hello Packet.  A router's hello-
            sending behavior varies depending on whether the router
            itself is eligible to become Designated Router.

            If the router is eligible to become Designated Router, it
            must periodically send Hello Packets to all neighbors that
            are also eligible.  In addition, if the router is itself the
            Designated Router or Backup Designated Router, it must also
            send periodic Hello Packets to all other neighbors.  This
            means that any two eligible routers are always exchanging
            Hello Packets, which is necessary for the correct operation
            of the Designated Router election algorithm.  To minimize
            the number of Hello Packets sent, the number of eligible
            routers on an NBMA network should be kept small.

            If the router is not eligible to become Designated Router,
            it must periodically send Hello Packets to both the
            Designated Router and the Backup Designated Router (if they
            exist).  It must also send an Hello Packet in reply to an
            Hello Packet received from any eligible neighbor (other than
            the current Designated Router and Backup Designated Router).
            This is needed to establish an initial bidirectional
            relationship with any potential Designated Router.

            When sending Hello packets periodically to any neighbor, the
            interval between Hello Packets is determined by the



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            neighbor's state.  If the neighbor is in state Down, Hello
            Packets are sent every PollInterval seconds.  Otherwise,
            Hello Packets are sent every HelloInterval seconds.


10.  The Neighbor Data Structure

    An OSPF router converses with its neighboring routers.  Each
    separate conversation is described by a "neighbor data structure".
    Each conversation is bound to a particular OSPF router interface,
    and is identified either by the neighboring router's OSPF Router ID
    or by its Neighbor IP address (see below).  Thus if the OSPF router
    and another router have multiple attached networks in common,
    multiple conversations ensue, each described by a unique neighbor
    data structure.  Each separate conversation is loosely referred to
    in the text as being a separate "neighbor".

    The neighbor data structure contains all information pertinent to
    the forming or formed adjacency between the two neighbors.
    (However, remember that not all neighbors become adjacent.)  An
    adjacency can be viewed as a highly developed conversation between
    two routers.


    State
        The functional level of the neighbor conversation.  This is
        described in more detail in Section 10.1.

    Inactivity Timer
        A single shot timer whose firing indicates that no Hello Packet
        has been seen from this neighbor recently.  The length of the
        timer is RouterDeadInterval seconds.

    Master/Slave
        When the two neighbors are exchanging databases, they form a
        master/slave relationship.  The master sends the first Database
        Description Packet, and is the only part that is allowed to
        retransmit.  The slave can only respond to the master's Database
        Description Packets.  The master/slave relationship is
        negotiated in state ExStart.





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    DD Sequence Number
        The DD Sequence number of the Database Description packet that
        is currently being sent to the neighbor.

    Last received Database Description packet
        The initialize(I), more (M) and master(MS) bits, Options field,
        and DD sequence number contained in the last Database
        Description packet received from the neighbor. Used to determine
        whether the next Database Description packet received from the
        neighbor is a duplicate.

    Neighbor ID
        The OSPF Router ID of the neighboring router.  The Neighbor ID
        is learned when Hello packets are received from the neighbor, or
        is configured if this is a virtual adjacency (see Section C.4).

    Neighbor Priority
        The Router Priority of the neighboring router.  Contained in the
        neighbor's Hello packets, this item is used when selecting the
        Designated Router for the attached network.

    Neighbor IP address
        The IP address of the neighboring router's interface to the
        attached network.  Used as the Destination IP address when
        protocol packets are sent as unicasts along this adjacency.
        Also used in router-LSAs as the Link ID for the attached network
        if the neighboring router is selected to be Designated Router
        (see Section 12.4.1).  The Neighbor IP address is learned when
        Hello packets are received from the neighbor.  For virtual
        links, the Neighbor IP address is learned during the routing
        table build process (see Section 15).

    Neighbor Options
        The optional OSPF capabilities supported by the neighbor.
        Learned during the Database Exchange process (see Section 10.6).
        The neighbor's optional OSPF capabilities are also listed in its
        Hello packets.  This enables received Hello Packets to be
        rejected (i.e., neighbor relationships will not even start to
        form) if there is a mismatch in certain crucial OSPF
        capabilities (see Section 10.5).  The optional OSPF capabilities
        are documented in Section 4.5.




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    Neighbor's Designated Router
        The neighbor's idea of the Designated Router.  If this is the
        neighbor itself, this is important in the local calculation of
        the Designated Router.  Defined only on broadcast and NBMA
        networks.

    Neighbor's Backup Designated Router
        The neighbor's idea of the Backup Designated Router.  If this is
        the neighbor itself, this is important in the local calculation
        of the Backup Designated Router.  Defined only on broadcast and
        NBMA networks.


    The next set of variables are lists of LSAs.  These lists describe
    subsets of the area link-state database.  This memo defines five
    distinct types of LSAs, all of which may be present in an area
    link-state database: router-LSAs, network-LSAs, and Type 3 and 4
    summary-LSAs (all stored in the area data structure), and AS-
    external-LSAs (stored in the global data structure).


    Link state retransmission list
        The list of LSAs that have been flooded but not acknowledged on
        this adjacency.  These will be retransmitted at intervals until
        they are acknowledged, or until the adjacency is destroyed.

    Database summary list
        The complete list of LSAs that make up the area link-state
        database, at the moment the neighbor goes into Database Exchange
        state.  This list is sent to the neighbor in Database
        Description packets.

    Link state request list
        The list of LSAs that need to be received from this neighbor in
        order to synchronize the two neighbors' link-state databases.
        This list is created as Database Description packets are
        received, and is then sent to the neighbor in Link State Request
        packets.  The list is depleted as appropriate Link State Update
        packets are received.






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    10.1.  Neighbor states

        The state of a neighbor (really, the state of a conversation
        being held with a neighboring router) is documented in the
        following sections.  The states are listed in order of
        progressing functionality.  For example, the inoperative state
        is listed first, followed by a list of intermediate states
        before the final, fully functional state is achieved.  The
        specification makes use of this ordering by sometimes making
        references such as "those neighbors/adjacencies in state greater
        than X".  Figures 12 and 13 show the graph of neighbor state
        changes.  The arcs of the graphs are labelled with the event
        causing the state change.  The neighbor events are documented in
        Section 10.2.

        The graph in Figure 12 shows the state changes effected by the
        Hello Protocol.  The Hello Protocol is responsible for neighbor
        acquisition and maintenance, and for ensuring two way
        communication between neighbors.

        The graph in Figure 13 shows the forming of an adjacency.  Not
        every two neighboring routers become adjacent (see Section
        10.4).  The adjacency starts to form when the neighbor is in
        state ExStart.  After the two routers discover their
        master/slave status, the state transitions to Exchange.  At this
        point the neighbor starts to be used in the flooding procedure,
        and the two neighboring routers begin synchronizing their
        databases.  When this synchronization is finished, the neighbor
        is in state Full and we say that the two routers are fully
        adjacent.  At this point the adjacency is listed in LSAs.

        For a more detailed description of neighbor state changes,
        together with the additional actions involved in each change,
        see Section 10.3.


        Down
            This is the initial state of a neighbor conversation.  It
            indicates that there has been no recent information received
            from the neighbor.  On NBMA networks, Hello packets may
            still be sent to "Down" neighbors, although at a reduced
            frequency (see Section 9.5.1).



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                                   +----+
                                   |Down|
                                   +----+
                                     |\
                                     | \Start
                                     |  \      +-------+
                             Hello   |   +---->|Attempt|
                            Received |         +-------+
                                     |             |
                             +----+<-+             |HelloReceived
                             |Init|<---------------+
                             +----+<--------+
                                |           |
                                |2-Way      |1-Way
                                |Received   |Received
                                |           |
              +-------+         |        +-----+
              |ExStart|<--------+------->|2-Way|
              +-------+                  +-----+

              Figure 12: Neighbor state changes (Hello Protocol)

                  In addition to the state transitions pictured,
                  Event KillNbr always forces Down State,
                  Event InactivityTimer always forces Down State,
                  Event LLDown always forces Down State


















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                                  +-------+
                                  |ExStart|
                                  +-------+
                                    |
                     NegotiationDone|
                                    +->+--------+
                                       |Exchange|
                                    +--+--------+
                                    |
                            Exchange|
                              Done  |
                    +----+          |      +-------+
                    |Full|<---------+----->|Loading|
                    +----+<-+              +-------+
                            |  LoadingDone     |
                            +------------------+

            Figure 13: Neighbor state changes (Database Exchange)

                In addition to the state transitions pictured,
                Event SeqNumberMismatch forces ExStart state,
                Event BadLSReq forces ExStart state,
                Event 1-Way forces Init state,
                Event KillNbr always forces Down State,
                Event InactivityTimer always forces Down State,
                Event LLDown always forces Down State,
                Event AdjOK? leads to adjacency forming/breaking

        Attempt
            This state is only valid for neighbors attached to NBMA
            networks.  It indicates that no recent information has been
            received from the neighbor, but that a more concerted effort
            should be made to contact the neighbor.  This is done by
            sending the neighbor Hello packets at intervals of
            HelloInterval (see Section 9.5.1).

        Init
            In this state, an Hello packet has recently been seen from
            the neighbor.  However, bidirectional communication has not
            yet been established with the neighbor (i.e., the router
            itself did not appear in the neighbor's Hello packet).  All




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            neighbors in this state (or higher) are listed in the Hello
            packets sent from the associated interface.

        2-Way
            In this state, communication between the two routers is
            bidirectional.  This has been assured by the operation of
            the Hello Protocol.  This is the most advanced state short
            of beginning adjacency establishment.  The (Backup)
            Designated Router is selected from the set of neighbors in
            state 2-Way or greater.

        ExStart
            This is the first step in creating an adjacency between the
            two neighboring routers.  The goal of this step is to decide
            which router is the master, and to decide upon the initial
            DD sequence number.  Neighbor conversations in this state or
            greater are called adjacencies.

        Exchange
            In this state the router is describing its entire link state
            database by sending Database Description packets to the
            neighbor.  Each Database Description Packet has a DD
            sequence number, and is explicitly acknowledged.  Only one
            Database Description Packet is allowed outstanding at any
            one time.  In this state, Link State Request Packets may
            also be sent asking for the neighbor's more recent LSAs.
            All adjacencies in Exchange state or greater are used by the
            flooding procedure.  In fact, these adjacencies are fully
            capable of transmitting and receiving all types of OSPF
            routing protocol packets.

        Loading
            In this state, Link State Request packets are sent to the
            neighbor asking for the more recent LSAs that have been
            discovered (but not yet received) in the Exchange state.

        Full
            In this state, the neighboring routers are fully adjacent.
            These adjacencies will now appear in router-LSAs and
            network-LSAs.





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    10.2.  Events causing neighbor state changes

        State changes can be effected by a number of events.  These
        events are shown in the labels of the arcs in Figures 12 and 13.
        The label definitions are as follows:


        HelloReceived
            An Hello packet has been received from the neighbor.

        Start
            This is an indication that Hello Packets should now be sent
            to the neighbor at intervals of HelloInterval seconds.  This
            event is generated only for neighbors associated with NBMA
            networks.

        2-WayReceived
            Bidirectional communication has been realized between the
            two neighboring routers.  This is indicated by the router
            seeing itself in the neighbor's Hello packet.

        NegotiationDone
            The Master/Slave relationship has been negotiated, and DD
            sequence numbers have been exchanged.  This signals the
            start of the sending/receiving of Database Description
            packets.  For more information on the generation of this
            event, consult Section 10.8.

        ExchangeDone
            Both routers have successfully transmitted a full sequence
            of Database Description packets.  Each router now knows what
            parts of its link state database are out of date.  For more
            information on the generation of this event, consult Section
            10.8.

        BadLSReq
            A Link State Request has been received for an LSA not
            contained in the database.  This indicates an error in the
            Database Exchange process.

        Loading Done
            Link State Updates have been received for all out-of-date



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            portions of the database.  This is indicated by the Link
            state request list becoming empty after the Database
            Exchange process has completed.

        AdjOK?
            A decision must be made as to whether an adjacency should be
            established/maintained with the neighbor.  This event will
            start some adjacencies forming, and destroy others.


        The following events cause well developed neighbors to revert to
        lesser states.  Unlike the above events, these events may occur
        when the neighbor conversation is in any of a number of states.


        SeqNumberMismatch
            A Database Description packet has been received that either
            a) has an unexpected DD sequence number, b) unexpectedly has
            the Init bit set or c) has an Options field differing from
            the last Options field received in a Database Description
            packet.  Any of these conditions indicate that some error
            has occurred during adjacency establishment.

        1-Way
            An Hello packet has been received from the neighbor, in
            which the router is not mentioned.  This indicates that
            communication with the neighbor is not bidirectional.

        KillNbr
            This  is  an  indication that  all  communication  with  the
            neighbor  is now  impossible,  forcing  the  neighbor  to
            revert  to  Down  state.

        InactivityTimer
            The inactivity Timer has fired.  This means that no Hello
            packets have been seen recently from the neighbor.  The
            neighbor reverts to Down state.

        LLDown
            This is an indication from the lower level protocols that
            the neighbor is now unreachable.  For example, on an X.25
            network this could be indicated by an X.25 clear indication



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            with appropriate cause and diagnostic fields.  This event
            forces the neighbor into Down state.


    10.3.  The Neighbor state machine

        A detailed description of the neighbor state changes follows.
        Each state change is invoked by an event (Section 10.2).  This
        event may produce different effects, depending on the current
        state of the neighbor.  For this reason, the state machine below
        is organized by current neighbor state and received event.  Each
        entry in the state machine describes the resulting new neighbor
        state and the required set of additional actions.

        When a neighbor's state changes, it may be necessary to rerun
        the Designated Router election algorithm.  This is determined by
        whether the interface NeighborChange event is generated (see
        Section 9.2).  Also, if the Interface is in DR state (the router
        is itself Designated Router), changes in neighbor state may
        cause a new network-LSA to be originated (see Section 12.4).

        When the neighbor state machine needs to invoke the interface
        state machine, it should be done as a scheduled task (see
        Section 4.4).  This simplifies things, by ensuring that neither
        state machine will be executed recursively.


         State(s):  Down

            Event:  Start

        New state:  Attempt

           Action:  Send an Hello Packet to the neighbor (this neighbor
                    is always associated with an NBMA network) and start
                    the Inactivity Timer for the neighbor.  The timer's
                    later firing would indicate that communication with
                    the neighbor was not attained.


         State(s):  Attempt




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            Event:  HelloReceived

        New state:  Init

           Action:  Restart the Inactivity Timer for the neighbor, since
                    the neighbor has now been heard from.


         State(s):  Down

            Event:  HelloReceived

        New state:  Init

           Action:  Start the Inactivity Timer for the neighbor.  The
                    timer's later firing would indicate that the
                    neighbor is dead.


         State(s):  Init or greater

            Event:  HelloReceived

        New state:  No state change.

           Action:  Restart the Inactivity Timer for the neighbor, since
                    the neighbor has again been heard from.


         State(s):  Init

            Event:  2-WayReceived

        New state:  Depends upon action routine.

           Action:  Determine whether an adjacency should be established
                    with the neighbor (see Section 10.4).  If not, the
                    new neighbor state is 2-Way.

                    Otherwise (an adjacency should be established) the
                    neighbor state transitions to ExStart.  Upon
                    entering this state, the router increments the DD



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                    sequence number in the neighbor data structure.  If
                    this is the first time that an adjacency has been
                    attempted, the DD sequence number should be assigned
                    some unique value (like the time of day clock).  It
                    then declares itself master (sets the master/slave
                    bit to master), and starts sending Database
                    Description Packets, with the initialize (I), more
                    (M) and master (MS) bits set.  This Database
                    Description Packet should be otherwise empty.  This
                    Database Description Packet should be retransmitted
                    at intervals of RxmtInterval until the next state is
                    entered (see Section 10.8).


         State(s):  ExStart

            Event:  NegotiationDone

        New state:  Exchange

           Action:  The router must list the contents of its entire area
                    link state database in the neighbor Database summary
                    list.  The area link state database consists of the
                    router-LSAs, network-LSAs and summary-LSAs contained
                    in the area structure, along with the AS-external-
                    LSAs contained in the global structure.  AS-
                    external-LSAs are omitted from a virtual neighbor's
                    Database summary list.  AS-external-LSAs are omitted
                    from the Database summary list if the area has been
                    configured as a stub (see Section 3.6).  LSAs whose
                    age is equal to MaxAge are instead added to the
                    neighbor's Link state retransmission list.  A
                    summary of the Database summary list will be sent to
                    the neighbor in Database Description packets.  Each
                    Database Description Packet has a DD sequence
                    number, and is explicitly acknowledged.  Only one
                    Database Description Packet is allowed outstanding
                    at any one time.  For more detail on the sending and
                    receiving of Database Description packets, see
                    Sections 10.8 and 10.6.





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         State(s):  Exchange

            Event:  ExchangeDone

        New state:  Depends upon action routine.

           Action:  If the neighbor Link state request list is empty,
                    the new neighbor state is Full.  No other action is
                    required.  This is an adjacency's final state.

                    Otherwise, the new neighbor state is Loading.  Start
                    (or continue) sending Link State Request packets to
                    the neighbor (see Section 10.9).  These are requests
                    for the neighbor's more recent LSAs (which were
                    discovered but not yet received in the Exchange
                    state).  These LSAs are listed in the Link state
                    request list associated with the neighbor.


         State(s):  Loading

            Event:  Loading Done

        New state:  Full

           Action:  No action required.  This is an adjacency's final
                    state.


         State(s):  2-Way

            Event:  AdjOK?

        New state:  Depends upon action routine.

           Action:  Determine whether an adjacency should be formed with
                    the neighboring router (see Section 10.4).  If not,
                    the neighbor state remains at 2-Way.  Otherwise,
                    transition the neighbor state to ExStart and perform
                    the actions associated with the above state machine
                    entry for state Init and event 2-WayReceived.




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         State(s):  ExStart or greater

            Event:  AdjOK?

        New state:  Depends upon action routine.

           Action:  Determine whether the neighboring router should
                    still be adjacent.  If yes, there is no state change
                    and no further action is necessary.

                    Otherwise, the (possibly partially formed) adjacency
                    must be destroyed.  The neighbor state transitions
                    to 2-Way.  The Link state retransmission list,
                    Database summary list and Link state request list
                    are cleared of LSAs.


         State(s):  Exchange or greater

            Event:  SeqNumberMismatch

        New state:  ExStart

           Action:  The (possibly partially formed) adjacency is torn
                    down, and then an attempt is made at
                    reestablishment.  The neighbor state first
                    transitions to ExStart.  The Link state
                    retransmission list, Database summary list and Link
                    state request list are cleared of LSAs.  Then the
                    router increments the DD sequence number in the
                    neighbor data structure, declares itself master
                    (sets the master/slave bit to master), and starts
                    sending Database Description Packets, with the
                    initialize (I), more (M) and master (MS) bits set.
                    This Database Description Packet should be otherwise
                    empty (see Section 10.8).


         State(s):  Exchange or greater

            Event:  BadLSReq




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        New state:  ExStart

           Action:  The action for event BadLSReq is exactly the same as
                    for the neighbor event SeqNumberMismatch.  The
                    (possibly partially formed) adjacency is torn down,
                    and then an attempt is made at reestablishment.  For
                    more information, see the neighbor state machine
                    entry that is invoked when event SeqNumberMismatch
                    is generated in state Exchange or greater.


         State(s):  Any state

            Event:  KillNbr

        New state:  Down

           Action:  The Link state retransmission list, Database summary
                    list and Link state request list are cleared of
                    LSAs.  Also, the Inactivity Timer is disabled.


         State(s):  Any state

            Event:  LLDown

        New state:  Down

           Action:  The Link state retransmission list, Database summary
                    list and Link state request list are cleared of
                    LSAs.  Also, the Inactivity Timer is disabled.


         State(s):  Any state

            Event:  InactivityTimer

        New state:  Down

           Action:  The Link state retransmission list, Database summary
                    list and Link state request list are cleared of
                    LSAs.



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         State(s):  2-Way or greater

            Event:  1-WayReceived

        New state:  Init

           Action:  The Link state retransmission list, Database summary
                    list and Link state request list are cleared of
                    LSAs.


         State(s):  2-Way or greater

            Event:  2-WayReceived

        New state:  No state change.

           Action:  No action required.


         State(s):  Init

            Event:  1-WayReceived

        New state:  No state change.

           Action:  No action required.


    10.4.  Whether to become adjacent

        Adjacencies are established with some subset of the router's
        neighbors.  Routers connected by point-to-point networks,
        Point-to-MultiPoint networks and virtual links always become
        adjacent.  On broadcast and NBMA networks, all routers become
        adjacent to both the Designated Router and the Backup Designated
        Router.

        The adjacency-forming decision occurs in two places in the
        neighbor state machine.  First, when bidirectional communication
        is initially established with the neighbor, and secondly, when
        the identity of the attached network's (Backup) Designated



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        Router changes.  If the decision is made to not attempt an
        adjacency, the state of the neighbor communication stops at 2-
        Way.

        An adjacency should be established with a bidirectional neighbor
        when at least one of the following conditions holds:


        o   The underlying network type is point-to-point

        o   The underlying network type is Point-to-MultiPoint

        o   The underlying network type is virtual link

        o   The router itself is the Designated Router

        o   The router itself is the Backup Designated Router

        o   The neighboring router is the Designated Router

        o   The neighboring router is the Backup Designated Router


    10.5.  Receiving Hello Packets

        This section explains the detailed processing of a received
        Hello Packet.  (See Section A.3.2 for the format of Hello
        packets.)  The generic input processing of OSPF packets will
        have checked the validity of the IP header and the OSPF packet
        header.  Next, the values of the Network Mask, HelloInterval,
        and RouterDeadInterval fields in the received Hello packet must
        be checked against the values configured for the receiving
        interface.  Any mismatch causes processing to stop and the
        packet to be dropped.  In other words, the above fields are
        really describing the attached network's configuration. However,
        there is one exception to the above rule: on point-to-point
        networks and on virtual links, the Network Mask in the received
        Hello Packet should be ignored.

        The receiving interface attaches to a single OSPF area (this
        could be the backbone).  The setting of the E-bit found in the
        Hello Packet's Options field must match this area's



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        ExternalRoutingCapability.  If AS-external-LSAs are not flooded
        into/throughout the area (i.e, the area is a "stub") the E-bit
        must be clear in received Hello Packets, otherwise the E-bit
        must be set.  A mismatch causes processing to stop and the
        packet to be dropped.  The setting of the rest of the bits in
        the Hello Packet's Options field should be ignored.

        At this point, an attempt is made to match the source of the
        Hello Packet to one of the receiving interface's neighbors.  If
        the receiving interface connects to a broadcast, Point-to-
        MultiPoint or NBMA network the source is identified by the IP
        source address found in the Hello's IP header.  If the receiving
        interface connects to a point-to-point link or a virtual link,
        the source is identified by the Router ID found in the Hello's
        OSPF packet header.  The interface's current list of neighbors
        is contained in the interface's data structure.  If a matching
        neighbor structure cannot be found, (i.e., this is the first
        time the neighbor has been detected), one is created.  The
        initial state of a newly created neighbor is set to Down.

        When receiving an Hello Packet from a neighbor on a broadcast,
        Point-to-MultiPoint or NBMA network, set the neighbor
        structure's Neighbor ID equal to the Router ID found in the
        packet's OSPF header.  For these network types, the neighbor
        structure's Router Priority field, Neighbor's Designated Router
        field, and Neighbor's Backup Designated Router field are also
        set equal to the corresponding fields found in the received
        Hello Packet; changes in these fields should be noted for
        possible use in the steps below.  When receiving an Hello on a
        point-to-point network (but not on a virtual link) set the
        neighbor structure's Neighbor IP address to the packet's IP
        source address.

        Now the rest of the Hello Packet is examined, generating events
        to be given to the neighbor and interface state machines.  These
        state machines are specified either to be executed or scheduled
        (see Section 4.4).  For example, by specifying below that the
        neighbor state machine be executed in line, several neighbor
        state transitions may be effected by a single received Hello:






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        o   Each Hello Packet causes the neighbor state machine to be
            executed with the event HelloReceived.

        o   Then the list of neighbors contained in the Hello Packet is
            examined.  If the router itself appears in this list, the
            neighbor state machine should be executed with the event 2-
            WayReceived.  Otherwise, the neighbor state machine should
            be executed with the event 1-WayReceived, and the processing
            of the packet stops.

        o   Next, if a change in the neighbor's Router Priority field
            was noted, the receiving interface's state machine is
            scheduled with the event NeighborChange.

        o   If the neighbor is both declaring itself to be Designated
            Router (Hello Packet's Designated Router field = Neighbor IP
            address) and the Backup Designated Router field in the
            packet is equal to 0.0.0.0 and the receiving interface is in
            state Waiting, the receiving interface's state machine is
            scheduled with the event BackupSeen.  Otherwise, if the
            neighbor is declaring itself to be Designated Router and it
            had not previously, or the neighbor is not declaring itself
            Designated Router where it had previously, the receiving
            interface's state machine is scheduled with the event
            NeighborChange.

        o   If the neighbor is declaring itself to be Backup Designated
            Router (Hello Packet's Backup Designated Router field =
            Neighbor IP address) and the receiving interface is in state
            Waiting, the receiving interface's state machine is
            scheduled with the event BackupSeen.  Otherwise, if the
            neighbor is declaring itself to be Backup Designated Router
            and it had not previously, or the neighbor is not declaring
            itself Backup Designated Router where it had previously, the
            receiving interface's state machine is scheduled with the
            event NeighborChange.

        On NBMA networks, receipt of an Hello Packet may also cause an
        Hello Packet to be sent back to the neighbor in response. See
        Section 9.5.1 for more details.





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    10.6.  Receiving Database Description Packets

        This section explains the detailed processing of a received
        Database Description Packet.  The incoming Database Description
        Packet has already been associated with a neighbor and receiving
        interface by the generic input packet processing (Section 8.2).
        Whether the Database Description packet should be accepted, and
        if so, how it should be further processed depends upon the
        neighbor state.

        If a Database Description packet is accepted, the following
        packet fields should be saved in the corresponding neighbor data
        structure under "last received Database Description packet":
        the packet's initialize(I), more (M) and master(MS) bits,
        Options field, and DD sequence number. If these fields are set
        identically in two consecutive Database Description packets
        received from the neighbor, the second Database Description
        packet is considered to be a "duplicate" in the processing
        described below.

        If the Interface MTU field in the Database Description packet
        indicates an IP datagram size that is larger than the router can
        accept on the receiving interface without fragmentation, the
        Database Description packet is rejected.  Otherwise, if the
        neighbor state is:

        Down
            The packet should be rejected.

        Attempt
            The packet should be rejected.

        Init
            The neighbor state machine should be executed with the event
            2-WayReceived.  This causes an immediate state change to
            either state 2-Way or state ExStart. If the new state is
            ExStart, the processing of the current packet should then
            continue in this new state by falling through to case
            ExStart below.






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        2-Way
            The packet should be ignored.  Database Description Packets
            are used only for the purpose of bringing up adjacencies.[7]

        ExStart
            If the received packet matches one of the following cases,
            then the neighbor state machine should be executed with the
            event NegotiationDone (causing the state to transition to
            Exchange), the packet's Options field should be recorded in
            the neighbor structure's Neighbor Options field and the
            packet should be accepted as next in sequence and processed
            further (see below).  Otherwise, the packet should be
            ignored.

            o   The initialize(I), more (M) and master(MS) bits are set,
                the contents of the packet are empty, and the neighbor's
                Router ID is larger than the router's own.  In this case
                the router is now Slave.  Set the master/slave bit to
                slave, and set the neighbor data structure's DD sequence
                number to that specified by the master.

            o   The initialize(I) and master(MS) bits are off, the
                packet's DD sequence number equals the neighbor data
                structure's DD sequence number (indicating
                acknowledgment) and the neighbor's Router ID is smaller
                than the router's own.  In this case the router is
                Master.

        Exchange
            Duplicate Database Description packets are discarded by the
            master, and cause the slave to retransmit the last Database
            Description packet that it had sent. Otherwise (the packet
            is not a duplicate):

            o   If the state of the MS-bit is inconsistent with the
                master/slave state of the connection, generate the
                neighbor event SeqNumberMismatch and stop processing the
                packet.

            o   If the initialize(I) bit is set, generate the neighbor
                event SeqNumberMismatch and stop processing the packet.




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            o   If the packet's Options field indicates a different set
                of optional OSPF capabilities than were previously
                received from the neighbor (recorded in the Neighbor
                Options field of the neighbor structure), generate the
                neighbor event SeqNumberMismatch and stop processing the
                packet.

            o   Database Description packets must be processed in
                sequence, as indicated by the packets' DD sequence
                numbers. If the router is master, the next packet
                received should have DD sequence number equal to the DD
                sequence number in the neighbor data structure. If the
                router is slave, the next packet received should have DD
                sequence number equal to one more than the DD sequence
                number stored in the neighbor data structure. In either
                case, if the packet is the next in sequence it should be
                accepted and its contents processed as specified below.

            o   Else, generate the neighbor event SeqNumberMismatch and
                stop processing the packet.

        Loading or Full
            In this state, the router has sent and received an entire
            sequence of Database Description Packets.  The only packets
            received should be duplicates (see above).  In particular,
            the packet's Options field should match the set of optional
            OSPF capabilities previously indicated by the neighbor
            (stored in the neighbor structure's Neighbor Options field).
            Any other packets received, including the reception of a
            packet with the Initialize(I) bit set, should generate the
            neighbor event SeqNumberMismatch.[8] Duplicates should be
            discarded by the master.  The slave must respond to
            duplicates by repeating the last Database Description packet
            that it had sent.


        When the router accepts a received Database Description Packet
        as the next in sequence the packet contents are processed as
        follows.  For each LSA listed, the LSA's LS type is checked for
        validity.  If the LS type is unknown (e.g., not one of the LS
        types 1-5 defined by this specification), or if this is an AS-
        external-LSA (LS type = 5) and the neighbor is associated with a



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        stub area, generate the neighbor event SeqNumberMismatch and
        stop processing the packet.  Otherwise, the router looks up the
        LSA in its database to see whether it also has an instance of
        the LSA.  If it does not, or if the database copy is less recent
        (see Section 13.1), the LSA is put on the Link state request
        list so that it can be requested (immediately or at some later
        time) in Link State Request Packets.

        When the router accepts a received Database Description Packet
        as the next in sequence, it also performs the following actions,
        depending on whether it is master or slave:


        Master
            Increments the DD sequence number in the neighbor data
            structure.  If the router has already sent its entire
            sequence of Database Description Packets, and the just
            accepted packet has the more bit (M) set to 0, the neighbor
            event ExchangeDone is generated.  Otherwise, it should send
            a new Database Description to the slave.

        Slave
            Sets the DD sequence number in the neighbor data structure
            to the DD sequence number appearing in the received packet.
            The slave must send a Database Description Packet in reply.
            If the received packet has the more bit (M) set to 0, and
            the packet to be sent by the slave will also have the M-bit
            set to 0, the neighbor event ExchangeDone is generated.
            Note that the slave always generates this event before the
            master.


    10.7.  Receiving Link State Request Packets

        This section explains the detailed processing of received Link
        State Request packets.  Received Link State Request Packets
        specify a list of LSAs that the neighbor wishes to receive.
        Link State Request Packets should be accepted when the neighbor
        is in states Exchange, Loading, or Full.  In all other states
        Link State Request Packets should be ignored.





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        Each LSA specified in the Link State Request packet should be
        located in the router's database, and copied into Link State
        Update packets for transmission to the neighbor.  These LSAs
        should NOT be placed on the Link state retransmission list for
        the neighbor.  If an LSA cannot be found in the database,
        something has gone wrong with the Database Exchange process, and
        neighbor event BadLSReq should be generated.


    10.8.  Sending Database Description Packets

        This section describes how Database Description Packets are sent
        to a neighbor. The Database Description packet's Interface MTU
        field is set to the size of the largest IP datagram that can be
        sent out the sending interface, without fragmentation.  Common
        MTUs in use in the Internet can be found in Table 7-1 of
        [Ref22]. Interface MTU should be set to 0 in Database
        Description packets sent over virtual links.

        The router's optional OSPF capabilities (see Section 4.5) are
        transmitted to the neighbor in the Options field of the Database
        Description packet.  The router should maintain the same set of
        optional capabilities throughout the Database Exchange and
        flooding procedures.  If for some reason the router's optional
        capabilities change, the Database Exchange procedure should be
        restarted by reverting to neighbor state ExStart.  One optional
        capability is defined in this specification (see Sections 4.5
        and A.2). The E-bit should be set if and only if the attached
        network belongs to a non-stub area. Unrecognized bits in the
        Options field should be set to zero.

        The sending of Database Description packets depends on the
        neighbor's state.  In state ExStart the router sends empty
        Database Description packets, with the initialize (I), more (M)
        and master (MS) bits set.  These packets are retransmitted every
        RxmtInterval seconds.

        In state Exchange the Database Description Packets actually
        contain summaries of the link state information contained in the
        router's database.  Each LSA in the area's link-state database
        (at the time the neighbor transitions into Exchange state) is
        listed in the neighbor Database summary list.  Each new Database



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        Description Packet copies its DD sequence number from the
        neighbor data structure and then describes the current top of
        the Database summary list.  Items are removed from the Database
        summary list when the previous packet is acknowledged.

        In state Exchange, the determination of when to send a Database
        Description packet depends on whether the router is master or
        slave:


        Master
            Database Description packets are sent when either a) the
            slave acknowledges the previous Database Description packet
            by echoing the DD sequence number or b) RxmtInterval seconds
            elapse without an acknowledgment, in which case the previous
            Database Description packet is retransmitted.

        Slave
            Database Description packets are sent only in response to
            Database Description packets received from the master.  If
            the Database Description packet received from the master is
            new, a new Database Description packet is sent, otherwise
            the previous Database Description packet is resent.


        In states Loading and Full the slave must resend its last
        Database Description packet in response to duplicate Database
        Description packets received from the master.  For this reason
        the slave must wait RouterDeadInterval seconds before freeing
        the last Database Description packet.  Reception of a Database
        Description packet from the master after this interval will
        generate a SeqNumberMismatch neighbor event.


    10.9.  Sending Link State Request Packets

        In neighbor states Exchange or Loading, the Link state request
        list contains a list of those LSAs that need to be obtained from
        the neighbor.  To request these LSAs, a router sends the
        neighbor the beginning of the Link state request list, packaged
        in a Link State Request packet.




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        When the neighbor responds to these requests with the proper
        Link State Update packet(s), the Link state request list is
        truncated and a new Link State Request packet is sent.  This
        process continues until the Link state request list becomes
        empty. LSAs on the Link state request list that have been
        requested, but not yet received, are packaged into Link State
        Request packets for retransmission at intervals of RxmtInterval.
        There should be at most one Link State Request packet
        outstanding at any one time.

        When the Link state request list becomes empty, and the neighbor
        state is Loading (i.e., a complete sequence of Database
        Description packets has been sent to and received from the
        neighbor), the Loading Done neighbor event is generated.


    10.10.  An Example

        Figure 14 shows an example of an adjacency forming.  Routers RT1
        and RT2 are both connected to a broadcast network.  It is
        assumed that RT2 is the Designated Router for the network, and
        that RT2 has a higher Router ID than Router RT1.

        The neighbor state changes realized by each router are listed on
        the sides of the figure.

        At the beginning of Figure 14, Router RT1's interface to the
        network becomes operational.  It begins sending Hello Packets,
        although it doesn't know the identity of the Designated Router
        or of any other neighboring routers.  Router RT2 hears this
        hello (moving the neighbor to Init state), and in its next Hello
        Packet indicates that it is itself the Designated Router and
        that it has heard Hello Packets from RT1.  This in turn causes
        RT1 to go to state ExStart, as it starts to bring up the
        adjacency.

        RT1 begins by asserting itself as the master.  When it sees that
        RT2 is indeed the master (because of RT2's higher Router ID),
        RT1 transitions to slave state and adopts its neighbor's DD
        sequence number.  Database Description packets are then
        exchanged, with polls coming from the master (RT2) and responses
        from the slave (RT1).  This sequence of Database Description



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            +---+                                         +---+
            |RT1|                                         |RT2|
            +---+                                         +---+

            Down                                          Down
                            Hello(DR=0,seen=0)
                       ------------------------------>
                         Hello (DR=RT2,seen=RT1,...)      Init
                       <------------------------------
            ExStart        D-D (Seq=x,I,M,Master)
                       ------------------------------>
                           D-D (Seq=y,I,M,Master)         ExStart
                       <------------------------------
            Exchange       D-D (Seq=y,M,Slave)
                       ------------------------------>
                           D-D (Seq=y+1,M,Master)         Exchange
                       <------------------------------
                           D-D (Seq=y+1,M,Slave)
                       ------------------------------>
                                     ...
                                     ...
                                     ...
                           D-D (Seq=y+n, Master)
                       <------------------------------
                           D-D (Seq=y+n, Slave)
             Loading   ------------------------------>
                                 LS Request                Full
                       ------------------------------>
                                 LS Update
                       <------------------------------
                                 LS Request
                       ------------------------------>
                                 LS Update
                       <------------------------------
             Full





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                   Figure 14: An adjacency bring-up example





        Packets ends when both the poll and associated response has the
        M-bit off.

        In this example, it is assumed that RT2 has a completely up to
        date database.  In that case, RT2 goes immediately into Full
        state.  RT1 will go into Full state after updating the necessary
        parts of its database.  This is done by sending Link State
        Request Packets, and receiving Link State Update Packets in
        response.  Note that, while RT1 has waited until a complete set
        of Database Description Packets has been received (from RT2)
        before sending any Link State Request Packets, this need not be
        the case.  RT1 could have interleaved the sending of Link State
        Request Packets with the reception of Database Description
        Packets.


11.  The Routing Table Structure

    The routing table data structure contains all the information
    necessary to forward an IP data packet toward its destination.  Each
    routing table entry describes the collection of best paths to a
    particular destination.  When forwarding an IP data packet, the
    routing table entry providing the best match for the packet's IP
    destination is located.  The matching routing table entry then
    provides the next hop towards the packet's destination.  OSPF also
    provides for the existence of a default route (Destination ID =
    DefaultDestination, Address Mask =  0x00000000).  When the default
    route exists, it matches all IP destinations (although any other
    matching entry is a better match).  Finding the routing table entry
    that best matches an IP destination is further described in Section
    11.1.

    There is a single routing table in each router.  Two sample routing
    tables are described in Sections 11.2 and 11.3.  The building of the
    routing table is discussed in Section 16.




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    The rest of this section defines the fields found in a routing table
    entry.  The first set of fields describes the routing table entry's
    destination.


    Destination Type
        Destination type is either "network" or "router". Only network
        entries are actually used when forwarding IP data traffic.
        Router routing table entries are used solely as intermediate
        steps in the routing table build process.

        A network is a range of IP addresses, to which IP data traffic
        may be forwarded.  This includes IP networks (class A, B, or C),
        IP subnets, IP supernets and single IP hosts.  The default route
        also falls into this category.

        Router entries are kept for area border routers and AS boundary
        routers.  Routing table entries for area border routers are used
        when calculating the inter-area routes (see Section 16.2), and
        when maintaining configured virtual links (see Section 15).
        Routing table entries for AS boundary routers are used when
        calculating the AS external routes (see Section 16.4).

    Destination ID
        The destination's identifier or name.  This depends on the
        Destination Type.  For networks, the identifier is their
        associated IP address.  For routers, the identifier is the OSPF
        Router ID.[9]

    Address Mask
        Only defined for networks.  The network's IP address together
        with its address mask defines a range of IP addresses.  For IP
        subnets, the address mask is referred to as the subnet mask.
        For host routes, the mask is "all ones" (0xffffffff).

    Optional Capabilities
        When the destination is a router this field indicates the
        optional OSPF capabilities supported by the destination router.
        The only optional capability defined by this specification is
        the ability to process AS-external-LSAs.  For a further
        discussion of OSPF's optional capabilities, see Section 4.5.




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    The set of paths to use for a destination may vary based on the OSPF
    area to which the paths belong.  This means that there may be
    multiple routing table entries for the same destination, depending
    on the values of the next field.


    Area
        This field indicates the area whose link state information has
        led to the routing table entry's collection of paths.  This is
        called the entry's associated area.  For sets of AS external
        paths, this field is not defined.  For destinations of type
        "router", there may be separate sets of paths (and therefore
        separate routing table entries) associated with each of several
        areas. For example, this will happen when two area border
        routers share multiple areas in common.  For destinations of
        type "network", only the set of paths associated with the best
        area (the one providing the preferred route) is kept.


    The rest of the routing table entry describes the set of paths to
    the destination.  The following fields pertain to the set of paths
    as a whole.  In other words, each one of the paths contained in a
    routing table entry is of the same path-type and cost (see below).


    Path-type
        There are four possible types of paths used to route traffic to
        the destination, listed here in decreasing order of preference:
        intra-area, inter-area, type 1 external or type 2 external.
        Intra-area paths indicate destinations belonging to one of the
        router's attached areas.  Inter-area paths are paths to
        destinations in other OSPF areas.  These are discovered through
        the examination of received summary-LSAs.  AS external paths are
        paths to destinations external to the AS.  These are detected
        through the examination of received AS-external-LSAs.

    Cost
        The link state cost of the path to the destination.  For all
        paths except type 2 external paths this describes the entire
        path's cost.  For Type 2 external paths, this field describes
        the cost of the portion of the path internal to the AS.  This




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        cost is calculated as the sum of the costs of the path's
        constituent links.

    Type 2 cost
        Only valid for type 2 external paths.  For these paths, this
        field indicates the cost of the path's external portion.  This
        cost has been advertised by an AS boundary router, and is the
        most significant part of the total path cost.  For example, a
        type 2 external path with type 2 cost of 5 is always preferred
        over a path with type 2 cost of 10, regardless of the cost of
        the two paths' internal components.

    Link State Origin
        Valid only for intra-area paths, this field indicates the LSA
        (router-LSA or network-LSA) that directly references the
        destination.  For example, if the destination is a transit
        network, this is the transit network's network-LSA.  If the
        destination is a stub network, this is the router-LSA for the
        attached router.  The LSA is discovered during the shortest-path
        tree calculation (see Section 16.1).  Multiple LSAs may
        reference the destination, however a tie-breaking scheme always
        reduces the choice to a single LSA. The Link State Origin field
        is not used by the OSPF protocol, but it is used by the routing
        table calculation in OSPF's Multicast routing extensions
        (MOSPF).

    When multiple paths of equal path-type and cost exist to a
    destination (called elsewhere "equal-cost" paths), they are stored
    in a single routing table entry.  Each one of the "equal-cost" paths
    is distinguished by the following fields:

    Next hop
        The outgoing router interface to use when forwarding traffic to
        the destination.  On broadcast, Point-to-MultiPoint and NBMA
        networks, the next hop also includes the IP address of the next
        router (if any) in the path towards the destination.

    Advertising router
        Valid only for inter-area and AS external paths.  This field
        indicates the Router ID of the router advertising the summary-
        LSA or AS-external-LSA that led to this path.




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    11.1.  Routing table lookup

        When an IP data packet is received, an OSPF router finds the
        routing table entry that best matches the packet's destination.
        This routing table entry then provides the outgoing interface
        and next hop router to use in forwarding the packet. This
        section describes the process of finding the best matching
        routing table entry.

        Before the lookup begins, "discard" routing table entries should
        be inserted into the routing table for each of the router's
        active area address ranges (see Section 3.5).  (An area range is
        considered "active" if the range contains one or more networks
        reachable by intra-area paths.) The destination of a "discard"
        entry is the set of addresses described by its associated active
        area address range, and the path type of each "discard" entry is
        set to "inter-area".[10]

        Several routing table entries may match the destination address.
        In this case, the "best match" is the routing table entry that
        provides the most specific (longest) match. Another way of
        saying this is to choose the entry that specifies the narrowest
        range of IP addresses.[11] For example, the entry for the
        address/mask pair of (128.185.1.0, 0xffffff00) is more specific
        than an entry for the pair (128.185.0.0, 0xffff0000). The
        default route is the least specific match, since it matches all
        destinations. (Note that for any single routing table entry,
        multiple paths may be possible. In these cases, the calculations
        in Sections 16.1, 16.2, and 16.4 always yield the paths having
        the most preferential path-type, as described in Section 11).

        If there is no matching routing table entry, or the best match
        routing table entry is one of the above "discard" routing table
        entries, then the packet's IP destination is considered
        unreachable. Instead of being forwarded, the packet should then
        be discarded and an ICMP destination unreachable message should
        be returned to the packet's source.

    11.2.  Sample routing table, without areas

        Consider the Autonomous System pictured in Figure 2.  No OSPF
        areas have been configured.  A single metric is shown per



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        outbound interface.  The calculation of Router RT6's routing
        table proceeds as described in Section 2.2.  The resulting
        routing table is shown in Table 12.  Destination types are
        abbreviated: Network as "N", Router as "R".

        There are no instances of multiple equal-cost shortest paths in
        this example.  Also, since there are no areas, there are no
        inter-area paths.

        Routers RT5 and RT7 are AS boundary routers.  Intra-area routes
        have been calculated to Routers RT5 and RT7.  This allows
        external routes to be calculated to the destinations advertised
        by RT5 and RT7 (i.e., Networks N12, N13, N14 and N15).  It is
        assumed all AS-external-LSAs originated by RT5 and RT7 are
        advertising type 1 external metrics.  This results in type 1
        external paths being calculated to destinations N12-N15.



    11.3.  Sample routing table, with areas

        Consider the previous example, this time split into OSPF areas.
        An OSPF area configuration is pictured in Figure 6.  Router
        RT4's routing table will be described for this area
        configuration.  Router RT4 has a connection to Area 1 and a
        backbone connection.  This causes Router RT4 to view the AS as
        the concatenation of the two graphs shown in Figures 7 and 8.
        The resulting routing table is displayed in Table 13.

        Again, Routers RT5 and RT7 are AS boundary routers.  Routers
        RT3, RT4, RT7, RT10 and RT11 are area border routers.  Note that
        there are two routing entries for the area border router RT3,
        since it has two areas in common with RT4 (Area 1 and the
        backbone).

        Backbone paths have been calculated to all area border routers.
        These are used when determining the inter-area routes.  Note
        that all of the inter-area routes are associated with the
        backbone; this is always the case when the calculating router is
        itself an area border router.  Routing information is condensed
        at area boundaries.  In this example, we assume that Area 3 has
        been defined so that networks N9-N11 and the host route to H1



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      Type   Dest   Area   Path  Type    Cost   Next     Adv.
                                                Hop(s)   Router(s)
      ____________________________________________________________
      N      N1     0      intra-area    10     RT3      *
      N      N2     0      intra-area    10     RT3      *
      N      N3     0      intra-area    7      RT3      *
      N      N4     0      intra-area    8      RT3      *
      N      Ib     0      intra-area    7      *        *
      N      Ia     0      intra-area    12     RT10     *
      N      N6     0      intra-area    8      RT10     *
      N      N7     0      intra-area    12     RT10     *
      N      N8     0      intra-area    10     RT10     *
      N      N9     0      intra-area    11     RT10     *
      N      N10    0      intra-area    13     RT10     *
      N      N11    0      intra-area    14     RT10     *
      N      H1     0      intra-area    21     RT10     *
      R      RT5    0      intra-area    6      RT5      *
      R      RT7    0      intra-area    8      RT10     *
      ____________________________________________________________
      N      N12    *      type 1 ext.   10     RT10     RT7
      N      N13    *      type 1 ext.   14     RT5      RT5
      N      N14    *      type 1 ext.   14     RT5      RT5
      N      N15    *      type 1 ext.   17     RT10     RT7


               Table 12: The routing table for Router RT6
                         (no configured areas).

        are all condensed to a single route when advertised into the
        backbone (by Router RT11).  Note that the cost of this route is
        the maximum of the set of costs to its individual components.

        There is a virtual link configured between Routers RT10 and
        RT11.  Without this configured virtual link, RT11 would be
        unable to advertise a route for networks N9-N11 and Host H1 into
        the backbone, and there would not be an entry for these networks
        in Router RT4's routing table.

        In this example there are two equal-cost paths to Network N12.
        However, they both use the same next hop (Router RT5).



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        Router RT4's routing table would improve (i.e., some of the
        paths in the routing table would become shorter) if an
        additional virtual link were configured between Router RT4 and
        Router RT3.  The new virtual link would itself be associated
        with the first entry for area border router RT3 in Table 13 (an
        intra-area path through Area 1).  This would yield a cost of 1
        for the virtual link.  The routing table entries changes that
        would be caused by the addition of this virtual link are shown


   Type   Dest        Area   Path  Type    Cost   Next      Adv.
                                                  Hops(s)   Router(s)
   __________________________________________________________________
   N      N1          1      intra-area    4      RT1       *
   N      N2          1      intra-area    4      RT2       *
   N      N3          1      intra-area    1      *         *
   N      N4          1      intra-area    3      RT3       *
   R      RT3         1      intra-area    1      *         *
   __________________________________________________________________
   N      Ib          0      intra-area    22     RT5       *
   N      Ia          0      intra-area    27     RT5       *
   R      RT3         0      intra-area    21     RT5       *
   R      RT5         0      intra-area    8      *         *
   R      RT7         0      intra-area    14     RT5       *
   R      RT10        0      intra-area    22     RT5       *
   R      RT11        0      intra-area    25     RT5       *
   __________________________________________________________________
   N      N6          0      inter-area    15     RT5       RT7
   N      N7          0      inter-area    19     RT5       RT7
   N      N8          0      inter-area    18     RT5       RT7
   N      N9-N11,H1   0      inter-area    36     RT5       RT11
   __________________________________________________________________
   N      N12         *      type 1 ext.   16     RT5       RT5,RT7
   N      N13         *      type 1 ext.   16     RT5       RT5
   N      N14         *      type 1 ext.   16     RT5       RT5
   N      N15         *      type 1 ext.   23     RT5       RT7


                  Table 13: Router RT4's routing table
                       in the presence of areas.





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        in Table 14.



12.  Link State Advertisements (LSAs)

    Each router in the Autonomous System originates one or more link
    state advertisements (LSAs).  This memo defines five distinct types
    of LSAs, which are described in Section 4.3.  The collection of LSAs
    forms the link-state database.  Each separate type of LSA has a
    separate function.  Router-LSAs and network-LSAs describe how an
    area's routers and networks are interconnected.  Summary-LSAs
    provide a way of condensing an area's routing information.  AS-
    external-LSAs provide a way of transparently advertising
    externally-derived routing information throughout the Autonomous
    System.

    Each LSA begins with a standard 20-byte header.  This LSA header is
    discussed below.







    Type   Dest        Area   Path  Type   Cost   Next     Adv.
                                                  Hop(s)   Router(s)
    ________________________________________________________________
    N      Ib          0      intra-area   16     RT3      *
    N      Ia          0      intra-area   21     RT3      *
    R      RT3         0      intra-area   1      *        *
    R      RT10        0      intra-area   16     RT3      *
    R      RT11        0      intra-area   19     RT3      *
    ________________________________________________________________
    N      N9-N11,H1   0      inter-area   30     RT3      RT11


                  Table 14: Changes resulting from an
                        additional virtual link.





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    12.1.  The LSA Header

        The LSA header contains the LS type, Link State ID and
        Advertising Router fields.  The combination of these three
        fields uniquely identifies the LSA.

        There may be several instances of an LSA present in the
        Autonomous System, all at the same time.  It must then be
        determined which instance is more recent.  This determination is
        made by examining the LS sequence, LS checksum and LS age
        fields.  These fields are also contained in the 20-byte LSA
        header.

        Several of the OSPF packet types list LSAs.  When the instance
        is not important, an LSA is referred to by its LS type, Link
        State ID and Advertising Router (see Link State Request
        Packets).  Otherwise, the LS sequence number, LS age and LS
        checksum fields must also be referenced.

        A detailed explanation of the fields contained in the LSA header
        follows.


        12.1.1.  LS age

            This field is the age of the LSA in seconds.  It should be
            processed as an unsigned 16-bit integer.  It is set to 0
            when the LSA is originated.  It must be incremented by
            InfTransDelay on every hop of the flooding procedure.  LSAs
            are also aged as they are held in each router's database.

            The age of an LSA is never incremented past MaxAge.  LSAs
            having age MaxAge are not used in the routing table
            calculation.  When an LSA's age first reaches MaxAge, it is
            reflooded.  An LSA of age MaxAge is finally flushed from the
            database when it is no longer needed to ensure database
            synchronization.  For more information on the aging of LSAs,
            consult Section 14.

            The LS age field is examined when a router receives two
            instances of an LSA, both having identical LS sequence
            numbers and LS checksums.  An instance of age MaxAge is then



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            always accepted as most recent; this allows old LSAs to be
            flushed quickly from the routing domain.  Otherwise, if the
            ages differ by more than MaxAgeDiff, the instance having the
            smaller age is accepted as most recent.[12] See Section 13.1
            for more details.


        12.1.2.  Options

            The Options field in the LSA header indicates which optional
            capabilities are associated with the LSA.  OSPF's optional
            capabilities are described in Section 4.5.  One optional
            capability is defined by this specification, represented by
            the E-bit found in the Options field.  The unrecognized bits
            in the Options field should be set to zero.

            The E-bit represents OSPF's ExternalRoutingCapability.  This
            bit should be set in all LSAs associated with the backbone,
            and all LSAs associated with non-stub areas (see Section
            3.6).  It should also be set in all AS-external-LSAs.  It
            should be reset in all router-LSAs, network-LSAs and
            summary-LSAs associated with a stub area.  For all LSAs, the
            setting of the E-bit is for informational purposes only; it
            does not affect the routing table calculation.


        12.1.3.  LS type

            The LS type field dictates the format and function of the
            LSA.  LSAs of different types have different names (e.g.,
            router-LSAs or network-LSAs).  All LSA types defined by this
            memo, except the AS-external-LSAs (LS type = 5), are flooded
            throughout a single area only.  AS-external-LSAs are flooded
            throughout the entire Autonomous System, excepting stub
            areas (see Section 3.6).  Each separate LSA type is briefly
            described below in Table 15.

        12.1.4.  Link State ID

            This field identifies the piece of the routing domain that
            is being described by the LSA.  Depending on the LSA's LS
            type, the Link State ID takes on the values listed in Table



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            LS Type   LSA description
            ________________________________________________
            1         These are the router-LSAs.
                      They describe the collected
                       states of the router's
                      interfaces. For more information,
                      consult Section 12.4.1.
            ________________________________________________
            2         These are the network-LSAs.
                      They describe the set of routers
                      attached to the network. For
                      more information, consult
                      Section 12.4.2.
            ________________________________________________
            3 or 4    These are the summary-LSAs.
                      They describe inter-area routes,
                      and enable the condensation of
                      routing information at area
                      borders. Originated by area border
                      routers, the Type 3 summary-LSAs
                      describe routes to networks while the
                      Type 4 summary-LSAs describe routes to
                      AS boundary routers.
            ________________________________________________
            5         These are the AS-external-LSAs.
                      Originated by AS boundary routers,
                      they describe routes
                      to destinations external to the
                      Autonomous System. A default route for
                      the Autonomous System can also be
                      described by an AS-external-LSA.


            Table 15: OSPF link state advertisements (LSAs).

            16.


            Actually, for Type 3 summary-LSAs (LS type = 3) and AS-
            external-LSAs (LS type = 5), the Link State ID may



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            LS Type   Link State ID
            _______________________________________________
            1         The originating router's Router ID.
            2         The IP interface address of the
                      network's Designated Router.
            3         The destination network's IP address.
            4         The Router ID of the described AS
                      boundary router.
            5         The destination network's IP address.


                   Table 16: The LSA's Link State ID.

            additionally have one or more of the destination network's
            "host" bits set. For example, when originating an AS-
            external-LSA for the network 10.0.0.0 with mask of
            255.0.0.0, the Link State ID can be set to anything in the
            range 10.0.0.0 through 10.255.255.255 inclusive (although
            10.0.0.0 should be used whenever possible). The freedom to
            set certain host bits allows a router to originate separate
            LSAs for two networks having the same address but different
            masks. See Appendix E for details.

            When the LSA is describing a network (LS type = 2, 3 or 5),
            the network's IP address is easily derived by masking the
            Link State ID with the network/subnet mask contained in the
            body of the LSA.  When the LSA is describing a router (LS
            type = 1 or 4), the Link State ID is always the described
            router's OSPF Router ID.

            When an AS-external-LSA (LS Type = 5) is describing a
            default route, its Link State ID is set to
            DefaultDestination (0.0.0.0).


        12.1.5.  Advertising Router

            This field specifies the OSPF Router ID of the LSA's
            originator.  For router-LSAs, this field is identical to the
            Link State ID field.  Network-LSAs are originated by the



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            network's Designated Router.  Summary-LSAs originated by
            area border routers.  AS-external-LSAs are originated by AS
            boundary routers.


        12.1.6.  LS sequence number

            The sequence number field is a signed 32-bit integer.  It is
            used to detect old and duplicate LSAs.  The space of
            sequence numbers is linearly ordered.  The larger the
            sequence number (when compared as signed 32-bit integers)
            the more recent the LSA.  To describe to sequence number
            space more precisely, let N refer in the discussion below to
            the constant 2**31.

            The sequence number -N (0x80000000) is reserved (and
            unused).  This leaves -N + 1 (0x80000001) as the smallest
            (and therefore oldest) sequence number; this sequence number
            is referred to as the constant InitialSequenceNumber. A
            router uses InitialSequenceNumber the first time it
            originates any LSA.  Afterwards, the LSA's sequence number
            is incremented each time the router originates a new
            instance of the LSA.  When an attempt is made to increment
            the sequence number past the maximum value of N - 1
            (0x7fffffff; also referred to as MaxSequenceNumber), the
            current instance of the LSA must first be flushed from the
            routing domain.  This is done by prematurely aging the LSA
            (see Section 14.1) and reflooding it.  As soon as this flood
            has been acknowledged by all adjacent neighbors, a new
            instance can be originated with sequence number of
            InitialSequenceNumber.

            The router may be forced to promote the sequence number of
            one of its LSAs when a more recent instance of the LSA is
            unexpectedly received during the flooding process.  This
            should be a rare event.  This may indicate that an out-of-
            date LSA, originated by the router itself before its last
            restart/reload, still exists in the Autonomous System.  For
            more information see Section 13.4.






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        12.1.7.  LS checksum

            This field is the checksum of the complete contents of the
            LSA, excepting the LS age field.  The LS age field is
            excepted so that an LSA's age can be incremented without
            updating the checksum.  The checksum used is the same that
            is used for ISO connectionless datagrams; it is commonly
            referred to as the Fletcher checksum.  It is documented in
            Annex B of [Ref6].  The LSA header also contains the length
            of the LSA in bytes; subtracting the size of the LS age
            field (two bytes) yields the amount of data to checksum.

            The checksum is used to detect data corruption of an LSA.
            This corruption can occur while an LSA is being flooded, or
            while it is being held in a router's memory.  The LS
            checksum field cannot take on the value of zero; the
            occurrence of such a value should be considered a checksum
            failure.  In other words, calculation of the checksum is not
            optional.

            The checksum of an LSA is verified in two cases:  a) when it
            is received in a Link State Update Packet and b) at times
            during the aging of the link state database.  The detection
            of a checksum failure leads to separate actions in each
            case.  See Sections 13 and 14 for more details.

            Whenever the LS sequence number field indicates that two
            instances of an LSA are the same, the LS checksum field is
            examined.  If there is a difference, the instance with the
            larger LS checksum is considered to be most recent.[13] See
            Section 13.1 for more details.


    12.2.  The link state database

        A router has a separate link state database for every area to
        which it belongs. All routers belonging to the same area have
        identical link state databases for the area.

        The databases for each individual area are always dealt with
        separately.  The shortest path calculation is performed
        separately for each area (see Section 16).  Components of the



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        area link-state database are flooded throughout the area only.
        Finally, when an adjacency (belonging to Area A) is being
        brought up, only the database for Area A is synchronized between
        the two routers.

        The area database is composed of router-LSAs, network-LSAs and
        summary-LSAs (all listed in the area data structure).  In
        addition, external routes (AS-external-LSAs) are included in all
        non-stub area databases (see Section 3.6).

        An implementation of OSPF must be able to access individual
        pieces of an area database.  This lookup function is based on an
        LSA's LS type, Link State ID and Advertising Router.[14] There
        will be a single instance (the most up-to-date) of each LSA in
        the database.  The database lookup function is invoked during
        the LSA flooding procedure (Section 13) and the routing table
        calculation (Section 16).  In addition, using this lookup
        function the router can determine whether it has itself ever
        originated a particular LSA, and if so, with what LS sequence
        number.

        An LSA is added to a router's database when either a) it is
        received during the flooding process (Section 13) or b) it is
        originated by the router itself (Section 12.4).  An LSA is
        deleted from a router's database when either a) it has been
        overwritten by a newer instance during the flooding process
        (Section 13) or b) the router originates a newer instance of one
        of its self-originated LSAs (Section 12.4) or c) the LSA ages
        out and is flushed from the routing domain (Section 14).
        Whenever an LSA is deleted from the database it must also be
        removed from all neighbors' Link state retransmission lists (see
        Section 10).


    12.3.  Representation of TOS

        For backward compatibility with previous versions of the OSPF
        specification ([Ref9]), TOS-specific information can be included
        in router-LSAs, summary-LSAs and AS-external-LSAs.  The encoding
        of TOS in OSPF LSAs is specified in Table 17. That table relates
        the OSPF encoding to the IP packet header's TOS field (defined
        in [Ref12]).  The OSPF encoding is expressed as a decimal



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        integer, and the IP packet header's TOS field is expressed in
        the binary TOS values used in [Ref12].



                    OSPF encoding   RFC 1349 TOS values
                    ___________________________________________
                    0               0000 normal service
                    2               0001 minimize monetary cost
                    4               0010 maximize reliability
                    6               0011
                    8               0100 maximize throughput
                    10              0101
                    12              0110
                    14              0111
                    16              1000 minimize delay
                    18              1001
                    20              1010
                    22              1011
                    24              1100
                    26              1101
                    28              1110
                    30              1111


                        Table 17: Representing TOS in OSPF.


    12.4.  Originating LSAs

        Into any given OSPF area, a router will originate several LSAs.
        Each router originates a router-LSA.  If the router is also the
        Designated Router for any of the area's networks, it will
        originate network-LSAs for those networks.

        Area border routers originate a single summary-LSA for each
        known inter-area destination.  AS boundary routers originate a
        single AS-external-LSA for each known AS external destination.
        Destinations are advertised one at a time so that the change in
        any single route can be flooded without reflooding the entire
        collection of routes.  During the flooding procedure, many LSAs
        can be carried by a single Link State Update packet.



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        As an example, consider Router RT4 in Figure 6.  It is an area
        border router, having a connection to Area 1 and the backbone.
        Router RT4 originates 5 distinct LSAs into the backbone (one
        router-LSA, and one summary-LSA for each of the networks N1-N4).
        Router RT4 will also originate 8 distinct LSAs into Area 1 (one
        router-LSA and seven summary-LSAs as pictured in Figure 7).  If
        RT4 has been selected as Designated Router for Network N3, it
        will also originate a network-LSA for N3 into Area 1.

        In this same figure, Router RT5 will be originating 3 distinct
        AS-external-LSAs (one for each of the networks N12-N14).  These
        will be flooded throughout the entire AS, assuming that none of
        the areas have been configured as stubs.  However, if area 3 has
        been configured as a stub area, the AS-external-LSAs for
        networks N12-N14 will not be flooded into area 3 (see Section
        3.6).  Instead, Router RT11 would originate a default summary-
        LSA that would be flooded throughout area 3 (see Section
        12.4.3).  This instructs all of area 3's internal routers to
        send their AS external traffic to RT11.

        Whenever a new instance of an LSA is originated, its LS sequence
        number is incremented, its LS age is set to 0, its LS checksum
        is calculated, and the LSA is added to the link state database
        and flooded out the appropriate interfaces.  See Section 13.2
        for details concerning the installation of the LSA into the link
        state database.  See Section 13.3 for details concerning the
        flooding of newly originated LSAs.


        The ten events that can cause a new instance of an LSA to be
        originated are:


        (1) The LS age field of one of the router's self-originated LSAs
            reaches the value LSRefreshTime. In this case, a new
            instance of the LSA is originated, even though the contents
            of the LSA (apart from the LSA header) will be the same.
            This guarantees periodic originations of all LSAs.  This
            periodic updating of LSAs adds robustness to the link state
            algorithm.  LSAs that solely describe unreachable
            destinations should not be refreshed, but should instead be
            flushed from the routing domain (see Section 14.1).



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        When whatever is being described by an LSA changes, a new LSA is
        originated.  However, two instances of the same LSA may not be
        originated within the time period MinLSInterval.  This may
        require that the generation of the next instance be delayed by
        up to MinLSInterval.  The following events may cause the
        contents of an LSA to change.  These events should cause new
        originations if and only if the contents of the new LSA would be
        different:


        (2) An interface's state changes (see Section 9.1).  This may
            mean that it is necessary to produce a new instance of the
            router-LSA.

        (3) An attached network's Designated Router changes.  A new
            router-LSA should be originated.  Also, if the router itself
            is now the Designated Router, a new network-LSA should be
            produced.  If the router itself is no longer the Designated
            Router, any network-LSA that it might have originated for
            the network should be flushed from the routing domain (see
            Section 14.1).

        (4) One of the neighboring routers changes to/from the FULL
            state.  This may mean that it is necessary to produce a new
            instance of the router-LSA.  Also, if the router is itself
            the Designated Router for the attached network, a new
            network-LSA should be produced.


        The next four events concern area border routers only:


        (5) An intra-area route has been added/deleted/modified in the
            routing table.  This may cause a new instance of a summary-
            LSA (for this route) to be originated in each attached area
            (possibly including the backbone).

        (6) An inter-area route has been added/deleted/modified in the
            routing table.  This may cause a new instance of a summary-
            LSA (for this route) to be originated in each attached area
            (but NEVER for the backbone).




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        (7) The router becomes newly attached to an area.  The router
            must then originate summary-LSAs into the newly attached
            area for all pertinent intra-area and inter-area routes in
            the router's routing table.  See Section 12.4.3 for more
            details.

        (8) When the state of one of the router's configured virtual
            links changes, it may be necessary to originate a new
            router-LSA into the virtual link's Transit area (see the
            discussion of the router-LSA's bit V in Section 12.4.1), as
            well as originating a new router-LSA into the backbone.


        The last two events concern AS boundary routers (and former AS
        boundary routers) only:


        (9) An external route gained through direct experience with an
            external routing protocol (like BGP) changes.  This will
            cause an AS boundary router to originate a new instance of
            an AS-external-LSA.

        (10)
            A router ceases to be an AS boundary router, perhaps after
            restarting. In this situation the router should flush all
            AS-external-LSAs that it had previously originated.  These
            LSAs can be flushed via the premature aging procedure
            specified in Section 14.1.


        The construction of each type of LSA is explained in detail
        below.  In general, these sections describe the contents of the
        LSA body (i.e., the part coming after the 20-byte LSA header).
        For information concerning the building of the LSA header, see
        Section 12.1.

        12.4.1.  Router-LSAs

            A router originates a router-LSA for each area that it
            belongs to.  Such an LSA describes the collected states of
            the router's links to the area.  The LSA is flooded
            throughout the particular area, and no further.



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                  ....................................
                  . 192.1.2                   Area 1 .
                  .     +                            .
                  .     |                            .
                  .     | 3+---+1                    .
                  .  N1 |--|RT1|-----+               .
                  .     |  +---+      \              .
                  .     |              \  _______N3  .
                  .     +               \/       \   .  1+---+
                  .                     * 192.1.1 *------|RT4|
                  .     +               /\_______/   .   +---+
                  .     |              /     |       .
                  .     | 3+---+1     /      |       .
                  .  N2 |--|RT2|-----+      1|       .
                  .     |  +---+           +---+8    .         6+---+
                  .     |                  |RT3|----------------|RT6|
                  .     +                  +---+     .          +---+
                  . 192.1.3                  |2      .   18.10.0.6|7
                  .                          |       .            |
                  .                   +------------+ .
                  .                     192.1.4 (N4) .
                  ....................................


                    Figure 15: Area 1 with IP addresses shown

            The format of a router-LSA is shown in Appendix A (Section
            A.4.2).  The first 20 bytes of the LSA consist of the
            generic LSA header that was discussed in Section 12.1.
            router-LSAs have LS type = 1.

            A router also indicates whether it is an area border router,
            or an AS boundary router, by setting the appropriate bits
            (bit B and bit E, respectively) in its router-LSAs. This
            enables paths to those types of routers to be saved in the
            routing table, for later processing of summary-LSAs and AS-
            external-LSAs.  Bit B should be set whenever the router is
            actively attached to two or more areas, even if the router
            is not currently attached to the OSPF backbone area.  Bit E
            should never be set in a router-LSA for a stub area (stub
            areas cannot contain AS boundary routers).



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            In addition, the router sets bit V in its router-LSA for
            Area A if and only if the router is the endpoint of one or
            more fully adjacent virtual links having Area A as their
            Transit area. The setting of bit V enables other routers in
            Area A to discover whether the area supports transit traffic
            (see TransitCapability in Section 6).

            The router-LSA then describes the router's working
            connections (i.e., interfaces or links) to the area.  Each
            link is typed according to the kind of attached network.
            Each link is also labelled with its Link ID.  This Link ID
            gives a name to the entity that is on the other end of the
            link.  Table 18 summarizes the values used for the Type and
            Link ID fields.



                   Link type   Description       Link ID
                   __________________________________________________
                   1           Point-to-point    Neighbor Router ID
                               link
                   2           Link to transit   Interface address of
                               network           Designated Router
                   3           Link to stub      IP network number
                               network
                   4           Virtual link      Neighbor Router ID


                           Table 18: Link descriptions in the
                                      router-LSA.


            In addition, the Link Data field is specified for each link.
            This field gives 32 bits of extra information for the link.
            For links to transit networks, numbered point-to-point links
            and virtual links, this field specifies the IP interface
            address of the associated router interface (this is needed
            by the routing table calculation, see Section 16.1.1).  For
            links to stub networks, this field specifies the stub
            network's IP address mask.  For unnumbered point-to-point
            links, the Link Data field should be set to the unnumbered
            interface's MIB-II [Ref8] ifIndex value.



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            Finally, the cost of using the link for output is specified.
            The output cost of a link is configurable.  With the
            exception of links to stub networks, the output cost must
            always be non-zero.

            To further describe the process of building the list of link
            descriptions, suppose a router wishes to build a router-LSA
            for Area A.  The router examines its collection of interface
            data structures.  For each interface, the following steps
            are taken:


            o   If the attached network does not belong to Area A, no
                links are added to the LSA, and the next interface
                should be examined.

            o   If the state of the interface is Down, no links are
                added.

            o   If the state of the interface is Loopback, add a Type 3
                link (stub network) as long as this is not an interface
                to an unnumbered point-to-point network.  The Link ID
                should be set to the IP interface address, the Link Data
                set to the mask 0xffffffff (indicating a host route),
                and the cost set to 0.

            o   Otherwise, the link descriptions added to the router-LSA
                depend on the OSPF interface type. Link descriptions
                used for point-to-point interfaces are specified in
                Section 12.4.1.1, for virtual links in Section 12.4.1.2,
                for broadcast and NBMA interfaces in 12.4.1.3, and for
                Point-to-MultiPoint interfaces in 12.4.1.4.

            After consideration of all the router interfaces, host links
            are added to the router-LSA by examining the list of
            attached hosts belonging to Area A.  A host route is
            represented as a Type 3 link (stub network) whose Link ID is
            the host's IP address, Link Data is the mask of all ones
            (0xffffffff), and cost the host's configured cost (see
            Section C.7).





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            12.4.1.1.  Describing point-to-point interfaces

                For point-to-point interfaces, one or more link
                descriptions are added to the router-LSA as follows:

                o   If the neighboring router is fully adjacent, add a
                    Type 1 link (point-to-point). The Link ID should be
                    set to the Router ID of the neighboring router. For
                    numbered point-to-point networks, the Link Data
                    should specify the IP interface address. For
                    unnumbered point-to-point networks, the Link Data
                    field should specify the interface's MIB-II [Ref8]
                    ifIndex value. The cost should be set to the output
                    cost of the point-to-point interface.

                o   In addition, as long as the state of the interface
                    is "Point-to-Point" (and regardless of the
                    neighboring router state), a Type 3 link (stub
                    network) should be added. There are two forms that
                    this stub link can take:

                    Option 1
                        Assuming that the neighboring router's IP
                        address is known, set the Link ID of the Type 3
                        link to the neighbor's IP address, the Link Data
                        to the mask 0xffffffff (indicating a host
                        route), and the cost to the interface's
                        configured output cost.[15]

                    Option 2
                        If a subnet has been assigned to the point-to-
                        point link, set the Link ID of the Type 3 link
                        to the subnet's IP address, the Link Data to the
                        subnet's mask, and the cost to the interface's
                        configured output cost.[16]


            12.4.1.2.  Describing broadcast and NBMA interfaces

                For operational broadcast and NBMA interfaces, a single
                link description is added to the router-LSA as follows:




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                o   If the state of the interface is Waiting, add a Type
                    3 link (stub network) with Link ID set to the IP
                    network number of the attached network, Link Data
                    set to the attached network's address mask, and cost
                    equal to the interface's configured output cost.

                o   Else, there has been a Designated Router elected for
                    the attached network.  If the router is fully
                    adjacent to the Designated Router, or if the router
                    itself is Designated Router and is fully adjacent to
                    at least one other router, add a single Type 2 link
                    (transit network) with Link ID set to the IP
                    interface address of the attached network's
                    Designated Router (which may be the router itself),
                    Link Data set to the router's own IP interface
                    address, and cost equal to the interface's
                    configured output cost.  Otherwise, add a link as if
                    the interface state were Waiting (see above).


            12.4.1.3.  Describing virtual links

                For virtual links, a link description is added to the
                router-LSA only when the virtual neighbor is fully
                adjacent. In this case, add a Type 4 link (virtual link)
                with Link ID set to the Router ID of the virtual
                neighbor, Link Data set to the IP interface address
                associated with the virtual link and cost set to the
                cost calculated for the virtual link during the routing
                table calculation (see Section 15).


            12.4.1.4.  Describing Point-to-MultiPoint interfaces

                For operational Point-to-MultiPoint interfaces, one or
                more link descriptions are added to the router-LSA as
                follows:

                o   A single Type 3 link (stub network) is added with
                    Link ID set to the router's own IP interface
                    address, Link Data set to the mask 0xffffffff
                    (indicating a host route), and cost set to 0.



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                o   For each fully adjacent neighbor associated with the
                    interface, add an additional Type 1 link (point-to-
                    point) with Link ID set to the Router ID of the
                    neighboring router, Link Data set to the IP
                    interface address and cost equal to the interface's
                    configured output cost.


            12.4.1.5.  Examples of router-LSAs

                Consider the router-LSAs generated by Router RT3, as
                pictured in Figure 6.  The area containing Router RT3
                (Area 1) has been redrawn, with actual network
                addresses, in Figure 15.  Assume that the last byte of
                all of RT3's interface addresses is 3, giving it the
                interface addresses 192.1.1.3 and 192.1.4.3, and that
                the other routers have similar addressing schemes.  In
                addition, assume that all links are functional, and that
                Router IDs are assigned as the smallest IP interface
                address.

                RT3 originates two router-LSAs, one for Area 1 and one
                for the backbone.  Assume that Router RT4 has been
                selected as the Designated router for network 192.1.1.0.
                RT3's router-LSA for Area 1 is then shown below.  It
                indicates that RT3 has two connections to Area 1, the
                first a link to the transit network 192.1.1.0 and the
                second a link to the stub network 192.1.4.0.  Note that
                the transit network is identified by the IP interface of
                its Designated Router (i.e., the Link ID = 192.1.1.4
                which is the Designated Router RT4's IP interface to
                192.1.1.0).  Note also that RT3 has indicated that it is
                an area border router.

        ; RT3's router-LSA for Area 1

        LS age = 0                     ;always true on origination
        Options = (E-bit)              ;
        LS type = 1                    ;indicates router-LSA
        Link State ID = 192.1.1.3      ;RT3's Router ID
        Advertising Router = 192.1.1.3 ;RT3's Router ID
        bit E = 0                      ;not an AS boundary router



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        bit B = 1                      ;area border router
        #links = 2
               Link ID = 192.1.1.4     ;IP address of Desig. Rtr.
               Link Data = 192.1.1.3   ;RT3's IP interface to net
               Type = 2                ;connects to transit network
               # TOS metrics = 0
               metric = 1

               Link ID = 192.1.4.0     ;IP Network number
               Link Data = 0xffffff00  ;Network mask
               Type = 3                ;connects to stub network
               # TOS metrics = 0
               metric = 2

                    Next RT3's router-LSA for the backbone is shown.  It
                    indicates that RT3 has a single attachment to the
                    backbone.  This attachment is via an unnumbered
                    point-to-point link to Router RT6.  RT3 has again
                    indicated that it is an area border router.

        ; RT3's router-LSA for the backbone

        LS age = 0                     ;always true on origination
        Options = (E-bit)              ;
        LS type = 1                    ;indicates router-LSA
        Link State ID = 192.1.1.3      ;RT3's router ID
        Advertising Router = 192.1.1.3 ;RT3's router ID
        bit E = 0                      ;not an AS boundary router
        bit B = 1                      ;area border router
        #links = 1
               Link ID = 18.10.0.6     ;Neighbor's Router ID
               Link Data = 0.0.0.3     ;MIB-II ifIndex of P-P link
               Type = 1                ;connects to router
               # TOS metrics = 0
               metric = 8

        12.4.2.  Network-LSAs

            A network-LSA is generated for every transit broadcast or
            NBMA network.  (A transit network is a network having two or
            more attached routers).  The network-LSA describes all the
            routers that are attached to the network.



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            The Designated Router for the network originates the LSA.
            The Designated Router originates the LSA only if it is fully
            adjacent to at least one other router on the network.  The
            network-LSA is flooded throughout the area that contains the
            transit network, and no further.  The network-LSA lists
            those routers that are fully adjacent to the Designated
            Router; each fully adjacent router is identified by its OSPF
            Router ID.  The Designated Router includes itself in this
            list.

            The Link State ID for a network-LSA is the IP interface
            address of the Designated Router.  This value, masked by the
            network's address mask (which is also contained in the
            network-LSA) yields the network's IP address.

            A router that has formerly been the Designated Router for a
            network, but is no longer, should flush the network-LSA that
            it had previously originated.  This LSA is no longer used in
            the routing table calculation.  It is flushed by prematurely
            incrementing the LSA's age to MaxAge and reflooding (see
            Section 14.1). In addition, in those rare cases where a
            router's Router ID has changed, any network-LSAs that were
            originated with the router's previous Router ID must be
            flushed. Since the router may have no idea what it's
            previous Router ID might have been, these network-LSAs are
            indicated by having their Link State ID equal to one of the
            router's IP interface addresses and their Advertising Router
            equal to some value other than the router's current Router
            ID (see Section 13.4 for more details).


            12.4.2.1.  Examples of network-LSAs

                Again consider the area configuration in Figure 6.
                Network-LSAs are originated for Network N3 in Area 1,
                Networks N6 and N8 in Area 2, and Network N9 in Area 3.
                Assuming that Router RT4 has been selected as the
                Designated Router for Network N3, the following
                network-LSA is generated by RT4 on behalf of Network N3
                (see Figure 15 for the address assignments):

        ; Network-LSA for Network N3



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        LS age = 0                     ;always true on origination
        Options = (E-bit)              ;
        LS type = 2                    ;indicates network-LSA
        Link State ID = 192.1.1.4      ;IP address of Desig. Rtr.
        Advertising Router = 192.1.1.4 ;RT4's Router ID
        Network Mask = 0xffffff00
               Attached Router = 192.1.1.4    ;Router ID
               Attached Router = 192.1.1.1    ;Router ID
               Attached Router = 192.1.1.2    ;Router ID
               Attached Router = 192.1.1.3    ;Router ID

        12.4.3.  Summary-LSAs

            The destination described by a summary-LSA is either an IP
            network, an AS boundary router or a range of IP addresses.
            Summary-LSAs are flooded throughout a single area only.  The
            destination described is one that is external to the area,
            yet still belongs to the Autonomous System.

            Summary-LSAs are originated by area border routers.  The
            precise summary routes to advertise into an area are
            determined by examining the routing table structure (see
            Section 11) in accordance with the algorithm described
            below. Note that only intra-area routes are advertised into
            the backbone, while both intra-area and inter-area routes
            are advertised into the other areas.

            To determine which routes to advertise into an attached Area
            A, each routing table entry is processed as follows.
            Remember that each routing table entry describes a set of
            equal-cost best paths to a particular destination:

            o   Only Destination Types of network and AS boundary router
                are advertised in summary-LSAs.  If the routing table
                entry's Destination Type is area border router, examine
                the next routing table entry.

            o   AS external routes are never advertised in summary-LSAs.
                If the routing table entry has Path-type of type 1
                external or type 2 external, examine the next routing
                table entry.




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            o   Else, if the area associated with this set of paths is
                the Area A itself, do not generate a summary-LSA for the
                route.[17]

            o   Else, if the next hops associated with this set of paths
                belong to Area A itself, do not generate a summary-LSA
                for the route.[18] This is the logical equivalent of a
                Distance Vector protocol's split horizon logic.

            o   Else, if the routing table cost equals or exceeds the
                value LSInfinity, a summary-LSA cannot be generated for
                this route.

            o   Else, if the destination of this route is an AS boundary
                router, a summary-LSA should be originated if and only
                if the routing table entry describes the preferred path
                to the AS boundary router (see Step 3 of Section 16.4).
                If so, a Type 4 summary-LSA is originated for the
                destination, with Link State ID equal to the AS boundary
                router's Router ID and metric equal to the routing table
                entry's cost. Note: these LSAs should not be generated
                if Area A has been configured as a stub area.

            o   Else, the Destination type is network. If this is an
                inter-area route, generate a Type 3 summary-LSA for the
                destination, with Link State ID equal to the network's
                address (if necessary, the Link State ID can also have
                one or more of the network's host bits set; see Appendix
                E for details) and metric equal to the routing table
                cost.

            o   The one remaining case is an intra-area route to a
                network.  This means that the network is contained in
                one of the router's directly attached areas.  In
                general, this information must be condensed before
                appearing in summary-LSAs.  Remember that an area has a
                configured list of address ranges, each range consisting
                of an [address,mask] pair and a status indication of
                either Advertise or DoNotAdvertise.  At most a single
                Type 3 summary-LSA is originated for each range. When
                the range's status indicates Advertise, a Type 3
                summary-LSA is generated with Link State ID equal to the



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                range's address (if necessary, the Link State ID can
                also have one or more of the range's "host" bits set;
                see Appendix E for details) and cost equal to the
                largest cost of any of the component networks. When the
                range's status indicates DoNotAdvertise, the Type 3
                summary-LSA is suppressed and the component networks
                remain hidden from other areas.

                By default, if a network is not contained in any
                explicitly configured address range, a Type 3 summary-
                LSA is generated with Link State ID equal to the
                network's address (if necessary, the Link State ID can
                also have one or more of the network's "host" bits set;
                see Appendix E for details) and metric equal to the
                network's routing table cost.

                If an area is capable of carrying transit traffic (i.e.,
                its TransitCapability is set to TRUE), routing
                information concerning backbone networks should not be
                condensed before being summarized into the area.  Nor
                should the advertisement of backbone networks into
                transit areas be suppressed.  In other words, the
                backbone's configured ranges should be ignored when
                originating summary-LSAs into transit areas.

            If a router advertises a summary-LSA for a destination which
            then becomes unreachable, the router must then flush the LSA
            from the routing domain by setting its age to MaxAge and
            reflooding (see Section 14.1).  Also, if the destination is
            still reachable, yet can no longer be advertised according
            to the above procedure (e.g., it is now an inter-area route,
            when it used to be an intra-area route associated with some
            non-backbone area; it would thus no longer be advertisable
            to the backbone), the LSA should also be flushed from the
            routing domain.


            12.4.3.1.  Originating summary-LSAs into stub areas

                The algorithm in Section 12.4.3 is optional when Area A
                is an OSPF stub area. Area border routers connecting to
                a stub area can originate summary-LSAs into the area



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                according to the Section 12.4.3's algorithm, or can
                choose to originate only a subset of the summary-LSAs,
                possibly under configuration control.  The fewer LSAs
                originated, the smaller the stub area's link state
                database, further reducing the demands on its routers'
                resources. However, omitting LSAs may also lead to sub-
                optimal inter-area routing, although routing will
                continue to function.

                As specified in Section 12.4.3, Type 4 summary-LSAs
                (ASBR-summary-LSAs) are never originated into stub
                areas.

                In a stub area, instead of importing external routes
                each area border router originates a "default summary-
                LSA" into the area. The Link State ID for the default
                summary-LSA is set to DefaultDestination, and the metric
                set to the (per-area) configurable parameter
                StubDefaultCost.  Note that StubDefaultCost need not be
                configured identically in all of the stub area's area
                border routers.


            12.4.3.2.  Examples of summary-LSAs

                Consider again the area configuration in Figure 6.
                Routers RT3, RT4, RT7, RT10 and RT11 are all area border
                routers, and therefore are originating summary-LSAs.
                Consider in particular Router RT4.  Its routing table
                was calculated as the example in Section 11.3.  RT4
                originates summary-LSAs into both the backbone and Area
                1.  Into the backbone, Router RT4 originates separate
                LSAs for each of the networks N1-N4.  Into Area 1,
                Router RT4 originates separate LSAs for networks N6-N8
                and the AS boundary routers RT5,RT7.  It also condenses
                host routes Ia and Ib into a single summary-LSA.
                Finally, the routes to networks N9,N10,N11 and Host H1
                are advertised by a single summary-LSA.  This
                condensation was originally performed by the router
                RT11.





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                These LSAs are illustrated graphically in Figures 7 and
                8.  Two of the summary-LSAs originated by Router RT4
                follow.  The actual IP addresses for the networks and
                routers in question have been assigned in Figure 15.

        ; Summary-LSA for Network N1,
        ; originated by Router RT4 into the backbone

        LS age = 0                  ;always true on origination
        Options = (E-bit)           ;
        LS type = 3                 ;Type 3 summary-LSA
        Link State ID = 192.1.2.0   ;N1's IP network number
        Advertising Router = 192.1.1.4       ;RT4's ID
        metric = 4

        ; Summary-LSA for AS boundary router RT7
        ; originated by Router RT4 into Area 1

        LS age = 0                  ;always true on origination
        Options = (E-bit)           ;
        LS type = 4                 ;Type 4 summary-LSA
        Link State ID = Router RT7's ID
        Advertising Router = 192.1.1.4       ;RT4's ID
        metric = 14

        12.4.4.  AS-external-LSAs

            AS-external-LSAs describe routes to destinations external to
            the Autonomous System.  Most AS-external-LSAs describe
            routes to specific external destinations; in these cases the
            LSA's Link State ID is set to the destination network's IP
            address (if necessary, the Link State ID can also have one
            or more of the network's "host" bits set; see Appendix E for
            details).  However, a default route for the Autonomous
            System can be described in an AS-external-LSA by setting the
            LSA's Link State ID to DefaultDestination (0.0.0.0).  AS-
            external-LSAs are originated by AS boundary routers.  An AS
            boundary router originates a single AS-external-LSA for each
            external route that it has learned, either through another
            routing protocol (such as BGP), or through configuration
            information.




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            AS-external-LSAs are the only type of LSAs that are flooded
            throughout the entire Autonomous System; all other types of
            LSAs are specific to a single area.  However, AS-external-
            LSAs are not flooded into/throughout stub areas (see Section
            3.6).  This enables a reduction in link state database size
            for routers internal to stub areas.

            The metric that is advertised for an external route can be
            one of two types.  Type 1 metrics are comparable to the link
            state metric.  Type 2 metrics are assumed to be larger than
            the cost of any intra-AS path.

            If a router advertises an AS-external-LSA for a destination
            which then becomes unreachable, the router must then flush
            the LSA from the routing domain by setting its age to MaxAge
            and reflooding (see Section 14.1).


            12.4.4.1.  Examples of AS-external-LSAs

                Consider once again the AS pictured in Figure 6.  There
                are two AS boundary routers: RT5 and RT7.  Router RT5
                originates three AS-external-LSAs, for networks N12-N14.
                Router RT7 originates two AS-external-LSAs, for networks
                N12 and N15.  Assume that RT7 has learned its route to
                N12 via BGP, and that it wishes to advertise a Type 2
                metric to the AS.  RT7 would then originate the
                following LSA for N12:

        ; AS-external-LSA for Network N12,
        ; originated by Router RT7

        LS age = 0                  ;always true on origination
        Options = (E-bit)           ;
        LS type = 5                 ;AS-external-LSA
        Link State ID = N12's IP network number
        Advertising Router = Router RT7's ID
        bit E = 1                   ;Type 2 metric
        metric = 2
        Forwarding address = 0.0.0.0





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                    In the above example, the forwarding address field
                    has been set to 0.0.0.0, indicating that packets for
                    the external destination should be forwarded to the
                    advertising OSPF router (RT7).  This is not always
                    desirable.  Consider the example pictured in Figure
                    16.  There are three OSPF routers (RTA, RTB and RTC)
                    connected to a common network.  Only one of these
                    routers, RTA, is exchanging BGP information with the
                    non-OSPF router RTX.  RTA must then originate AS-
                    external-LSAs for those destinations it has learned
                    from RTX.  By using the AS-external-LSA's forwarding
                    address field, RTA can specify that packets for
                    these destinations be forwarded directly to RTX.
                    Without this feature, Routers RTB and RTC would take
                    an extra hop to get to these destinations.

                    Note that when the forwarding address field is non-
                    zero, it should point to a router belonging to
                    another Autonomous System.

                    A forwarding address can also be specified for the
                    default route.  For example, in figure 16 RTA may
                    want to specify that all externally-destined packets
                    should by default be forwarded to its BGP peer RTX.
                    The resulting AS-external-LSA is pictured below.
                    Note that the Link State ID is set to
                    DefaultDestination.

        ; Default route, originated by Router RTA
        ; Packets forwarded through RTX

        LS age = 0                  ;always true on origination
        Options = (E-bit)           ;
        LS type = 5                 ;AS-external-LSA
        Link State ID = DefaultDestination  ; default route
        Advertising Router = Router RTA's ID
        bit E = 1                   ;Type 2 metric
        metric = 1
        Forwarding address = RTX's IP address

                    In figure 16, suppose instead that both RTA and RTB
                    exchange BGP information with RTX.  In this case,



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                    RTA and RTB would originate the same set of AS-
                    external-LSAs.  These LSAs, if they specify the same
                    metric, would be functionally equivalent since they
                    would specify the same destination and forwarding
                    address (RTX).  This leads to a clear duplication of
                    effort.  If only one of RTA or RTB originated the
                    set of AS-external-LSAs, the routing would remain
                    the same, and the size of the link state database
                    would decrease.  However, it must be unambiguously
                    defined as to which router originates the LSAs
                    (otherwise neither may, or the identity of the
                    originator may oscillate).  The following rule is
                    thereby established: if two routers, both reachable
                    from one another, originate functionally equivalent
                    AS-external-LSAs (i.e., same destination, cost and
                    non-zero forwarding address), then the LSA
                    originated by the router having the highest OSPF
                    Router ID is used.  The router having the lower OSPF
                    Router ID can then flush its LSA.  Flushing an LSA
                    is discussed in Section 14.1.


                                +
                                |
                      +---+.....|.BGP
                      |RTA|-----|.....+---+
                      +---+     |-----|RTX|
                                |     +---+
                      +---+     |
                      |RTB|-----|
                      +---+     |
                                |
                      +---+     |
                      |RTC|-----|
                      +---+     |
                                |
                                +


               Figure 16: Forwarding address example





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13.  The Flooding Procedure

    Link State Update packets provide the mechanism for flooding LSAs.
    A Link State Update packet may contain several distinct LSAs, and
    floods each LSA one hop further from its point of origination.  To
    make the flooding procedure reliable, each LSA must be acknowledged
    separately.  Acknowledgments are transmitted in Link State
    Acknowledgment packets.  Many separate acknowledgments can also be
    grouped together into a single packet.

    The flooding procedure starts when a Link State Update packet has
    been received.  Many consistency checks have been made on the
    received packet before being handed to the flooding procedure (see
    Section 8.2).  In particular, the Link State Update packet has been
    associated with a particular neighbor, and a particular area.  If
    the neighbor is in a lesser state than Exchange, the packet should
    be dropped without further processing.

    All types of LSAs, other than AS-external-LSAs, are associated with
    a specific area.  However, LSAs do not contain an area field.  An
    LSA's area must be deduced from the Link State Update packet header.

    For each LSA contained in a Link State Update packet, the following
    steps are taken:


    (1) Validate the LSA's LS checksum.  If the checksum turns out to be
        invalid, discard the LSA and get the next one from the Link
        State Update packet.

    (2) Examine the LSA's LS type.  If the LS type is unknown, discard
        the LSA and get the next one from the Link State Update Packet.
        This specification defines LS types 1-5 (see Section 4.3).

    (3) Else if this is an AS-external-LSA (LS type = 5), and the area
        has been configured as a stub area, discard the LSA and get the
        next one from the Link State Update Packet.  AS-external-LSAs
        are not flooded into/throughout stub areas (see Section 3.6).

    (4) Else if the LSA's LS age is equal to MaxAge, and there is
        currently no instance of the LSA in the router's link state
        database, and none of router's neighbors are in states Exchange



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        or Loading, then take the following actions: a) Acknowledge the
        receipt of the LSA by sending a Link State Acknowledgment packet
        back to the sending neighbor (see Section 13.5), and b) Discard
        the LSA and examine the next LSA (if any) listed in the Link
        State Update packet.

    (5) Otherwise, find the instance of this LSA that is currently
        contained in the router's link state database.  If there is no
        database copy, or the received LSA is more recent than the
        database copy (see Section 13.1 below for the determination of
        which LSA is more recent) the following steps must be performed:

        (a) If there is already a database copy, and if the database
            copy was received via flooding and installed less than
            MinLSArrival seconds ago, discard the new LSA (without
            acknowledging it) and examine the next LSA (if any) listed
            in the Link State Update packet.

        (b) Otherwise immediately flood the new LSA out some subset of
            the router's interfaces (see Section 13.3).  In some cases
            (e.g., the state of the receiving interface is DR and the
            LSA was received from a router other than the Backup DR) the
            LSA will be flooded back out the receiving interface.  This
            occurrence should be noted for later use by the
            acknowledgment process (Section 13.5).

        (c) Remove the current database copy from all neighbors' Link
            state retransmission lists.

        (d) Install the new LSA in the link state database (replacing
            the current database copy).  This may cause the routing
            table calculation to be scheduled.  In addition, timestamp
            the new LSA with the current time (i.e., the time it was
            received).  The flooding procedure cannot overwrite the
            newly installed LSA until MinLSArrival seconds have elapsed.
            The LSA installation process is discussed further in Section
            13.2.

        (e) Possibly acknowledge the receipt of the LSA by sending a
            Link State Acknowledgment packet back out the receiving
            interface.  This is explained below in Section 13.5.




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        (f) If this new LSA indicates that it was originated by the
            receiving router itself (i.e., is considered a self-
            originated LSA), the router must take special action, either
            updating the LSA or in some cases flushing it from the
            routing domain. For a description of how self-originated
            LSAs are detected and subsequently handled, see Section
            13.4.

    (6) Else, if there is an instance of the LSA on the sending
        neighbor's Link state request list, an error has occurred in the
        Database Exchange process.  In this case, restart the Database
        Exchange process by generating the neighbor event BadLSReq for
        the sending neighbor and stop processing the Link State Update
        packet.

    (7) Else, if the received LSA is the same instance as the database
        copy (i.e., neither one is more recent) the following two steps
        should be performed:

        (a) If the LSA is listed in the Link state retransmission list
            for the receiving adjacency, the router itself is expecting
            an acknowledgment for this LSA.  The router should treat the
            received LSA as an acknowledgment by removing the LSA from
            the Link state retransmission list.  This is termed an
            "implied acknowledgment".  Its occurrence should be noted
            for later use by the acknowledgment process (Section 13.5).

        (b) Possibly acknowledge the receipt of the LSA by sending a
            Link State Acknowledgment packet back out the receiving
            interface.  This is explained below in Section 13.5.

    (8) Else, the database copy is more recent.  If the database copy
        has LS age equal to MaxAge and LS sequence number equal to
        MaxSequenceNumber, simply discard the received LSA without
        acknowledging it. (In this case, the LSA's LS sequence number is
        wrapping, and the MaxSequenceNumber LSA must be completely
        flushed before any new LSA instance can be introduced).
        Otherwise, as long as the database copy has not been sent in a
        Link State Update within the last MinLSArrival seconds, send the
        database copy back to the sending neighbor, encapsulated within
        a Link State Update Packet. The Link State Update Packet should
        be sent directly to the neighbor. In so doing, do not put the



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        database copy of the LSA on the neighbor's link state
        retransmission list, and do not acknowledge the received (less
        recent) LSA instance.


    13.1.  Determining which LSA is newer

        When a router encounters two instances of an LSA, it must
        determine which is more recent.  This occurred above when
        comparing a received LSA to its database copy.  This comparison
        must also be done during the Database Exchange procedure which
        occurs during adjacency bring-up.

        An LSA is identified by its LS type, Link State ID and
        Advertising Router.  For two instances of the same LSA, the LS
        sequence number, LS age, and LS checksum fields are used to
        determine which instance is more recent:


        o   The LSA having the newer LS sequence number is more recent.
            See Section 12.1.6 for an explanation of the LS sequence
            number space.  If both instances have the same LS sequence
            number, then:

        o   If the two instances have different LS checksums, then the
            instance having the larger LS checksum (when considered as a
            16-bit unsigned integer) is considered more recent.

        o   Else, if only one of the instances has its LS age field set
            to MaxAge, the instance of age MaxAge is considered to be
            more recent.

        o   Else, if the LS age fields of the two instances differ by
            more than MaxAgeDiff, the instance having the smaller
            (younger) LS age is considered to be more recent.

        o   Else, the two instances are considered to be identical.








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    13.2.  Installing LSAs in the database

        Installing a new LSA in the database, either as the result of
        flooding or a newly self-originated LSA, may cause the OSPF
        routing table structure to be recalculated.  The contents of the
        new LSA should be compared to the old instance, if present.  If
        there is no difference, there is no need to recalculate the
        routing table. When comparing an LSA to its previous instance,
        the following are all considered to be differences in contents:

            o   The LSA's Options field has changed.

            o   One of the LSA instances has LS age set to MaxAge, and
                the other does not.

            o   The length field in the LSA header has changed.

            o   The body of the LSA (i.e., anything outside the 20-byte
                LSA header) has changed. Note that this excludes changes
                in LS Sequence Number and LS Checksum.

        If the contents are different, the following pieces of the
        routing table must be recalculated, depending on the new LSA's
        LS type field:


        Router-LSAs and network-LSAs
            The entire routing table must be recalculated, starting with
            the shortest path calculations for each area (not just the
            area whose link-state database has changed).  The reason
            that the shortest path calculation cannot be restricted to
            the single changed area has to do with the fact that AS
            boundary routers may belong to multiple areas.  A change in
            the area currently providing the best route may force the
            router to use an intra-area route provided by a different
            area.[19]

        Summary-LSAs
            The best route to the destination described by the summary-
            LSA must be recalculated (see Section 16.5).  If this
            destination is an AS boundary router, it may also be
            necessary to re-examine all the AS-external-LSAs.



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        AS-external-LSAs
            The best route to the destination described by the AS-
            external-LSA must be recalculated (see Section 16.6).


        Also, any old instance of the LSA must be removed from the
        database when the new LSA is installed.  This old instance must
        also be removed from all neighbors' Link state retransmission
        lists (see Section 10).


    13.3.  Next step in the flooding procedure

        When a new (and more recent) LSA has been received, it must be
        flooded out some set of the router's interfaces.  This section
        describes the second part of flooding procedure (the first part
        being the processing that occurred in Section 13), namely,
        selecting the outgoing interfaces and adding the LSA to the
        appropriate neighbors' Link state retransmission lists.  Also
        included in this part of the flooding procedure is the
        maintenance of the neighbors' Link state request lists.

        This section is equally applicable to the flooding of an LSA
        that the router itself has just originated (see Section 12.4).
        For these LSAs, this section provides the entirety of the
        flooding procedure (i.e., the processing of Section 13 is not
        performed, since, for example, the LSA has not been received
        from a neighbor and therefore does not need to be acknowledged).

        Depending upon the LSA's LS type, the LSA can be flooded out
        only certain interfaces.  These interfaces, defined by the
        following, are called the eligible interfaces:


        AS-external-LSAs (LS Type = 5)
            AS-external-LSAs are flooded throughout the entire AS, with
            the exception of stub areas (see Section 3.6).  The eligible
            interfaces are all the router's interfaces, excluding
            virtual links and those interfaces attaching to stub areas.

        All other LS types
            All other types are specific to a single area (Area A).  The



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            eligible interfaces are all those interfaces attaching to
            the Area A.  If Area A is the backbone, this includes all
            the virtual links.


        Link state databases must remain synchronized over all
        adjacencies associated with the above eligible interfaces.  This
        is accomplished by executing the following steps on each
        eligible interface.  It should be noted that this procedure may
        decide not to flood an LSA out a particular interface, if there
        is a high probability that the attached neighbors have already
        received the LSA.  However, in these cases the flooding
        procedure must be absolutely sure that the neighbors eventually
        do receive the LSA, so the LSA is still added to each
        adjacency's Link state retransmission list.  For each eligible
        interface:


        (1) Each of the neighbors attached to this interface are
            examined, to determine whether they must receive the new
            LSA.  The following steps are executed for each neighbor:

            (a) If the neighbor is in a lesser state than Exchange, it
                does not participate in flooding, and the next neighbor
                should be examined.

            (b) Else, if the adjacency is not yet full (neighbor state
                is Exchange or Loading), examine the Link state request
                list associated with this adjacency.  If there is an
                instance of the new LSA on the list, it indicates that
                the neighboring router has an instance of the LSA
                already.  Compare the new LSA to the neighbor's copy:

                o   If the new LSA is less recent, then examine the next
                    neighbor.

                o   If the two copies are the same instance, then delete
                    the LSA from the Link state request list, and
                    examine the next neighbor.[20]

                o   Else, the new LSA is more recent.  Delete the LSA
                    from the Link state request list.



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            (c) If the new LSA was received from this neighbor, examine
                the next neighbor.

            (d) At this point we are not positive that the neighbor has
                an up-to-date instance of this new LSA.  Add the new LSA
                to the Link state retransmission list for the adjacency.
                This ensures that the flooding procedure is reliable;
                the LSA will be retransmitted at intervals until an
                acknowledgment is seen from the neighbor.

        (2) The router must now decide whether to flood the new LSA out
            this interface.  If in the previous step, the LSA was NOT
            added to any of the Link state retransmission lists, there
            is no need to flood the LSA out the interface and the next
            interface should be examined.

        (3) If the new LSA was received on this interface, and it was
            received from either the Designated Router or the Backup
            Designated Router, chances are that all the neighbors have
            received the LSA already.  Therefore, examine the next
            interface.

        (4) If the new LSA was received on this interface, and the
            interface state is Backup (i.e., the router itself is the
            Backup Designated Router), examine the next interface.  The
            Designated Router will do the flooding on this interface.
            However, if the Designated Router fails the router (i.e.,
            the Backup Designated Router) will end up retransmitting the
            updates.

        (5) If this step is reached, the LSA must be flooded out the
            interface.  Send a Link State Update packet (including the
            new LSA as contents) out the interface.  The LSA's LS age
            must be incremented by InfTransDelay (which must be > 0)
            when it is copied into the outgoing Link State Update packet
            (until the LS age field reaches the maximum value of
            MaxAge).

            On broadcast networks, the Link State Update packets are
            multicast.  The destination IP address specified for the
            Link State Update Packet depends on the state of the
            interface.  If the interface state is DR or Backup, the



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            address AllSPFRouters should be used.  Otherwise, the
            address AllDRouters should be used.

            On non-broadcast networks, separate Link State Update
            packets must be sent, as unicasts, to each adjacent neighbor
            (i.e., those in state Exchange or greater).  The destination
            IP addresses for these packets are the neighbors' IP
            addresses.


    13.4.  Receiving self-originated LSAs

        It is a common occurrence for a router to receive self-
        originated LSAs via the flooding procedure. A self-originated
        LSA is detected when either 1) the LSA's Advertising Router is
        equal to the router's own Router ID or 2) the LSA is a network-
        LSA and its Link State ID is equal to one of the router's own IP
        interface addresses.

        However, if the received self-originated LSA is newer than the
        last instance that the router actually originated, the router
        must take special action.  The reception of such an LSA
        indicates that there are LSAs in the routing domain that were
        originated by the router before the last time it was restarted.
        In most cases, the router must then advance the LSA's LS
        sequence number one past the received LS sequence number, and
        originate a new instance of the LSA.

        It may be the case the router no longer wishes to originate the
        received LSA. Possible examples include: 1) the LSA is a
        summary-LSA or AS-external-LSA and the router no longer has an
        (advertisable) route to the destination, 2) the LSA is a
        network-LSA but the router is no longer Designated Router for
        the network or 3) the LSA is a network-LSA whose Link State ID
        is one of the router's own IP interface addresses but whose
        Advertising Router is not equal to the router's own Router ID
        (this latter case should be rare, and it indicates that the
        router's Router ID has changed since originating the LSA).  In
        all these cases, instead of updating the LSA, the LSA should be
        flushed from the routing domain by incrementing the received
        LSA's LS age to MaxAge and reflooding (see Section 14.1).




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    13.5.  Sending Link State Acknowledgment packets

        Each newly received LSA must be acknowledged.  This is usually
        done by sending Link State Acknowledgment packets.  However,
        acknowledgments can also be accomplished implicitly by sending
        Link State Update packets (see step 7a of Section 13).

        Many acknowledgments may be grouped together into a single Link
        State Acknowledgment packet.  Such a packet is sent back out the
        interface which received the LSAs.  The packet can be sent in
        one of two ways: delayed and sent on an interval timer, or sent
        directly to a particular neighbor.  The particular
        acknowledgment strategy used depends on the circumstances
        surrounding the receipt of the LSA.

        Sending delayed acknowledgments accomplishes several things: 1)
        it facilitates the packaging of multiple acknowledgments in a
        single Link State Acknowledgment packet, 2) it enables a single
        Link State Acknowledgment packet to indicate acknowledgments to
        several neighbors at once (through multicasting) and 3) it
        randomizes the Link State Acknowledgment packets sent by the
        various routers attached to a common network.  The fixed
        interval between a router's delayed transmissions must be short
        (less than RxmtInterval) or needless retransmissions will ensue.

        Direct acknowledgments are sent directly to a particular
        neighbor in response to the receipt of duplicate LSAs. Direct
        acknowledgments are sent immediately when the duplicate is
        received. On multi-access networks, these acknowledgments are
        sent directly to the neighbor's IP address.

        The precise procedure for sending Link State Acknowledgment
        packets is described in Table 19.  The circumstances surrounding
        the receipt of the LSA are listed in the left column.  The
        acknowledgment action then taken is listed in one of the two
        right columns.  This action depends on the state of the
        concerned interface; interfaces in state Backup behave
        differently from interfaces in all other states.  Delayed
        acknowledgments must be delivered to all adjacent routers
        associated with the interface.  On broadcast networks, this is
        accomplished by sending the delayed Link State Acknowledgment
        packets as multicasts.  The Destination IP address used depends



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                                     Action taken in state
   Circumstances            Backup                All other states
   _________________________________________________________________
   LSA  has                 No  acknowledgment    No  acknowledgment
   been  flooded back       sent.                 sent.
   out receiving  in-
   terface  (see Sec-
   tion 13, step 5b).
   _________________________________________________________________
   LSA   is                 Delayed acknowledg-   Delayed       ack-
   more  recent  than       ment sent if adver-   nowledgment sent.
   database copy, but       tisement   received
   was   not  flooded       from    Designated
   back out receiving       Router,  otherwise
   interface                do nothing
   _________________________________________________________________
   LSA is a                 Delayed acknowledg-   No  acknowledgment
   duplicate, and was       ment sent if adver-   sent.
   treated as an  im-       tisement   received
   plied  acknowledg-       from    Designated
   ment (see  Section       Router,  otherwise
   13, step 7a).            do nothing
   _________________________________________________________________
   LSA is a                 Direct acknowledg-    Direct acknowledg-
   duplicate, and was       ment sent.            ment sent.
   not treated as  an
   implied       ack-
   nowledgment.
   _________________________________________________________________
   LSA's LS                 Direct acknowledg-    Direct acknowledg-
   age is equal to          ment sent.            ment sent.
   MaxAge, and there is
   no current instance
   of the LSA
   in the link state
   database, and none
   of router's neighbors
   are in states Exchange



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   or Loading (see
   Section 13, step 4).


             Table 19: Sending link state acknowledgements.




        on the state of the interface.  If the interface state is DR or
        Backup, the destination AllSPFRouters is used.  In all other
        states, the destination AllDRouters is used.  On non-broadcast
        networks, delayed Link State Acknowledgment packets must be
        unicast separately over each adjacency (i.e., neighbor whose
        state is >= Exchange).

        The reasoning behind sending the above packets as multicasts is
        best explained by an example.  Consider the network
        configuration depicted in Figure 15.  Suppose RT4 has been
        elected as Designated Router, and RT3 as Backup Designated
        Router for the network N3.  When Router RT4 floods a new LSA to
        Network N3, it is received by routers RT1, RT2, and RT3.  These
        routers will not flood the LSA back onto net N3, but they still
        must ensure that their link-state databases remain synchronized
        with their adjacent neighbors.  So RT1, RT2, and RT4 are waiting
        to see an acknowledgment from RT3.  Likewise, RT4 and RT3 are
        both waiting to see acknowledgments from RT1 and RT2.  This is
        best achieved by sending the acknowledgments as multicasts.

        The reason that the acknowledgment logic for Backup DRs is
        slightly different is because they perform differently during
        the flooding of LSAs (see Section 13.3, step 4).



    13.6.  Retransmitting LSAs

        LSAs flooded out an adjacency are placed on the adjacency's Link
        state retransmission list.  In order to ensure that flooding is
        reliable, these LSAs are retransmitted until they are
        acknowledged.  The length of time between retransmissions is a
        configurable per-interface value, RxmtInterval.  If this is set



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        too low for an interface, needless retransmissions will ensue.
        If the value is set too high, the speed of the flooding, in the
        face of lost packets, may be affected.

        Several retransmitted LSAs may fit into a single Link State
        Update packet.  When LSAs are to be retransmitted, only the
        number fitting in a single Link State Update packet should be
        sent.  Another packet of retransmissions can be sent whenever
        some of the LSAs are acknowledged, or on the next firing of the
        retransmission timer.

        Link State Update Packets carrying retransmissions are always
        sent directly to the neighbor. On multi-access networks, this
        means that retransmissions are sent directly to the neighbor's
        IP address.  Each LSA's LS age must be incremented by
        InfTransDelay (which must be > 0) when it is copied into the
        outgoing Link State Update packet (until the LS age field
        reaches the maximum value of MaxAge).

        If an adjacent router goes down, retransmissions may occur until
        the adjacency is destroyed by OSPF's Hello Protocol.  When the
        adjacency is destroyed, the Link state retransmission list is
        cleared.


    13.7.  Receiving link state acknowledgments

        Many consistency checks have been made on a received Link State
        Acknowledgment packet before it is handed to the flooding
        procedure.  In particular, it has been associated with a
        particular neighbor.  If this neighbor is in a lesser state than
        Exchange, the Link State Acknowledgment packet is discarded.

        Otherwise, for each acknowledgment in the Link State
        Acknowledgment packet, the following steps are performed:


        o   Does the LSA acknowledged have an instance on the Link state
            retransmission list for the neighbor?  If not, examine the
            next acknowledgment.  Otherwise:





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        o   If the acknowledgment is for the same instance that is
            contained on the list, remove the item from the list and
            examine the next acknowledgment.  Otherwise:

        o   Log the questionable acknowledgment, and examine the next
            one.


14.  Aging The Link State Database

    Each LSA has an LS age field.  The LS age is expressed in seconds.
    An LSA's LS age field is incremented while it is contained in a
    router's database.  Also, when copied into a Link State Update
    Packet for flooding out a particular interface, the LSA's LS age is
    incremented by InfTransDelay.

    An LSA's LS age is never incremented past the value MaxAge.  LSAs
    having age MaxAge are not used in the routing table calculation.  As
    a router ages its link state database, an LSA's LS age may reach
    MaxAge.[21] At this time, the router must attempt to flush the LSA
    from the routing domain.  This is done simply by reflooding the
    MaxAge LSA just as if it was a newly originated LSA (see Section
    13.3).

    When creating a Database summary list for a newly forming adjacency,
    any MaxAge LSAs present in the link state database are added to the
    neighbor's Link state retransmission list instead of the neighbor's
    Database summary list.  See Section 10.3 for more details.

    A MaxAge LSA must be removed immediately from the router's link
    state database as soon as both a) it is no longer contained on any
    neighbor Link state retransmission lists and b) none of the router's
    neighbors are in states Exchange or Loading.

    When, in the process of aging the link state database, an LSA's LS
    age hits a multiple of CheckAge, its LS checksum should be verified.
    If the LS checksum is incorrect, a program or memory error has been
    detected, and at the very least the router itself should be
    restarted.






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    14.1.  Premature aging of LSAs

        An LSA can be flushed from the routing domain by setting its LS
        age to MaxAge, while leaving its LS sequence number alone, and
        then reflooding the LSA.  This procedure follows the same course
        as flushing an LSA whose LS age has naturally reached the value
        MaxAge (see Section 14).  In particular, the MaxAge LSA is
        removed from the router's link state database as soon as a) it
        is no longer contained on any neighbor Link state retransmission
        lists and b) none of the router's neighbors are in states
        Exchange or Loading.  We call the setting of an LSA's LS age to
        MaxAge "premature aging".

        Premature aging is used when it is time for a self-originated
        LSA's sequence number field to wrap.  At this point, the current
        LSA instance (having LS sequence number MaxSequenceNumber) must
        be prematurely aged and flushed from the routing domain before a
        new instance with sequence number equal to InitialSequenceNumber
        can be originated.  See Section 12.1.6 for more information.

        Premature aging can also be used when, for example, one of the
        router's previously advertised external routes is no longer
        reachable.  In this circumstance, the router can flush its AS-
        external-LSA from the routing domain via premature aging. This
        procedure is preferable to the alternative, which is to
        originate a new LSA for the destination specifying a metric of
        LSInfinity.  Premature aging is also be used when unexpectedly
        receiving self-originated LSAs during the flooding procedure
        (see Section 13.4).

        A router may only prematurely age its own self-originated LSAs.
        The router may not prematurely age LSAs that have been
        originated by other routers. An LSA is considered self-
        originated when either 1) the LSA's Advertising Router is equal
        to the router's own Router ID or 2) the LSA is a network-LSA and
        its Link State ID is equal to one of the router's own IP
        interface addresses.








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15.  Virtual Links

    The single backbone area (Area ID = 0.0.0.0) cannot be disconnected,
    or some areas of the Autonomous System will become unreachable.  To
    establish/maintain connectivity of the backbone, virtual links can
    be configured through non-backbone areas.  Virtual links serve to
    connect physically separate components of the backbone.  The two
    endpoints of a virtual link are area border routers.  The virtual
    link must be configured in both routers.  The configuration
    information in each router consists of the other virtual endpoint
    (the other area border router), and the non-backbone area the two
    routers have in common (called the Transit area).  Virtual links
    cannot be configured through stub areas (see Section 3.6).

    The virtual link is treated as if it were an unnumbered point-to-
    point network belonging to the backbone and joining the two area
    border routers.  An attempt is made to establish an adjacency over
    the virtual link.  When this adjacency is established, the virtual
    link will be included in backbone router-LSAs, and OSPF packets
    pertaining to the backbone area will flow over the adjacency.  Such
    an adjacency has been referred to in this document as a "virtual
    adjacency".

    In each endpoint router, the cost and viability of the virtual link
    is discovered by examining the routing table entry for the other
    endpoint router.  (The entry's associated area must be the
    configured Transit area).  This is called the virtual link's
    corresponding routing table entry.  The InterfaceUp event occurs for
    a virtual link when its corresponding routing table entry becomes
    reachable.  Conversely, the InterfaceDown event occurs when its
    routing table entry becomes unreachable.  In other words, the
    virtual link's viability is determined by the existence of an
    intra-area path, through the Transit area, between the two
    endpoints.  Note that a virtual link whose underlying path has cost
    greater than hexadecimal 0xffff (the maximum size of an interface
    cost in a router-LSA) should be considered inoperational (i.e.,
    treated the same as if the path did not exist).

    The other details concerning virtual links are as follows:

    o   AS-external-LSAs are NEVER flooded over virtual adjacencies.
        This would be duplication of effort, since the same AS-



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        external-LSAs are already flooded throughout the virtual link's
        Transit area.  For this same reason, AS-external-LSAs are not
        summarized over virtual adjacencies during the Database Exchange
        process.

    o   The cost of a virtual link is NOT configured.  It is defined to
        be the cost of the intra-area path between the two defining area
        border routers.  This cost appears in the virtual link's
        corresponding routing table entry.  When the cost of a virtual
        link changes, a new router-LSA should be originated for the
        backbone area.

    o   Just as the virtual link's cost and viability are determined by
        the routing table build process (through construction of the
        routing table entry for the other endpoint), so are the IP
        interface address for the virtual interface and the virtual
        neighbor's IP address.  These are used when sending OSPF
        protocol packets over the virtual link. Note that when one (or
        both) of the virtual link endpoints connect to the Transit area
        via an unnumbered point-to-point link, it may be impossible to
        calculate either the virtual interface's IP address and/or the
        virtual neighbor's IP address, thereby causing the virtual link
        to fail.

    o   In each endpoint's router-LSA for the backbone, the virtual link
        is represented as a Type 4 link whose Link ID is set to the
        virtual neighbor's OSPF Router ID and whose Link Data is set to
        the virtual interface's IP address.  See Section 12.4.1 for more
        information.

    o   A non-backbone area can carry transit data traffic (i.e., is
        considered a "transit area") if and only if it serves as the
        Transit area for one or more fully adjacent virtual links (see
        TransitCapability in Sections 6 and 16.1). Such an area requires
        special treatment when summarizing backbone networks into it
        (see Section 12.4.3), and during the routing calculation (see
        Section 16.3).

    o   The time between link state retransmissions, RxmtInterval, is
        configured for a virtual link.  This should be well over the
        expected round-trip delay between the two routers.  This may be




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        hard to estimate for a virtual link; it is better to err on the
        side of making it too large.


16.  Calculation of the routing table

    This section details the OSPF routing table calculation.  Using its
    attached areas' link state databases as input, a router runs the
    following algorithm, building its routing table step by step.  At
    each step, the router must access individual pieces of the link
    state databases (e.g., a router-LSA originated by a certain router).
    This access is performed by the lookup function discussed in Section
    12.2.  The lookup process may return an LSA whose LS age is equal to
    MaxAge.  Such an LSA should not be used in the routing table
    calculation, and is treated just as if the lookup process had
    failed.

    The OSPF routing table's organization is explained in Section 11.
    Two examples of the routing table build process are presented in
    Sections 11.2 and 11.3.  This process can be broken into the
    following steps:

    (1) The present routing table is invalidated.  The routing table is
        built again from scratch.  The old routing table is saved so
        that changes in routing table entries can be identified.

    (2) The intra-area routes are calculated by building the shortest-
        path tree for each attached area.  In particular, all routing
        table entries whose Destination Type is "area border router" are
        calculated in this step.  This step is described in two parts.
        At first the tree is constructed by only considering those links
        between routers and transit networks.  Then the stub networks
        are incorporated into the tree. During the area's shortest-path
        tree calculation, the area's TransitCapability is also
        calculated for later use in Step 4.

    (3) The inter-area routes are calculated, through examination of
        summary-LSAs.  If the router is attached to multiple areas
        (i.e., it is an area border router), only backbone summary-LSAs
        are examined.





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    (4) In area border routers connecting to one or more transit areas
        (i.e, non-backbone areas whose TransitCapability is found to be
        TRUE), the transit areas' summary-LSAs are examined to see
        whether better paths exist using the transit areas than were
        found in Steps 2-3 above.

    (5) Routes to external destinations are calculated, through
        examination of AS-external-LSAs.  The locations of the AS
        boundary routers (which originate the AS-external-LSAs) have
        been determined in steps 2-4.


    Steps 2-5 are explained in further detail below.

    Changes made to routing table entries as a result of these
    calculations can cause the OSPF protocol to take further actions.
    For example, a change to an intra-area route will cause an area
    border router to originate new summary-LSAs (see Section 12.4).  See
    Section 16.7 for a complete list of the OSPF protocol actions
    resulting from routing table changes.


    16.1.  Calculating the shortest-path tree for an area

        This calculation yields the set of intra-area routes associated
        with an area (called hereafter Area A).  A router calculates the
        shortest-path tree using itself as the root.[22] The formation
        of the shortest path tree is done here in two stages.  In the
        first stage, only links between routers and transit networks are
        considered.  Using the Dijkstra algorithm, a tree is formed from
        this subset of the link state database.  In the second stage,
        leaves are added to the tree by considering the links to stub
        networks.

        The procedure will be explained using the graph terminology that
        was introduced in Section 2.  The area's link state database is
        represented as a directed graph.  The graph's vertices are
        routers, transit networks and stub networks.  The first stage of
        the procedure concerns only the transit vertices (routers and
        transit networks) and their connecting links.  Throughout the
        shortest path calculation, the following data is also associated
        with each transit vertex:



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        Vertex (node) ID
            A 32-bit number which together with the vertex type (router
            or network) uniquely identifies the vertex.  For router
            vertices the Vertex ID is the router's OSPF Router ID.  For
            network vertices, it is the IP address of the network's
            Designated Router.

        An LSA
            Each transit vertex has an associated LSA.  For router
            vertices, this is a router-LSA.  For transit networks, this
            is a network-LSA (which is actually originated by the
            network's Designated Router).  In any case, the LSA's Link
            State ID is always equal to the above Vertex ID.

        List of next hops
            The list of next hops for the current set of shortest paths
            from the root to this vertex.  There can be multiple
            shortest paths due to the equal-cost multipath capability.
            Each next hop indicates the outgoing router interface to use
            when forwarding traffic to the destination.  On broadcast,
            Point-to-MultiPoint and NBMA networks, the next hop also
            includes the IP address of the next router (if any) in the
            path towards the destination.

        Distance from root
            The link state cost of the current set of shortest paths
            from the root to the vertex.  The link state cost of a path
            is calculated as the sum of the costs of the path's
            constituent links (as advertised in router-LSAs and
            network-LSAs).  One path is said to be "shorter" than
            another if it has a smaller link state cost.


        The first stage of the procedure (i.e., the Dijkstra algorithm)
        can now be summarized as follows. At each iteration of the
        algorithm, there is a list of candidate vertices.  Paths from
        the root to these vertices have been found, but not necessarily
        the shortest ones.  However, the paths to the candidate vertex
        that is closest to the root are guaranteed to be shortest; this
        vertex is added to the shortest-path tree, removed from the
        candidate list, and its adjacent vertices are examined for
        possible addition to/modification of the candidate list.  The



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        algorithm then iterates again.  It terminates when the candidate
        list becomes empty.

        The following steps describe the algorithm in detail.  Remember
        that we are computing the shortest path tree for Area A.  All
        references to link state database lookup below are from Area A's
        database.

        (1) Initialize the algorithm's data structures.  Clear the list
            of candidate vertices.  Initialize the shortest-path tree to
            only the root (which is the router doing the calculation).
            Set Area A's TransitCapability to FALSE.

        (2) Call the vertex just added to the tree vertex V.  Examine
            the LSA associated with vertex V.  This is a lookup in the
            Area A's link state database based on the Vertex ID.  If
            this is a router-LSA, and bit V of the router-LSA (see
            Section A.4.2) is set, set Area A's TransitCapability to
            TRUE.  In any case, each link described by the LSA gives the
            cost to an adjacent vertex.  For each described link, (say
            it joins vertex V to vertex W):

            (a) If this is a link to a stub network, examine the next
                link in V's LSA.  Links to stub networks will be
                considered in the second stage of the shortest path
                calculation.

            (b) Otherwise, W is a transit vertex (router or transit
                network).  Look up the vertex W's LSA (router-LSA or
                network-LSA) in Area A's link state database.  If the
                LSA does not exist, or its LS age is equal to MaxAge, or
                it does not have a link back to vertex V, examine the
                next link in V's LSA.[23]

            (c) If vertex W is already on the shortest-path tree,
                examine the next link in the LSA.

            (d) Calculate the link state cost D of the resulting path
                from the root to vertex W.  D is equal to the sum of the
                link state cost of the (already calculated) shortest
                path to vertex V and the advertised cost of the link
                between vertices V and W.  If D is:



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                o   Greater than the value that already appears for
                    vertex W on the candidate list, then examine the
                    next link.

                o   Equal to the value that appears for vertex W on the
                    candidate list, calculate the set of next hops that
                    result from using the advertised link.  Input to
                    this calculation is the destination (W), and its
                    parent (V).  This calculation is shown in Section
                    16.1.1.  This set of hops should be added to the
                    next hop values that appear for W on the candidate
                    list.

                o   Less than the value that appears for vertex W on the
                    candidate list, or if W does not yet appear on the
                    candidate list, then set the entry for W on the
                    candidate list to indicate a distance of D from the
                    root.  Also calculate the list of next hops that
                    result from using the advertised link, setting the
                    next hop values for W accordingly.  The next hop
                    calculation is described in Section 16.1.1; it takes
                    as input the destination (W) and its parent (V).

        (3) If at this step the candidate list is empty, the shortest-
            path tree (of transit vertices) has been completely built
            and this stage of the procedure terminates.  Otherwise,
            choose the vertex belonging to the candidate list that is
            closest to the root, and add it to the shortest-path tree
            (removing it from the candidate list in the process). Note
            that when there is a choice of vertices closest to the root,
            network vertices must be chosen before router vertices in
            order to necessarily find all equal-cost paths. This is
            consistent with the tie-breakers that were introduced in the
            modified Dijkstra algorithm used by OSPF's Multicast routing
            extensions (MOSPF).

        (4) Possibly modify the routing table.  For those routing table
            entries modified, the associated area will be set to Area A,
            the path type will be set to intra-area, and the cost will
            be set to the newly discovered shortest path's calculated
            distance.




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            If the newly added vertex is an area border router or AS
            boundary router, a routing table entry is added whose
            destination type is "router".  The Options field found in
            the associated router-LSA is copied into the routing table
            entry's Optional capabilities field. Call the newly added
            vertex Router X.  If Router X is the endpoint of one of the
            calculating router's virtual links, and the virtual link
            uses Area A as Transit area:  the virtual link is declared
            up, the IP address of the virtual interface is set to the IP
            address of the outgoing interface calculated above for
            Router X, and the virtual neighbor's IP address is set to
            Router X's interface address (contained in Router X's
            router-LSA) that points back to the root of the shortest-
            path tree; equivalently, this is the interface that points
            back to Router X's parent vertex on the shortest-path tree
            (similar to the calculation in Section 16.1.1).

            If the newly added vertex is a transit network, the routing
            table entry for the network is located.  The entry's
            Destination ID is the IP network number, which can be
            obtained by masking the Vertex ID (Link State ID) with its
            associated subnet mask (found in the body of the associated
            network-LSA).  If the routing table entry already exists
            (i.e., there is already an intra-area route to the
            destination installed in the routing table), multiple
            vertices have mapped to the same IP network.  For example,
            this can occur when a new Designated Router is being
            established.  In this case, the current routing table entry
            should be overwritten if and only if the newly found path is
            just as short and the current routing table entry's Link
            State Origin has a smaller Link State ID than the newly
            added vertex' LSA.

            If there is no routing table entry for the network (the
            usual case), a routing table entry for the IP network should
            be added.  The routing table entry's Link State Origin
            should be set to the newly added vertex' LSA.

        (5) Iterate the algorithm by returning to Step 2.






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        The stub networks are added to the tree in the procedure's
        second stage.  In this stage, all router vertices are again
        examined.  Those that have been determined to be unreachable in
        the above first phase are discarded.  For each reachable router
        vertex (call it V), the associated router-LSA is found in the
        link state database.  Each stub network link appearing in the
        LSA is then examined, and the following steps are executed:

        (1) Calculate the distance D of stub network from the root.  D
            is equal to the distance from the root to the router vertex
            (calculated in stage 1), plus the stub network link's
            advertised cost.  Compare this distance to the current best
            cost to the stub network.  This is done by looking up the
            stub network's current routing table entry.  If the
            calculated distance D is larger, go on to examine the next
            stub network link in the LSA.

        (2) If this step is reached, the stub network's routing table
            entry must be updated.  Calculate the set of next hops that
            would result from using the stub network link.  This
            calculation is shown in Section 16.1.1; input to this
            calculation is the destination (the stub network) and the
            parent vertex (the router vertex).  If the distance D is the
            same as the current routing table cost, simply add this set
            of next hops to the routing table entry's list of next hops.
            In this case, the routing table already has a Link State
            Origin.  If this Link State Origin is a router-LSA whose
            Link State ID is smaller than V's Router ID, reset the Link
            State Origin to V's router-LSA.

            Otherwise D is smaller than the routing table cost.
            Overwrite the current routing table entry by setting the
            routing table entry's cost to D, and by setting the entry's
            list of next hops to the newly calculated set.  Set the
            routing table entry's Link State Origin to V's router-LSA.
            Then go on to examine the next stub network link.


        For all routing table entries added/modified in the second
        stage, the associated area will be set to Area A and the path
        type will be set to intra-area.  When the list of reachable
        router-LSAs is exhausted, the second stage is completed.  At



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        this time, all intra-area routes associated with Area A have
        been determined.

        The specification does not require that the above two stage
        method be used to calculate the shortest path tree.  However, if
        another algorithm is used, an identical tree must be produced.
        For this reason, it is important to note that links between
        transit vertices must be bidirectional in order to be included
        in the above tree.  It should also be mentioned that more
        efficient algorithms exist for calculating the tree; for
        example, the incremental SPF algorithm described in [Ref1].


        16.1.1.  The next hop calculation

            This section explains how to calculate the current set of
            next hops to use for a destination.  Each next hop consists
            of the outgoing interface to use in forwarding packets to
            the destination together with the IP address of the next hop
            router (if any).  The next hop calculation is invoked each
            time a shorter path to the destination is discovered.  This
            can happen in either stage of the shortest-path tree
            calculation (see Section 16.1).  In stage 1 of the
            shortest-path tree calculation a shorter path is found as
            the destination is added to the candidate list, or when the
            destination's entry on the candidate list is modified (Step
            2d of Stage 1).  In stage 2 a shorter path is discovered
            each time the destination's routing table entry is modified
            (Step 2 of Stage 2).

            The set of next hops to use for the destination may be
            recalculated several times during the shortest-path tree
            calculation, as shorter and shorter paths are discovered.
            In the end, the destination's routing table entry will
            always reflect the next hops resulting from the absolute
            shortest path(s).

            Input to the next hop calculation is a) the destination and
            b) its parent in the current shortest path between the root
            (the calculating router) and the destination.  The parent is
            always a transit vertex (i.e., always a router or a transit
            network).



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            If there is at least one intervening router in the current
            shortest path between the destination and the root, the
            destination simply inherits the set of next hops from the
            parent.  Otherwise, there are two cases.  In the first case,
            the parent vertex is the root (the calculating router
            itself).  This means that the destination is either a
            directly connected network or directly connected router.
            The outgoing interface in this case is simply the OSPF
            interface connecting to the destination network/router. If
            the destination is a router which connects to the
            calculating router via a Point-to-MultiPoint network, the
            destination's next hop IP address(es) can be determined by
            examining the destination's router-LSA: each link pointing
            back to the calculating router and having a Link Data field
            belonging to the Point-to-MultiPoint network provides an IP
            address of the next hop router. If the destination is a
            directly connected network, or a router which connects to
            the calculating router via a point-to-point interface, no
            next hop IP address is required. If the destination is a
            router connected to the calculating router via a virtual
            link, the setting of the next hop should be deferred until
            the calculation in Section 16.3.

            In the second case, the parent vertex is a network that
            directly connects the calculating router to the destination
            router.  The list of next hops is then determined by
            examining the destination's router-LSA.  For each link in
            the router-LSA that points back to the parent network, the
            link's Link Data field provides the IP address of a next hop
            router.  The outgoing interface to use can then be derived
            from the next hop IP address (or it can be inherited from
            the parent network).


    16.2.  Calculating the inter-area routes

        The inter-area routes are calculated by examining summary-LSAs.
        If the router has active attachments to multiple areas, only
        backbone summary-LSAs are examined.  Routers attached to a
        single area examine that area's summary-LSAs.  In either case,
        the summary-LSAs examined below are all part of a single area's
        link state database (call it Area A).



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        Summary-LSAs are originated by the area border routers.  Each
        summary-LSA in Area A is considered in turn.  Remember that the
        destination described by a summary-LSA is either a network (Type
        3 summary-LSAs) or an AS boundary router (Type 4 summary-LSAs).
        For each summary-LSA:


        (1) If the cost specified by the LSA is LSInfinity, or if the
            LSA's LS age is equal to MaxAge, then examine the the next
            LSA.

        (2) If the LSA was originated by the calculating router itself,
            examine the next LSA.

        (3) If it is a Type 3 summary-LSA, and the collection of
            destinations described by the summary-LSA equals one of the
            router's configured area address ranges (see Section 3.5),
            and the particular area address range is active, then the
            summary-LSA should be ignored.  "Active" means that there
            are one or more reachable (by intra-area paths) networks
            contained in the area range.

        (4) Else, call the destination described by the LSA N (for Type
            3 summary-LSAs, N's address is obtained by masking the LSA's
            Link State ID with the network/subnet mask contained in the
            body of the LSA), and the area border originating the LSA
            BR.  Look up the routing table entry for BR having Area A as
            its associated area.  If no such entry exists for router BR
            (i.e., BR is unreachable in Area A), do nothing with this
            LSA and consider the next in the list.  Else, this LSA
            describes an inter-area path to destination N, whose cost is
            the distance to BR plus the cost specified in the LSA. Call
            the cost of this inter-area path IAC.

        (5) Next, look up the routing table entry for the destination N.
            (If N is an AS boundary router, look up the "router" routing
            table entry associated with Area A).  If no entry exists for
            N or if the entry's path type is "type 1 external" or "type
            2 external", then install the inter-area path to N, with
            associated area Area A, cost IAC, next hop equal to the list
            of next hops to router BR, and Advertising router equal to
            BR.



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        (6) Else, if the paths present in the table are intra-area
            paths, do nothing with the LSA (intra-area paths are always
            preferred).

        (7) Else, the paths present in the routing table are also
            inter-area paths.  Install the new path through BR if it is
            cheaper, overriding the paths in the routing table.
            Otherwise, if the new path is the same cost, add it to the
            list of paths that appear in the routing table entry.

    16.3.  Examining transit areas' summary-LSAs

        This step is only performed by area border routers attached to
        one or more non-backbone areas that are capable of carrying
        transit traffic (i.e., "transit areas", or those areas whose
        TransitCapability parameter has been set to TRUE in Step 2 of
        the Dijkstra algorithm (see Section 16.1).

        The purpose of the calculation below is to examine the transit
        areas to see whether they provide any better (shorter) paths
        than the paths previously calculated in Sections 16.1 and 16.2.
        Any paths found that are better than or equal to previously
        discovered paths are installed in the routing table.

        The calculation also determines the actual next hop(s) for those
        destinations whose next hop was calculated as a virtual link in
        Sections 16.1 and 16.2.  After completion of the calculation
        below, any paths calculated in Sections 16.1 and 16.2 that still
        have unresolved virtual next hops should be discarded.

        The calculation proceeds as follows. All the transit areas'
        summary-LSAs are examined in turn.  Each such summary-LSA
        describes a route through a transit area Area A to a Network N
        (N's address is obtained by masking the LSA's Link State ID with
        the network/subnet mask contained in the body of the LSA) or in
        the case of a Type 4 summary-LSA, to an AS boundary router N.
        Suppose also that the summary-LSA was originated by an area
        border router BR.

        (1) If the cost advertised by the summary-LSA is LSInfinity, or
            if the LSA's LS age is equal to MaxAge, then examine the
            next LSA.



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        (2) If the summary-LSA was originated by the calculating router
            itself, examine the next LSA.

        (3) Look up the routing table entry for N. (If N is an AS
            boundary router, look up the "router" routing table entry
            associated with the backbone area). If it does not exist, or
            if the route type is other than intra-area or inter-area, or
            if the area associated with the routing table entry is not
            the backbone area, then examine the next LSA. In other
            words, this calculation only updates backbone intra-area
            routes found in Section 16.1 and inter-area routes found in
            Section 16.2.

        (4) Look up the routing table entry for the advertising router
            BR associated with the Area A. If it is unreachable, examine
            the next LSA. Otherwise, the cost to destination N is the
            sum of the cost in BR's Area A routing table entry and the
            cost advertised in the LSA. Call this cost IAC.

        (5) If this cost is less than the cost occurring in N's routing
            table entry, overwrite N's list of next hops with those used
            for BR, and set N's routing table cost to IAC. Else, if IAC
            is the same as N's current cost, add BR's list of next hops
            to N's list of next hops. In any case, the area associated
            with N's routing table entry must remain the backbone area,
            and the path type (either intra-area or inter-area) must
            also remain the same.

        It is important to note that the above calculation never makes
        unreachable destinations reachable, but instead just potentially
        finds better paths to already reachable destinations.  The
        calculation installs any better cost found into the routing
        table entry, from which it may be readvertised in summary-LSAs
        to other areas.

        As an example of the calculation, consider the Autonomous System
        pictured in Figure 17.  There is a single non-backbone area
        (Area 1) that physically divides the backbone into two separate
        pieces. To maintain connectivity of the backbone, a virtual link
        has been configured between routers RT1 and RT4. On the right
        side of the figure, Network N1 belongs to the backbone. The
        dotted lines indicate that there is a much shorter intra-area



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                      ........................
                      . Area 1 (transit)     .            +
                      .                      .            |
                      .      +---+1        1+---+100      |
                      .      |RT2|----------|RT4|=========|
                      .    1/+---+********* +---+         |
                      .    /*******          .            |
                      .  1/*Virtual          .            |
                   1+---+/*  Link            .         Net|work
             =======|RT1|*                   .            | N1
                    +---+\                   .            |
                      .   \                  .            |
                      .    \                 .            |
                      .    1\+---+1        1+---+20       |
                      .      |RT3|----------|RT5|=========|
                      .      +---+          +---+         |
                      .                      .            |
                      ........................            +

                    Figure 17: Routing through transit areas

        backbone path between router RT5 and Network N1 (cost 20) than
        there is between Router RT4 and Network N1 (cost 100). Both
        Router RT4 and Router RT5 will inject summary-LSAs for Network
        N1 into Area 1.

        After the shortest-path tree has been calculated for the
        backbone in Section 16.1, Router RT1 (left end of the virtual
        link) will have calculated a path through Router RT4 for all
        data traffic destined for Network N1. However, since Router RT5
        is so much closer to Network N1, all routers internal to Area 1
        (e.g., Routers RT2 and RT3) will forward their Network N1
        traffic towards Router RT5, instead of RT4. And indeed, after
        examining Area 1's summary-LSAs by the above calculation, Router
        RT1 will also forward Network N1 traffic towards RT5. Note that
        in this example the virtual link enables transit data traffic to
        be forwarded through Area 1, but the actual path the transit
        data traffic takes does not follow the virtual link.  In other
        words, virtual links allow transit traffic to be forwarded
        through an area, but do not dictate the precise path that the
        traffic will take.



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    16.4.  Calculating AS external routes

        AS external routes are calculated by examining AS-external-LSAs.
        Each of the AS-external-LSAs is considered in turn.  Most AS-
        external-LSAs describe routes to specific IP destinations.  An
        AS-external-LSA can also describe a default route for the
        Autonomous System (Destination ID = DefaultDestination,
        network/subnet mask = 0x00000000).  For each AS-external-LSA:


        (1) If the cost specified by the LSA is LSInfinity, or if the
            LSA's LS age is equal to MaxAge, then examine the next LSA.

        (2) If the LSA was originated by the calculating router itself,
            examine the next LSA.

        (3) Call the destination described by the LSA N.  N's address is
            obtained by masking the LSA's Link State ID with the
            network/subnet mask contained in the body of the LSA.  Look
            up the routing table entries (potentially one per attached
            area) for the AS boundary router (ASBR) that originated the
            LSA. If no entries exist for router ASBR (i.e., ASBR is
            unreachable), do nothing with this LSA and consider the next
            in the list.

            Else, this LSA describes an AS external path to destination
            N.  Examine the forwarding address specified in the AS-
            external-LSA.  This indicates the IP address to which
            packets for the destination should be forwarded.

            If the forwarding address is set to 0.0.0.0, packets should
            be sent to the ASBR itself. Among the multiple routing table
            entries for the ASBR, select the preferred entry as follows.
            If RFC1583Compatibility is set to "disabled", prune the set
            of routing table entries for the ASBR as described in
            Section 16.4.1. In any case, among the remaining routing
            table entries, select the routing table entry with the least
            cost; when there are multiple least cost routing table
            entries the entry whose associated area has the largest OSPF
            Area ID (when considered as an unsigned 32-bit integer) is
            chosen.




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            If the forwarding address is non-zero, look up the
            forwarding address in the routing table.[24] The matching
            routing table entry must specify an intra-area or inter-area
            path; if no such path exists, do nothing with the LSA and
            consider the next in the list.

        (4) Let X be the cost specified by the preferred routing table
            entry for the ASBR/forwarding address, and Y the cost
            specified in the LSA.  X is in terms of the link state
            metric, and Y is a type 1 or 2 external metric.

        (5) Look up the routing table entry for the destination N.  If
            no entry exists for N, install the AS external path to N,
            with next hop equal to the list of next hops to the
            forwarding address, and advertising router equal to ASBR.
            If the external metric type is 1, then the path-type is set
            to type 1 external and the cost is equal to X+Y.  If the
            external metric type is 2, the path-type is set to type 2
            external, the link state component of the route's cost is X,
            and the type 2 cost is Y.

        (6) Compare the AS external path described by the LSA with the
            existing paths in N's routing table entry, as follows. If
            the new path is preferred, it replaces the present paths in
            N's routing table entry.  If the new path is of equal
            preference, it is added to N's routing table entry's list of
            paths.

            (a) Intra-area and inter-area paths are always preferred
                over AS external paths.

            (b) Type 1 external paths are always preferred over type 2
                external paths. When all paths are type 2 external
                paths, the paths with the smallest advertised type 2
                metric are always preferred.

            (c) If the new AS external path is still indistinguishable
                from the current paths in the N's routing table entry,
                and RFC1583Compatibility is set to "disabled", select
                the preferred paths based on the intra-AS paths to the
                ASBR/forwarding addresses, as specified in Section
                16.4.1.



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            (d) If the new AS external path is still indistinguishable
                from the current paths in the N's routing table entry,
                select the preferred path based on a least cost
                comparison.  Type 1 external paths are compared by
                looking at the sum of the distance to the forwarding
                address and the advertised type 1 metric (X+Y).  Type 2
                external paths advertising equal type 2 metrics are
                compared by looking at the distance to the forwarding
                addresses.

        16.4.1.  External path preferences

            When multiple intra-AS paths are available to
            ASBRs/forwarding addresses, the following rules indicate
            which paths are preferred. These rules apply when the same
            ASBR is reachable through multiple areas, or when trying to
            decide which of several AS-external-LSAs should be
            preferred. In the former case the paths all terminate at the
            same ASBR, while in the latter the paths terminate at
            separate ASBRs/forwarding addresses. In either case, each
            path is represented by a separate routing table entry as
            defined in Section 11.

            This section only applies when RFC1583Compatibility is set
            to "disabled".

            The path preference rules, stated from highest to lowest
            preference, are as follows. Note that as a result of these
            rules, there may still be multiple paths of the highest
            preference. In this case, the path to use must be determined
            based on cost, as described in Section 16.4.

            o   Intra-area paths using non-backbone areas are always the
                most preferred.

            o   The other paths, intra-area backbone paths and inter-
                area paths, are of equal preference.

    16.5.  Incremental updates -- summary-LSAs

        When a new summary-LSA is received, it is not necessary to
        recalculate the entire routing table.  Call the destination



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        described by the summary-LSA N (N's address is obtained by
        masking the LSA's Link State ID with the network/subnet mask
        contained in the body of the LSA), and let Area A be the area to
        which the LSA belongs. There are then two separate cases:

        Case 1: Area A is the backbone and/or the router is not an area
            border router.
            In this case, the following calculations must be performed.
            First, if there is presently an inter-area route to the
            destination N, N's routing table entry is invalidated,
            saving the entry's values for later comparisons. Then the
            calculation in Section 16.2 is run again for the single
            destination N. In this calculation, all of Area A's
            summary-LSAs that describe a route to N are examined.  In
            addition, if the router is an area border router attached to
            one or more transit areas, the calculation in Section 16.3
            must be run again for the single destination.  If the
            results of these calculations have changed the cost/path to
            an AS boundary router (as would be the case for a Type 4
            summary-LSA) or to any forwarding addresses, all AS-
            external-LSAs will have to be reexamined by rerunning the
            calculation in Section 16.4.  Otherwise, if N is now newly
            unreachable, the calculation in Section 16.4 must be rerun
            for the single destination N, in case an alternate external
            route to N exists.

        Case 2: Area A is a transit area and the router is an area
            border router.
            In this case, the following calculations must be performed.
            First, if N's routing table entry presently contains one or
            more inter-area paths that utilize the transit area Area A,
            these paths should be removed. If this removes all paths
            from the routing table entry, the entry should be
            invalidated.  The entry's old values should be saved for
            later comparisons. Next the calculation in Section 16.3 must
            be run again for the single destination N. If the results of
            this calculation have caused the cost to N to increase, the
            complete routing table calculation must be rerun starting
            with the Dijkstra algorithm specified in Section 16.1.
            Otherwise, if the cost/path to an AS boundary router (as
            would be the case for a Type 4 summary-LSA) or to any
            forwarding addresses has changed, all AS-external-LSAs will



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            have to be reexamined by rerunning the calculation in
            Section 16.4.  Otherwise, if N is now newly unreachable, the
            calculation in Section 16.4 must be rerun for the single
            destination N, in case an alternate external route to N
            exists.

    16.6.  Incremental updates -- AS-external-LSAs

        When a new AS-external-LSA is received, it is not necessary to
        recalculate the entire routing table.  Call the destination
        described by the AS-external-LSA N.  N's address is obtained by
        masking the LSA's Link State ID with the network/subnet mask
        contained in the body of the LSA. If there is already an intra-
        area or inter-area route to the destination, no recalculation is
        necessary (internal routes take precedence).

        Otherwise, the procedure in Section 16.4 will have to be
        performed, but only for those AS-external-LSAs whose destination
        is N.  Before this procedure is performed, the present routing
        table entry for N should be invalidated.

    16.7.  Events generated as a result of routing table changes

        Changes to routing table entries sometimes cause the OSPF area
        border routers to take additional actions.  These routers need
        to act on the following routing table changes:

        o   The cost or path type of a routing table entry has changed.
            If the destination described by this entry is a Network or
            AS boundary router, and this is not simply a change of AS
            external routes, new summary-LSAs may have to be generated
            (potentially one for each attached area, including the
            backbone).  See Section 12.4.3 for more information.  If a
            previously advertised entry has been deleted, or is no
            longer advertisable to a particular area, the LSA must be
            flushed from the routing domain by setting its LS age to
            MaxAge and reflooding (see Section 14.1).

        o   A routing table entry associated with a configured virtual
            link has changed.  The destination of such a routing table
            entry is an area border router.  The change indicates a
            modification to the virtual link's cost or viability.



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            If the entry indicates that the area border router is newly
            reachable, the corresponding virtual link is now
            operational.  An InterfaceUp event should be generated for
            the virtual link, which will cause a virtual adjacency to
            begin to form (see Section 10.3).  At this time the virtual
            link's IP interface address and the virtual neighbor's
            Neighbor IP address are also calculated.

            If the entry indicates that the area border router is no
            longer reachable, the virtual link and its associated
            adjacency should be destroyed.  This means an InterfaceDown
            event should be generated for the associated virtual link.

            If the cost of the entry has changed, and there is a fully
            established virtual adjacency, a new router-LSA for the
            backbone must be originated.  This in turn may cause further
            routing table changes.

    16.8.  Equal-cost multipath

        The OSPF protocol maintains multiple equal-cost routes to all
        destinations.  This can be seen in the steps used above to
        calculate the routing table, and in the definition of the
        routing table structure.

        Each one of the multiple routes will be of the same type
        (intra-area, inter-area, type 1 external or type 2 external),
        cost, and will have the same associated area.  However, each
        route may specify a separate next hop and Advertising router.

        There is no requirement that a router running OSPF keep track of
        all possible equal-cost routes to a destination.  An
        implementation may choose to keep only a fixed number of routes
        to any given destination.  This does not affect any of the
        algorithms presented in this specification.










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Footnotes


    [1]The graph's vertices represent either routers, transit networks,
    or stub networks.  Since routers may belong to multiple areas, it is
    not possible to color the graph's vertices.

    [2]It is possible for all of a router's interfaces to be unnumbered
    point-to-point links.  In this case, an IP address must be assigned
    to the router.  This address will then be advertised in the router's
    router-LSA as a host route.

    [3]Note that in these cases both interfaces, the non-virtual and the
    virtual, would have the same IP address.

    [4]Note that no host route is generated for, and no IP packets can
    be addressed to, interfaces to unnumbered point-to-point networks.
    This is regardless of such an interface's state.

    [5]It is instructive to see what happens when the Designated Router
    for the network crashes.  Call the Designated Router for the network
    RT1, and the Backup Designated Router RT2.  If Router RT1 crashes
    (or maybe its interface to the network dies), the other routers on
    the network will detect RT1's absence within RouterDeadInterval
    seconds.  All routers may not detect this at precisely the same
    time; the routers that detect RT1's absence before RT2 does will,
    for a time, select RT2 to be both Designated Router and Backup
    Designated Router.  When RT2 detects that RT1 is gone it will move
    itself to Designated Router.  At this time, the remaining router
    having highest Router Priority will be selected as Backup Designated
    Router.

    [6]On point-to-point networks, the lower level protocols indicate
    whether the neighbor is up and running.  Likewise, existence of the
    neighbor on virtual links is indicated by the routing table
    calculation.  However, in both these cases, the Hello Protocol is
    still used.  This ensures that communication between the neighbors
    is bidirectional, and that each of the neighbors has a functioning
    routing protocol layer.

    [7]When the identity of the Designated Router is changing, it may be
    quite common for a neighbor in this state to send the router a



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    Database Description packet; this means that there is some momentary
    disagreement on the Designated Router's identity.

    [8]Note that it is possible for a router to resynchronize any of its
    fully established adjacencies by setting the adjacency's state back
    to ExStart.  This will cause the other end of the adjacency to
    process a SeqNumberMismatch event, and therefore to also go back to
    ExStart state.

    [9]The address space of IP networks and the address space of OSPF
    Router IDs may overlap.  That is, a network may have an IP address
    which is identical (when considered as a 32-bit number) to some
    router's Router ID.

    [10]"Discard" entries are necessary to ensure that route
    summarization at area boundaries will not cause packet looping.

    [11]It is assumed that, for two different address ranges matching
    the destination, one range is more specific than the other. Non-
    contiguous subnet masks can be configured to violate this
    assumption. Such subnet mask configurations cannot be handled by the
    OSPF protocol.

    [12]MaxAgeDiff is an architectural constant.  It indicates the
    maximum dispersion of ages, in seconds, that can occur for a single
    LSA instance as it is flooded throughout the routing domain.  If two
    LSAs differ by more than this, they are assumed to be different
    instances of the same LSA.  This can occur when a router restarts
    and loses track of the LSA's previous LS sequence number.  See
    Section 13.4 for more details.

    [13]When two LSAs have different LS checksums, they are assumed to
    be separate instances.  This can occur when a router restarts, and
    loses track of the LSA's previous LS sequence number.  In the case
    where the two LSAs have the same LS sequence number, it is not
    possible to determine which LSA is actually newer.  However, if the
    wrong LSA is accepted as newer, the originating router will simply
    originate another instance.  See Section 13.4 for further details.

    [14]There is one instance where a lookup must be done based on
    partial information.  This is during the routing table calculation,
    when a network-LSA must be found based solely on its Link State ID.



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    The lookup in this case is still well defined, since no two
    network-LSAs can have the same Link State ID.

    [15]This is the way RFC 1583 specified point-to-point
    representation.  It has three advantages: a) it does not require
    allocating a subnet to the point-to-point link, b) it tends to bias
    the routing so that packets destined for the point-to-point
    interface will actually be received over the interface (which is
    useful for diagnostic purposes) and c) it allows network
    bootstrapping of a neighbor, without requiring that the bootstrap
    program contain an OSPF implementation.

    [16]This is the more traditional point-to-point representation used
    by protocols such as RIP.

    [17]This clause covers the case: Inter-area routes are not
    summarized to the backbone.  This is because inter-area routes are
    always associated with the backbone area.

    [18]This clause is only invoked when a non-backbone Area A supports
    transit data traffic (i.e., has TransitCapability set to TRUE).  For
    example, in the area configuration of Figure 6, Area 2 can support
    transit traffic due to the configured virtual link between Routers
    RT10 and RT11. As a result, Router RT11 need only originate a single
    summary-LSA into Area 2 (having the collapsed destination N9-
    N11,H1), since all of Router RT11's other eligible routes have next
    hops belonging to Area 2 itself (and as such only need be advertised
    by other area border routers; in this case, Routers RT10 and RT7).

    [19]By keeping more information in the routing table, it is possible
    for an implementation to recalculate the shortest path tree for only
    a single area.  In fact, there are incremental algorithms that allow
    an implementation to recalculate only a portion of a single area's
    shortest path tree [Ref1].  However, these algorithms are beyond the
    scope of this specification.

    [20]This is how the Link state request list is emptied, which
    eventually causes the neighbor state to transition to Full.  See
    Section 10.9 for more details.

    [21]It should be a relatively rare occurrence for an LSA's LS age to
    reach MaxAge in this fashion.  Usually, the LSA will be replaced by



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    a more recent instance before it ages out.

    [22]Strictly speaking, because of equal-cost multipath, the
    algorithm does not create a tree.  We continue to use the "tree"
    terminology because that is what occurs most often in the existing
    literature.

    [23]Note that the presence of any link back to V is sufficient; it
    need not be the matching half of the link under consideration from V
    to W. This is enough to ensure that, before data traffic flows
    between a pair of neighboring routers, their link state databases
    will be synchronized.

    [24]When the forwarding address is non-zero, it should point to a
    router belonging to another Autonomous System.  See Section 12.4.4
    for more details.





























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References

    [Ref1]  McQuillan, J., I. Richer and E. Rosen, "ARPANET Routing
            Algorithm Improvements", BBN Technical Report 3803, April
            1978.

    [Ref2]  Digital Equipment Corporation, "Information processing
            systems -- Data communications -- Intermediate System to
            Intermediate System Intra-Domain Routing Protocol", October
            1987.

    [Ref3]  McQuillan, J., et.al., "The New Routing Algorithm for the
            ARPANET", IEEE Transactions on Communications, May 1980.

    [Ref4]  Perlman, R., "Fault-Tolerant Broadcast of Routing
            Information", Computer Networks, December 1983.

    [Ref5]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.

    [Ref6]  McKenzie, A., "ISO Transport Protocol specification ISO DP
            8073", RFC 905, April 1984.

    [Ref7]  Deering, S., "Host extensions for IP multicasting", STD 5,
            RFC 1112, May 1988.

    [Ref8]  McCloghrie, K., and M. Rose, "Management Information Base
            for network management of TCP/IP-based internets: MIB-II",
            STD 17, RFC 1213, March 1991.

    [Ref9]  Moy, J., "OSPF Version 2", RFC 1583, March 1994.

    [Ref10] Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless
            Inter-Domain Routing (CIDR): an Address Assignment and
            Aggregation Strategy", RFC1519, September 1993.

    [Ref11] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
            1700, October 1994.

    [Ref12] Almquist, P., "Type of Service in the Internet Protocol
            Suite", RFC 1349, July 1992.




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    [Ref13] Leiner, B., et.al., "The DARPA Internet Protocol Suite", DDN
            Protocol Handbook, April 1985.

    [Ref14] Bradley, T., and C. Brown, "Inverse Address Resolution
            Protocol", RFC 1293, January 1992.

    [Ref15] deSouza, O., and M. Rodrigues, "Guidelines for Running OSPF
            Over Frame Relay Networks", RFC 1586, March 1994.

    [Ref16] Bellovin, S., "Security Problems in the TCP/IP Protocol
            Suite", ACM Computer Communications Review, Volume 19,
            Number 2, pp. 32-38, April 1989.

    [Ref17] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
            April 1992.

    [Ref18] Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
            1994.

    [Ref19] Coltun, R., and V. Fuller, "The OSPF NSSA Option", RFC 1587,
            March 1994.

    [Ref20] Ferguson, D., "The OSPF External Attributes LSA", work in
            progress.

    [Ref21] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
            1793, April 1995.

    [Ref22] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
            November 1990.

    [Ref23] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-
            4)", RFC 1771, March 1995.

    [Ref24] Hinden, R., "Internet Routing Protocol Standardization
            Criteria", BBN, October 1991.

    [Ref25] Moy, J., "OSPF Version 2", RFC 2178, July 1997.

    [Ref26] Rosen, E., "Vulnerabilities of Network Control Protocols: An
            Example", Computer Communication Review, July 1981.




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A. OSPF data formats

    This appendix describes the format of OSPF protocol packets and OSPF
    LSAs.  The OSPF protocol runs directly over the IP network layer.
    Before any data formats are described, the details of the OSPF
    encapsulation are explained.

    Next the OSPF Options field is described.  This field describes
    various capabilities that may or may not be supported by pieces of
    the OSPF routing domain. The OSPF Options field is contained in OSPF
    Hello packets, Database Description packets and in OSPF LSAs.

    OSPF packet formats are detailed in Section A.3.  A description of
    OSPF LSAs appears in Section A.4.

A.1 Encapsulation of OSPF packets

    OSPF runs directly over the Internet Protocol's network layer.  OSPF
    packets are therefore encapsulated solely by IP and local data-link
    headers.

    OSPF does not define a way to fragment its protocol packets, and
    depends on IP fragmentation when transmitting packets larger than
    the network MTU. If necessary, the length of OSPF packets can be up
    to 65,535 bytes (including the IP header).  The OSPF packet types
    that are likely to be large (Database Description Packets, Link
    State Request, Link State Update, and Link State Acknowledgment
    packets) can usually be split into several separate protocol
    packets, without loss of functionality.  This is recommended; IP
    fragmentation should be avoided whenever possible.  Using this
    reasoning, an attempt should be made to limit the sizes of OSPF
    packets sent over virtual links to 576 bytes unless Path MTU
    Discovery is being performed (see [Ref22]).

    The other important features of OSPF's IP encapsulation are:

    o   Use of IP multicast.  Some OSPF messages are multicast, when
        sent over broadcast networks.  Two distinct IP multicast
        addresses are used.  Packets sent to these multicast addresses
        should never be forwarded; they are meant to travel a single hop
        only.  To ensure that these packets will not travel multiple
        hops, their IP TTL must be set to 1.



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        AllSPFRouters
            This multicast address has been assigned the value
            224.0.0.5.  All routers running OSPF should be prepared to
            receive packets sent to this address.  Hello packets are
            always sent to this destination.  Also, certain OSPF
            protocol packets are sent to this address during the
            flooding procedure.

        AllDRouters
            This multicast address has been assigned the value
            224.0.0.6.  Both the Designated Router and Backup Designated
            Router must be prepared to receive packets destined to this
            address.  Certain OSPF protocol packets are sent to this
            address during the flooding procedure.

    o   OSPF is IP protocol number 89.  This number has been registered
        with the Network Information Center.  IP protocol number
        assignments are documented in [Ref11].

    o   All OSPF routing protocol packets are sent using the normal
        service TOS value of binary 0000 defined in [Ref12].

    o   Routing protocol packets are sent with IP precedence set to
        Internetwork Control.  OSPF protocol packets should be given
        precedence over regular IP data traffic, in both sending and
        receiving.  Setting the IP precedence field in the IP header to
        Internetwork Control [Ref5] may help implement this objective.


















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A.2 The Options field

    The OSPF Options field is present in OSPF Hello packets, Database
    Description packets and all LSAs.  The Options field enables OSPF
    routers to support (or not support) optional capabilities, and to
    communicate their capability level to other OSPF routers.  Through
    this mechanism routers of differing capabilities can be mixed within
    an OSPF routing domain.

    When used in Hello packets, the Options field allows a router to
    reject a neighbor because of a capability mismatch.  Alternatively,
    when capabilities are exchanged in Database Description packets a
    router can choose not to forward certain LSAs to a neighbor because
    of its reduced functionality.  Lastly, listing capabilities in LSAs
    allows routers to forward traffic around reduced functionality
    routers, by excluding them from parts of the routing table
    calculation.

    Five bits of the OSPF Options field have been assigned, although
    only one (the E-bit) is described completely by this memo. Each bit
    is described briefly below. Routers should reset (i.e.  clear)
    unrecognized bits in the Options field when sending Hello packets or
    Database Description packets and when originating LSAs. Conversely,
    routers encountering unrecognized Option bits in received Hello
    Packets, Database Description packets or LSAs should ignore the
    capability and process the packet/LSA normally.

                       +------------------------------------+
                       | * | * | DC | EA | N/P | MC | E | * |
                       +------------------------------------+

                             The Options field


    E-bit
        This bit describes the way AS-external-LSAs are flooded, as
        described in Sections 3.6, 9.5, 10.8 and 12.1.2 of this memo.

    MC-bit
        This bit describes whether IP multicast datagrams are forwarded
        according to the specifications in [Ref18].




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    N/P-bit
        This bit describes the handling of Type-7 LSAs, as specified in
        [Ref19].

    EA-bit
        This bit describes the router's willingness to receive and
        forward External-Attributes-LSAs, as specified in [Ref20].

    DC-bit
        This bit describes the router's handling of demand circuits, as
        specified in [Ref21].


































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A.3 OSPF Packet Formats

    There are five distinct OSPF packet types.  All OSPF packet types
    begin with a standard 24 byte header.  This header is described
    first.  Each packet type is then described in a succeeding section.
    In these sections each packet's division into fields is displayed,
    and then the field definitions are enumerated.

    All OSPF packet types (other than the OSPF Hello packets) deal with
    lists of LSAs.  For example, Link State Update packets implement the
    flooding of LSAs throughout the OSPF routing domain.  Because of
    this, OSPF protocol packets cannot be parsed unless the format of
    LSAs is also understood.  The format of LSAs is described in Section
    A.4.

    The receive processing of OSPF packets is detailed in Section 8.2.
    The sending of OSPF packets is explained in Section 8.1.




























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A.3.1 The OSPF packet header

    Every OSPF packet starts with a standard 24 byte header.  This
    header contains all the information necessary to determine whether
    the packet should be accepted for further processing.  This
    determination is described in Section 8.2 of the specification.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |     Type      |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



    Version #
        The OSPF version number.  This specification documents version 2
        of the protocol.

    Type
        The OSPF packet types are as follows. See Sections A.3.2 through
        A.3.6 for details.












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                          Type   Description
                          ________________________________
                          1      Hello
                          2      Database Description
                          3      Link State Request
                          4      Link State Update
                          5      Link State Acknowledgment




    Packet length
        The length of the OSPF protocol packet in bytes.  This length
        includes the standard OSPF header.

    Router ID
        The Router ID of the packet's source.

    Area ID
        A 32 bit number identifying the area that this packet belongs
        to.  All OSPF packets are associated with a single area.  Most
        travel a single hop only.  Packets travelling over a virtual
        link are labelled with the backbone Area ID of 0.0.0.0.

    Checksum
        The standard IP checksum of the entire contents of the packet,
        starting with the OSPF packet header but excluding the 64-bit
        authentication field.  This checksum is calculated as the 16-bit
        one's complement of the one's complement sum of all the 16-bit
        words in the packet, excepting the authentication field.  If the
        packet's length is not an integral number of 16-bit words, the
        packet is padded with a byte of zero before checksumming.  The
        checksum is considered to be part of the packet authentication
        procedure; for some authentication types the checksum
        calculation is omitted.

    AuType
        Identifies the authentication procedure to be used for the
        packet.  Authentication is discussed in Appendix D of the
        specification.  Consult Appendix D for a list of the currently
        defined authentication types.



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    Authentication
        A 64-bit field for use by the authentication scheme. See
        Appendix D for details.










































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A.3.2 The Hello packet

    Hello packets are OSPF packet type 1.  These packets are sent
    periodically on all interfaces (including virtual links) in order to
    establish and maintain neighbor relationships.  In addition, Hello
    Packets are multicast on those physical networks having a multicast
    or broadcast capability, enabling dynamic discovery of neighboring
    routers.

    All routers connected to a common network must agree on certain
    parameters (Network mask, HelloInterval and RouterDeadInterval).
    These parameters are included in Hello packets, so that differences
    can inhibit the forming of neighbor relationships.  A detailed
    explanation of the receive processing for Hello packets is presented
    in Section 10.5.  The sending of Hello packets is covered in Section
    9.5.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       1       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Network Mask                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         HelloInterval         |    Options    |    Rtr Pri    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     RouterDeadInterval                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Designated Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Backup Designated Router                    |



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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Neighbor                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |


    Network mask
        The network mask associated with this interface.  For example,
        if the interface is to a class B network whose third byte is
        used for subnetting, the network mask is 0xffffff00.

    Options
        The optional capabilities supported by the router, as documented
        in Section A.2.

    HelloInterval
        The number of seconds between this router's Hello packets.

    Rtr Pri
        This router's Router Priority.  Used in (Backup) Designated
        Router election.  If set to 0, the router will be ineligible to
        become (Backup) Designated Router.

    RouterDeadInterval
        The number of seconds before declaring a silent router down.

    Designated Router
        The identity of the Designated Router for this network, in the
        view of the sending router.  The Designated Router is identified
        here by its IP interface address on the network.  Set to 0.0.0.0
        if there is no Designated Router.

    Backup Designated Router
        The identity of the Backup Designated Router for this network,
        in the view of the sending router.  The Backup Designated Router
        is identified here by its IP interface address on the network.
        Set to 0.0.0.0 if there is no Backup Designated Router.

    Neighbor
        The Router IDs of each router from whom valid Hello packets have
        been seen recently on the network.  Recently means in the last
        RouterDeadInterval seconds.



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A.3.3 The Database Description packet

    Database Description packets are OSPF packet type 2.  These packets
    are exchanged when an adjacency is being initialized.  They describe
    the contents of the link-state database.  Multiple packets may be
    used to describe the database.  For this purpose a poll-response
    procedure is used.  One of the routers is designated to be the
    master, the other the slave.  The master sends Database Description
    packets (polls) which are acknowledged by Database Description
    packets sent by the slave (responses).  The responses are linked to
    the polls via the packets' DD sequence numbers.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       2       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Interface MTU         |    Options    |0|0|0|0|0|I|M|MS
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     DD sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                                                             -+
       |                                                               |
       +-                      An LSA Header                          -+
       |                                                               |
       +-                                                             -+
       |                                                               |
       +-                                                             -+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |



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    The format of the Database Description packet is very similar to
    both the Link State Request and Link State Acknowledgment packets.
    The main part of all three is a list of items, each item describing
    a piece of the link-state database.  The sending of Database
    Description Packets is documented in Section 10.8.  The reception of
    Database Description packets is documented in Section 10.6.

    Interface MTU
        The size in bytes of the largest IP datagram that can be sent
        out the associated interface, without fragmentation.  The MTUs
        of common Internet link types can be found in Table 7-1 of
        [Ref22]. Interface MTU should be set to 0 in Database
        Description packets sent over virtual links.

    Options
        The optional capabilities supported by the router, as documented
        in Section A.2.

    I-bit
        The Init bit.  When set to 1, this packet is the first in the
        sequence of Database Description Packets.

    M-bit
        The More bit.  When set to 1, it indicates that more Database
        Description Packets are to follow.

    MS-bit
        The Master/Slave bit.  When set to 1, it indicates that the
        router is the master during the Database Exchange process.
        Otherwise, the router is the slave.

    DD sequence number
        Used to sequence the collection of Database Description Packets.
        The initial value (indicated by the Init bit being set) should
        be unique.  The DD sequence number then increments until the
        complete database description has been sent.

    The rest of the packet consists of a (possibly partial) list of the
    link-state database's pieces.  Each LSA in the database is described
    by its LSA header.  The LSA header is documented in Section A.4.1.
    It contains all the information required to uniquely identify both
    the LSA and the LSA's current instance.



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A.3.4 The Link State Request packet

    Link State Request packets are OSPF packet type 3.  After exchanging
    Database Description packets with a neighboring router, a router may
    find that parts of its link-state database are out-of-date.  The
    Link State Request packet is used to request the pieces of the
    neighbor's database that are more up-to-date.  Multiple Link State
    Request packets may need to be used.

    A router that sends a Link State Request packet has in mind the
    precise instance of the database pieces it is requesting. Each
    instance is defined by its LS sequence number, LS checksum, and LS
    age, although these fields are not specified in the Link State
    Request Packet itself.  The router may receive even more recent
    instances in response.

    The sending of Link State Request packets is documented in Section
    10.9.  The reception of Link State Request packets is documented in
    Section 10.7.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       3       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          LS type                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Link State ID                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |



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    Each LSA requested is specified by its LS type, Link State ID, and
    Advertising Router.  This uniquely identifies the LSA, but not its
    instance.  Link State Request packets are understood to be requests
    for the most recent instance (whatever that might be).









































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A.3.5 The Link State Update packet

    Link State Update packets are OSPF packet type 4.  These packets
    implement the flooding of LSAs.  Each Link State Update packet
    carries a collection of LSAs one hop further from their origin.
    Several LSAs may be included in a single packet.

    Link State Update packets are multicast on those physical networks
    that support multicast/broadcast.  In order to make the flooding
    procedure reliable, flooded LSAs are acknowledged in Link State
    Acknowledgment packets.  If retransmission of certain LSAs is
    necessary, the retransmitted LSAs are always sent directly to the
    neighbor.  For more information on the reliable flooding of LSAs,
    consult Section 13.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       4       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            # LSAs                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                                                            +-+
       |                             LSAs                              |
       +-                                                            +-+
       |                              ...                              |








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    # LSAs
        The number of LSAs included in this update.


    The body of the Link State Update packet consists of a list of LSAs.
    Each LSA begins with a common 20 byte header, described in Section
    A.4.1. Detailed formats of the different types of LSAs are described
    in Section A.4.





































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A.3.6 The Link State Acknowledgment packet

    Link State Acknowledgment Packets are OSPF packet type 5.  To make
    the flooding of LSAs reliable, flooded LSAs are explicitly
    acknowledged.  This acknowledgment is accomplished through the
    sending and receiving of Link State Acknowledgment packets.
    Multiple LSAs can be acknowledged in a single Link State
    Acknowledgment packet.

    Depending on the state of the sending interface and the sender of
    the corresponding Link State Update packet, a Link State
    Acknowledgment packet is sent either to the multicast address
    AllSPFRouters, to the multicast address AllDRouters, or as a
    unicast.  The sending of Link State Acknowledgement packets is
    documented in Section 13.5.  The reception of Link State
    Acknowledgement packets is documented in Section 13.7.

    The format of this packet is similar to that of the Data Description
    packet.  The body of both packets is simply a list of LSA headers.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       5       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                                                             -+
       |                                                               |
       +-                         An LSA Header                       -+
       |                                                               |
       +-                                                             -+



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       |                                                               |
       +-                                                             -+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |


    Each acknowledged LSA is described by its LSA header.  The LSA
    header is documented in Section A.4.1.  It contains all the
    information required to uniquely identify both the LSA and the LSA's
    current instance.


































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A.4 LSA formats

    This memo defines five distinct types of LSAs.  Each LSA begins with
    a standard 20 byte LSA header.  This header is explained in Section
    A.4.1.  Succeeding sections then diagram the separate LSA types.

    Each LSA describes a piece of the OSPF routing domain.  Every router
    originates a router-LSA.  In addition, whenever the router is
    elected Designated Router, it originates a network-LSA.  Other types
    of LSAs may also be originated (see Section 12.4).  All LSAs are
    then flooded throughout the OSPF routing domain.  The flooding
    algorithm is reliable, ensuring that all routers have the same
    collection of LSAs.  (See Section 13 for more information concerning
    the flooding algorithm).  This collection of LSAs is called the
    link-state database.

    From the link state database, each router constructs a shortest path
    tree with itself as root.  This yields a routing table (see Section
    11).  For the details of the routing table build process, see
    Section 16.

























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A.4.1 The LSA header

    All LSAs begin with a common 20 byte header.  This header contains
    enough information to uniquely identify the LSA (LS type, Link State
    ID, and Advertising Router).  Multiple instances of the LSA may
    exist in the routing domain at the same time.  It is then necessary
    to determine which instance is more recent.  This is accomplished by
    examining the LS age, LS sequence number and LS checksum fields that
    are also contained in the LSA header.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |    Options    |    LS type    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



    LS age
        The time in seconds since the LSA was originated.

    Options
        The optional capabilities supported by the described portion of
        the routing domain.  OSPF's optional capabilities are documented
        in Section A.2.

    LS type
        The type of the LSA.  Each LSA type has a separate advertisement
        format.  The LSA types defined in this memo are as follows (see
        Section 12.1.3 for further explanation):






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                        LS Type   Description
                        ___________________________________
                        1         Router-LSAs
                        2         Network-LSAs
                        3         Summary-LSAs (IP network)
                        4         Summary-LSAs (ASBR)
                        5         AS-external-LSAs




    Link State ID
        This field identifies the portion of the internet environment
        that is being described by the LSA.  The contents of this field
        depend on the LSA's LS type.  For example, in network-LSAs the
        Link State ID is set to the IP interface address of the
        network's Designated Router (from which the network's IP address
        can be derived).  The Link State ID is further discussed in
        Section 12.1.4.

    Advertising Router
        The Router ID of the router that originated the LSA.  For
        example, in network-LSAs this field is equal to the Router ID of
        the network's Designated Router.

    LS sequence number
        Detects old or duplicate LSAs.  Successive instances of an LSA
        are given successive LS sequence numbers.  See Section 12.1.6
        for more details.

    LS checksum
        The Fletcher checksum of the complete contents of the LSA,
        including the LSA header but excluding the LS age field. See
        Section 12.1.7 for more details.

    length
        The length in bytes of the LSA.  This includes the 20 byte LSA
        header.






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A.4.2 Router-LSAs

    Router-LSAs are the Type 1 LSAs.  Each router in an area originates
    a router-LSA.  The LSA describes the state and cost of the router's
    links (i.e., interfaces) to the area.  All of the router's links to
    the area must be described in a single router-LSA.  For details
    concerning the construction of router-LSAs, see Section 12.4.1.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |       1       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    0    |V|E|B|        0      |            # links            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Link ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Link Data                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     # TOS     |            metric             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      TOS      |        0      |          TOS  metric          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Link ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Link Data                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |






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    In router-LSAs, the Link State ID field is set to the router's OSPF
    Router ID. Router-LSAs are flooded throughout a single area only.

    bit V
        When set, the router is an endpoint of one or more fully
        adjacent virtual links having the described area as Transit area
        (V is for virtual link endpoint).

    bit E
        When set, the router is an AS boundary router (E is for
        external).

    bit B
        When set, the router is an area border router (B is for border).

    # links
        The number of router links described in this LSA.  This must be
        the total collection of router links (i.e., interfaces) to the
        area.


    The following fields are used to describe each router link (i.e.,
    interface). Each router link is typed (see the below Type field).
    The Type field indicates the kind of link being described.  It may
    be a link to a transit network, to another router or to a stub
    network.  The values of all the other fields describing a router
    link depend on the link's Type.  For example, each link has an
    associated 32-bit Link Data field.  For links to stub networks this
    field specifies the network's IP address mask.  For other link types
    the Link Data field specifies the router interface's IP address.


    Type
        A quick description of the router link.  One of the following.
        Note that host routes are classified as links to stub networks
        with network mask of 0xffffffff.









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                 Type   Description
                 __________________________________________________
                 1      Point-to-point connection to another router
                 2      Connection to a transit network
                 3      Connection to a stub network
                 4      Virtual link




    Link ID
        Identifies the object that this router link connects to.  Value
        depends on the link's Type.  When connecting to an object that
        also originates an LSA (i.e., another router or a transit
        network) the Link ID is equal to the neighboring LSA's Link
        State ID.  This provides the key for looking up the neighboring
        LSA in the link state database during the routing table
        calculation. See Section 12.2 for more details.



                       Type   Link ID
                       ______________________________________
                       1      Neighboring router's Router ID
                       2      IP address of Designated Router
                       3      IP network/subnet number
                       4      Neighboring router's Router ID




    Link Data
        Value again depends on the link's Type field. For connections to
        stub networks, Link Data specifies the network's IP address
        mask. For unnumbered point-to-point connections, it specifies
        the interface's MIB-II [Ref8] ifIndex value. For the other link
        types it specifies the router interface's IP address. This
        latter piece of information is needed during the routing table
        build process, when calculating the IP address of the next hop.
        See Section 16.1.1 for more details.




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    # TOS
        The number of different TOS metrics given for this link, not
        counting the required link metric (referred to as the TOS 0
        metric in [Ref9]).  For example, if no additional TOS metrics
        are given, this field is set to 0.

    metric
        The cost of using this router link.


    Additional TOS-specific information may also be included, for
    backward compatibility with previous versions of the OSPF
    specification ([Ref9]). Within each link, and for each desired TOS,
    TOS TOS-specific link information may be encoded as follows:

    TOS IP Type of Service that this metric refers to.  The encoding of
        TOS in OSPF LSAs is described in Section 12.3.

    TOS metric
        TOS-specific metric information.

























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A.4.3 Network-LSAs

    Network-LSAs are the Type 2 LSAs.  A network-LSA is originated for
    each broadcast and NBMA network in the area which supports two or
    more routers.  The network-LSA is originated by the network's
    Designated Router.  The LSA describes all routers attached to the
    network, including the Designated Router itself.  The LSA's Link
    State ID field lists the IP interface address of the Designated
    Router.

    The distance from the network to all attached routers is zero.  This
    is why metric fields need not be specified in the network-LSA.  For
    details concerning the construction of network-LSAs, see Section
    12.4.2.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |      Options  |      2        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Network Mask                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Attached Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |



    Network Mask
        The IP address mask for the network.  For example, a class A
        network would have the mask 0xff000000.





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    Attached Router
        The Router IDs of each of the routers attached to the network.
        Actually, only those routers that are fully adjacent to the
        Designated Router are listed.  The Designated Router includes
        itself in this list.  The number of routers included can be
        deduced from the LSA header's length field.







































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A.4.4 Summary-LSAs

    Summary-LSAs are the Type 3 and 4 LSAs.  These LSAs are originated
    by area border routers. Summary-LSAs describe inter-area
    destinations.  For details concerning the construction of summary-
    LSAs, see Section 12.4.3.

    Type 3 summary-LSAs are used when the destination is an IP network.
    In this case the LSA's Link State ID field is an IP network number
    (if necessary, the Link State ID can also have one or more of the
    network's "host" bits set; see Appendix E for details). When the
    destination is an AS boundary router, a Type 4 summary-LSA is used,
    and the Link State ID field is the AS boundary router's OSPF Router
    ID.  (To see why it is necessary to advertise the location of each
    ASBR, consult Section 16.4.)  Other than the difference in the Link
    State ID field, the format of Type 3 and 4 summary-LSAs is
    identical.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |    3 or 4     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Network Mask                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      0        |                  metric                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     TOS       |                TOS  metric                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |






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    For stub areas, Type 3 summary-LSAs can also be used to describe a
    (per-area) default route.  Default summary routes are used in stub
    areas instead of flooding a complete set of external routes.  When
    describing a default summary route, the summary-LSA's Link State ID
    is always set to DefaultDestination (0.0.0.0) and the Network Mask
    is set to 0.0.0.0.

    Network Mask
        For Type 3 summary-LSAs, this indicates the destination
        network's IP address mask.  For example, when advertising the
        location of a class A network the value 0xff000000 would be
        used.  This field is not meaningful and must be zero for Type 4
        summary-LSAs.

    metric
        The cost of this route.  Expressed in the same units as the
        interface costs in the router-LSAs.

    Additional TOS-specific information may also be included, for
    backward compatibility with previous versions of the OSPF
    specification ([Ref9]). For each desired TOS, TOS-specific
    information is encoded as follows:

    TOS IP Type of Service that this metric refers to.  The encoding of
        TOS in OSPF LSAs is described in Section 12.3.

    TOS metric
        TOS-specific metric information.

















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A.4.5 AS-external-LSAs

    AS-external-LSAs are the Type 5 LSAs.  These LSAs are originated by
    AS boundary routers, and describe destinations external to the AS.
    For details concerning the construction of AS-external-LSAs, see
    Section 12.4.3.

    AS-external-LSAs usually describe a particular external destination.
    For these LSAs the Link State ID field specifies an IP network
    number (if necessary, the Link State ID can also have one or more of
    the network's "host" bits set; see Appendix E for details).  AS-
    external-LSAs are also used to describe a default route.  Default
    routes are used when no specific route exists to the destination.
    When describing a default route, the Link State ID is always set to
    DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |      5        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Network Mask                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |E|     0       |                  metric                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Forwarding address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      External Route Tag                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |E|    TOS      |                TOS  metric                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Forwarding address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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       |                      External Route Tag                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |



    Network Mask
        The IP address mask for the advertised destination.  For
        example, when advertising a class A network the mask 0xff000000
        would be used.

    bit E
        The type of external metric.  If bit E is set, the metric
        specified is a Type 2 external metric.  This means the metric is
        considered larger than any link state path.  If bit E is zero,
        the specified metric is a Type 1 external metric.  This means
        that it is expressed in the same units as the link state metric
        (i.e., the same units as interface cost).

    metric
        The cost of this route.  Interpretation depends on the external
        type indication (bit E above).

    Forwarding address
        Data traffic for the advertised destination will be forwarded to
        this address.  If the Forwarding address is set to 0.0.0.0, data
        traffic will be forwarded instead to the LSA's originator (i.e.,
        the responsible AS boundary router).

    External Route Tag
        A 32-bit field attached to each external route.  This is not
        used by the OSPF protocol itself.  It may be used to communicate
        information between AS boundary routers; the precise nature of
        such information is outside the scope of this specification.

    Additional TOS-specific information may also be included, for
    backward compatibility with previous versions of the OSPF
    specification ([Ref9]). For each desired TOS, TOS-specific
    information is encoded as follows:

    TOS The Type of Service that the following fields concern.  The
        encoding of TOS in OSPF LSAs is described in Section 12.3.



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    bit E
        For backward-compatibility with [Ref9].

    TOS metric
        TOS-specific metric information.

    Forwarding address
        For backward-compatibility with [Ref9].

    External Route Tag
        For backward-compatibility with [Ref9].


































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B. Architectural Constants

    Several OSPF protocol parameters have fixed architectural values.
    These parameters have been referred to in the text by names such as
    LSRefreshTime.  The same naming convention is used for the
    configurable protocol parameters.  They are defined in Appendix C.

    The name of each architectural constant follows, together with its
    value and a short description of its function.


    LSRefreshTime
        The maximum time between distinct originations of any particular
        LSA.  If the LS age field of one of the router's self-originated
        LSAs reaches the value LSRefreshTime, a new instance of the LSA
        is originated, even though the contents of the LSA (apart from
        the LSA header) will be the same.  The value of LSRefreshTime is
        set to 30 minutes.

    MinLSInterval
        The minimum time between distinct originations of any particular
        LSA.  The value of MinLSInterval is set to 5 seconds.

    MinLSArrival
        For any particular LSA, the minimum time that must elapse
        between reception of new LSA instances during flooding. LSA
        instances received at higher frequencies are discarded. The
        value of MinLSArrival is set to 1 second.

    MaxAge
        The maximum age that an LSA can attain. When an LSA's LS age
        field reaches MaxAge, it is reflooded in an attempt to flush the
        LSA from the routing domain (See Section 14). LSAs of age MaxAge
        are not used in the routing table calculation.  The value of
        MaxAge is set to 1 hour.

    CheckAge
        When the age of an LSA in the link state database hits a
        multiple of CheckAge, the LSA's checksum is verified.  An
        incorrect checksum at this time indicates a serious error.  The
        value of CheckAge is set to 5 minutes.




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    MaxAgeDiff
        The maximum time dispersion that can occur, as an LSA is flooded
        throughout the AS.  Most of this time is accounted for by the
        LSAs sitting on router output queues (and therefore not aging)
        during the flooding process.  The value of MaxAgeDiff is set to
        15 minutes.

    LSInfinity
        The metric value indicating that the destination described by an
        LSA is unreachable. Used in summary-LSAs and AS-external-LSAs as
        an alternative to premature aging (see Section 14.1). It is
        defined to be the 24-bit binary value of all ones: 0xffffff.

    DefaultDestination
        The Destination ID that indicates the default route.  This route
        is used when no other matching routing table entry can be found.
        The default destination can only be advertised in AS-external-
        LSAs and in stub areas' type 3 summary-LSAs.  Its value is the
        IP address 0.0.0.0. Its associated Network Mask is also always
        0.0.0.0.

    InitialSequenceNumber
        The value used for LS Sequence Number when originating the first
        instance of any LSA. Its value is the signed 32-bit integer
        0x80000001.

    MaxSequenceNumber
        The maximum value that LS Sequence Number can attain.  Its value
        is the signed 32-bit integer 0x7fffffff.
















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C. Configurable Constants

    The OSPF protocol has quite a few configurable parameters.  These
    parameters are listed below.  They are grouped into general
    functional categories (area parameters, interface parameters, etc.).
    Sample values are given for some of the parameters.

    Some parameter settings need to be consistent among groups of
    routers.  For example, all routers in an area must agree on that
    area's parameters, and all routers attached to a network must agree
    on that network's IP network number and mask.

    Some parameters may be determined by router algorithms outside of
    this specification (e.g., the address of a host connected to the
    router via a SLIP line).  From OSPF's point of view, these items are
    still configurable.

    C.1 Global parameters

        In general, a separate copy of the OSPF protocol is run for each
        area.  Because of this, most configuration parameters are
        defined on a per-area basis.  The few global configuration
        parameters are listed below.


        Router ID
            This is a 32-bit number that uniquely identifies the router
            in the Autonomous System.  One algorithm for Router ID
            assignment is to choose the largest or smallest IP address
            assigned to the router.  If a router's OSPF Router ID is
            changed, the router's OSPF software should be restarted
            before the new Router ID takes effect. Before restarting in
            order to change its Router ID, the router should flush its
            self-originated LSAs from the routing domain (see Section
            14.1), or they will persist for up to MaxAge minutes.

        RFC1583Compatibility
            Controls the preference rules used in Section 16.4 when
            choosing among multiple AS-external-LSAs advertising the
            same destination. When set to "enabled", the preference
            rules remain those specified by RFC 1583 ([Ref9]). When set
            to "disabled", the preference rules are those stated in



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            Section 16.4.1, which prevent routing loops when AS-
            external-LSAs for the same destination have been originated
            from different areas. Set to "enabled" by default.

            In order to minimize the chance of routing loops, all OSPF
            routers in an OSPF routing domain should have
            RFC1583Compatibility set identically. When there are routers
            present that have not been updated with the functionality
            specified in Section 16.4.1 of this memo, all routers should
            have RFC1583Compatibility set to "enabled". Otherwise, all
            routers should have RFC1583Compatibility set to "disabled",
            preventing all routing loops.

    C.2 Area parameters

        All routers belonging to an area must agree on that area's
        configuration.  Disagreements between two routers will lead to
        an inability for adjacencies to form between them, with a
        resulting hindrance to the flow of routing protocol and data
        traffic.  The following items must be configured for an area:


        Area ID
            This is a 32-bit number that identifies the area.  The Area
            ID of 0.0.0.0 is reserved for the backbone.  If the area
            represents a subnetted network, the IP network number of the
            subnetted network may be used for the Area ID.

        List of address ranges
            An OSPF area is defined as a list of address ranges. Each
            address range consists of the following items:

            [IP address, mask]
                    Describes the collection of IP addresses contained
                    in the address range. Networks and hosts are
                    assigned to an area depending on whether their
                    addresses fall into one of the area's defining
                    address ranges.  Routers are viewed as belonging to
                    multiple areas, depending on their attached
                    networks' area membership.





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            Status  Set to either Advertise or DoNotAdvertise.  Routing
                    information is condensed at area boundaries.
                    External to the area, at most a single route is
                    advertised (via a summary-LSA) for each address
                    range. The route is advertised if and only if the
                    address range's Status is set to Advertise.
                    Unadvertised ranges allow the existence of certain
                    networks to be intentionally hidden from other
                    areas. Status is set to Advertise by default.

            As an example, suppose an IP subnetted network is to be its
            own OSPF area.  The area would be configured as a single
            address range, whose IP address is the address of the
            subnetted network, and whose mask is the natural class A, B,
            or C address mask.  A single route would be advertised
            external to the area, describing the entire subnetted
            network.

        ExternalRoutingCapability
            Whether AS-external-LSAs will be flooded into/throughout the
            area.  If AS-external-LSAs are excluded from the area, the
            area is called a "stub".  Internal to stub areas, routing to
            external destinations will be based solely on a default
            summary route.  The backbone cannot be configured as a stub
            area.  Also, virtual links cannot be configured through stub
            areas.  For more information, see Section 3.6.

        StubDefaultCost
            If the area has been configured as a stub area, and the
            router itself is an area border router, then the
            StubDefaultCost indicates the cost of the default summary-
            LSA that the router should advertise into the area.

    C.3 Router interface parameters

        Some of the configurable router interface parameters (such as IP
        interface address and subnet mask) actually imply properties of
        the attached networks, and therefore must be consistent across
        all the routers attached to that network.  The parameters that
        must be configured for a router interface are:





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        IP interface address
            The IP protocol address for this interface.  This uniquely
            identifies the router over the entire internet.  An IP
            address is not required on point-to-point networks.  Such a
            point-to-point network is called "unnumbered".

        IP interface mask
            Also referred to as the subnet/network mask, this indicates
            the portion of the IP interface address that identifies the
            attached network.  Masking the IP interface address with the
            IP interface mask yields the IP network number of the
            attached network.  On point-to-point networks and virtual
            links, the IP interface mask is not defined. On these
            networks, the link itself is not assigned an IP network
            number, and so the addresses of each side of the link are
            assigned independently, if they are assigned at all.

        Area ID
            The OSPF area to which the attached network belongs.

        Interface output cost
            The cost of sending a packet on the interface, expressed in
            the link state metric.  This is advertised as the link cost
            for this interface in the router's router-LSA. The interface
            output cost must always be greater than 0.

        RxmtInterval
            The number of seconds between LSA retransmissions, for
            adjacencies belonging to this interface.  Also used when
            retransmitting Database Description and Link State Request
            Packets.  This should be well over the expected round-trip
            delay between any two routers on the attached network.  The
            setting of this value should be conservative or needless
            retransmissions will result.  Sample value for a local area
            network: 5 seconds.

        InfTransDelay
            The estimated number of seconds it takes to transmit a Link
            State Update Packet over this interface.  LSAs contained in
            the update packet must have their age incremented by this
            amount before transmission.  This value should take into
            account the transmission and propagation delays of the



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            interface.  It must be greater than 0.  Sample value for a
            local area network: 1 second.

        Router Priority
            An 8-bit unsigned integer.  When two routers attached to a
            network both attempt to become Designated Router, the one
            with the highest Router Priority takes precedence.  If there
            is still a tie, the router with the highest Router ID takes
            precedence.  A router whose Router Priority is set to 0 is
            ineligible to become Designated Router on the attached
            network.  Router Priority is only configured for interfaces
            to broadcast and NBMA networks.

        HelloInterval
            The length of time, in seconds, between the Hello Packets
            that the router sends on the interface.  This value is
            advertised in the router's Hello Packets.  It must be the
            same for all routers attached to a common network.  The
            smaller the HelloInterval, the faster topological changes
            will be detected; however, more OSPF routing protocol
            traffic will ensue.  Sample value for a X.25 PDN network: 30
            seconds.  Sample value for a local area network: 10 seconds.

        RouterDeadInterval
            After ceasing to hear a router's Hello Packets, the number
            of seconds before its neighbors declare the router down.
            This is also advertised in the router's Hello Packets in
            their RouterDeadInterval field.  This should be some
            multiple of the HelloInterval (say 4).  This value again
            must be the same for all routers attached to a common
            network.

        AuType
            Identifies the authentication procedure to be used on the
            attached network.  This value must be the same for all
            routers attached to the network.  See Appendix D for a
            discussion of the defined authentication types.

        Authentication key
            This configured data allows the authentication procedure to
            verify OSPF protocol packets received over the interface.
            For example, if the AuType indicates simple password, the



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            Authentication key would be a clear 64-bit password.
            Authentication keys associated with the other OSPF
            authentication types are discussed in Appendix D.

    C.4 Virtual link parameters

        Virtual links are used to restore/increase connectivity of the
        backbone.  Virtual links may be configured between any pair of
        area border routers having interfaces to a common (non-backbone)
        area.  The virtual link appears as an unnumbered point-to-point
        link in the graph for the backbone.  The virtual link must be
        configured in both of the area border routers.

        A virtual link appears in router-LSAs (for the backbone) as if
        it were a separate router interface to the backbone.  As such,
        it has all of the parameters associated with a router interface
        (see Section C.3).  Although a virtual link acts like an
        unnumbered point-to-point link, it does have an associated IP
        interface address.  This address is used as the IP source in
        OSPF protocol packets it sends along the virtual link, and is
        set dynamically during the routing table build process.
        Interface output cost is also set dynamically on virtual links
        to be the cost of the intra-area path between the two routers.
        The parameter RxmtInterval must be configured, and should be
        well over the expected round-trip delay between the two routers.
        This may be hard to estimate for a virtual link; it is better to
        err on the side of making it too large.  Router Priority is not
        used on virtual links.

        A virtual link is defined by the following two configurable
        parameters: the Router ID of the virtual link's other endpoint,
        and the (non-backbone) area through which the virtual link runs
        (referred to as the virtual link's Transit area).  Virtual links
        cannot be configured through stub areas.

    C.5 NBMA network parameters

        OSPF treats an NBMA network much like it treats a broadcast
        network.  Since there may be many routers attached to the
        network, a Designated Router is selected for the network.  This
        Designated Router then originates a network-LSA, which lists all
        routers attached to the NBMA network.



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        However, due to the lack of broadcast capabilities, it may be
        necessary to use configuration parameters in the Designated
        Router selection.  These parameters will only need to be
        configured in those routers that are themselves eligible to
        become Designated Router (i.e., those router's whose Router
        Priority for the network is non-zero), and then only if no
        automatic procedure for discovering neighbors exists:


        List of all other attached routers
            The list of all other routers attached to the NBMA network.
            Each router is listed by its IP interface address on the
            network.  Also, for each router listed, that router's
            eligibility to become Designated Router must be defined.
            When an interface to a NBMA network comes up, the router
            sends Hello Packets only to those neighbors eligible to
            become Designated Router, until the identity of the
            Designated Router is discovered.

        PollInterval
            If a neighboring router has become inactive (Hello Packets
            have not been seen for RouterDeadInterval seconds), it may
            still be necessary to send Hello Packets to the dead
            neighbor.  These Hello Packets will be sent at the reduced
            rate PollInterval, which should be much larger than
            HelloInterval.  Sample value for a PDN X.25 network: 2
            minutes.

    C.6 Point-to-MultiPoint network parameters

        On Point-to-MultiPoint networks, it may be necessary to
        configure the set of neighbors that are directly reachable over
        the Point-to-MultiPoint network. Each neighbor is identified by
        its IP address on the Point-to-MultiPoint network. Designated
        Routers are not elected on Point-to-MultiPoint networks, so the
        Designated Router eligibility of configured neighbors is
        undefined.

        Alternatively, neighbors on Point-to-MultiPoint networks may be
        dynamically discovered by lower-level protocols such as Inverse
        ARP ([Ref14]).




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    C.7 Host route parameters

        Host routes are advertised in router-LSAs as stub networks with
        mask 0xffffffff.  They indicate either router interfaces to
        point-to-point networks, looped router interfaces, or IP hosts
        that are directly connected to the router (e.g., via a SLIP
        line).  For each host directly connected to the router, the
        following items must be configured:


        Host IP address
            The IP address of the host.

        Cost of link to host
            The cost of sending a packet to the host, in terms of the
            link state metric.  However, since the host probably has
            only a single connection to the internet, the actual
            configured cost in many cases is unimportant (i.e., will
            have no effect on routing).

        Area ID
            The OSPF area to which the host belongs.























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D. Authentication

    All OSPF protocol exchanges are authenticated.  The OSPF packet
    header (see Section A.3.1) includes an authentication type field,
    and 64-bits of data for use by the appropriate authentication scheme
    (determined by the type field).

    The authentication type is configurable on a per-interface (or
    equivalently, on a per-network/subnet) basis.  Additional
    authentication data is also configurable on a per-interface basis.

    Authentication types 0, 1 and 2 are defined by this specification.
    All other authentication types are reserved for definition by the
    IANA (iana@ISI.EDU).  The current list of authentication types is
    described below in Table 20.



                  AuType       Description
                  ___________________________________________
                  0            Null authentication
                  1            Simple password
                  2            Cryptographic authentication
                  All others   Reserved for assignment by the
                               IANA (iana@ISI.EDU)


                      Table 20: OSPF authentication types.



    D.1 Null authentication

        Use of this authentication type means that routing exchanges
        over the network/subnet are not authenticated.  The 64-bit
        authentication field in the OSPF header can contain anything; it
        is not examined on packet reception. When employing Null
        authentication, the entire contents of each OSPF packet (other
        than the 64-bit authentication field) are checksummed in order
        to detect data corruption.





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    D.2 Simple password authentication

        Using this authentication type, a 64-bit field is configured on
        a per-network basis.  All packets sent on a particular network
        must have this configured value in their OSPF header 64-bit
        authentication field.  This essentially serves as a "clear" 64-
        bit password. In addition, the entire contents of each OSPF
        packet (other than the 64-bit authentication field) are
        checksummed in order to detect data corruption.

        Simple password authentication guards against routers
        inadvertently joining the routing domain; each router must first
        be configured with its attached networks' passwords before it
        can participate in routing.  However, simple password
        authentication is vulnerable to passive attacks currently
        widespread in the Internet (see [Ref16]). Anyone with physical
        access to the network can learn the password and compromise the
        security of the OSPF routing domain.

    D.3 Cryptographic authentication

        Using this authentication type, a shared secret key is
        configured in all routers attached to a common network/subnet.
        For each OSPF protocol packet, the key is used to
        generate/verify a "message digest" that is appended to the end
        of the OSPF packet. The message digest is a one-way function of
        the OSPF protocol packet and the secret key. Since the secret
        key is never sent over the network in the clear, protection is
        provided against passive attacks.

        The algorithms used to generate and verify the message digest
        are specified implicitly by the secret key. This specification
        completely defines the use of OSPF Cryptographic authentication
        when the MD5 algorithm is used.

        In addition, a non-decreasing sequence number is included in
        each OSPF protocol packet to protect against replay attacks.
        This provides long term protection; however, it is still
        possible to replay an OSPF packet until the sequence number
        changes. To implement this feature, each neighbor data structure
        contains a new field called the "cryptographic sequence number".
        This field is initialized to zero, and is also set to zero



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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              0                |    Key ID     | Auth Data Len |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Cryptographic sequence number                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 18: Usage of the Authentication field
                   in the OSPF packet header when Cryptographic
                          Authentication is employed

        whenever the neighbor's state transitions to "Down". Whenever an
        OSPF packet is accepted as authentic, the cryptographic sequence
        number is set to the received packet's sequence number.

        This specification does not provide a rollover procedure for the
        cryptographic sequence number. When the cryptographic sequence
        number that the router is sending hits the maximum value, the
        router should reset the cryptographic sequence number that it is
        sending back to 0. After this is done, the router's neighbors
        will reject the router's OSPF packets for a period of
        RouterDeadInterval, and then the router will be forced to
        reestablish all adjacencies over the interface. However, it is
        expected that many implementations will use "seconds since
        reboot" (or "seconds since 1960", etc.) as the cryptographic
        sequence number. Such a choice will essentially prevent
        rollover, since the cryptographic sequence number field is 32
        bits in length.

        The OSPF Cryptographic authentication option does not provide
        confidentiality.

        When cryptographic authentication is used, the 64-bit
        Authentication field in the standard OSPF packet header is
        redefined as shown in Figure 18. The new field definitions are
        as follows:






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        Key ID
            This field identifies the algorithm and secret key used to
            create the message digest appended to the OSPF packet. Key
            Identifiers are unique per-interface (or equivalently, per-
            subnet).

        Auth Data Len
            The length in bytes of the message digest appended to the
            OSPF packet.

        Cryptographic sequence number
            An unsigned 32-bit non-decreasing sequence number. Used to
            guard against replay attacks.

        The message digest appended to the OSPF packet is not actually
        considered part of the OSPF protocol packet: the message digest
        is not included in the OSPF header's packet length, although it
        is included in the packet's IP header length field.

        Each key is identified by the combination of interface and Key
        ID. An interface may have multiple keys active at any one time.
        This enables smooth transition from one key to another. Each key
        has four time constants associated with it. These time constants
        can be expressed in terms of a time-of-day clock, or in terms of
        a router's local clock (e.g., number of seconds since last
        reboot):

        KeyStartAccept
            The time that the router will start accepting packets that
            have been created with the given key.

        KeyStartGenerate
            The time that the router will start using the key for packet
            generation.

        KeyStopGenerate
            The time that the router will stop using the key for packet
            generation.

        KeyStopAccept
            The time that the router will stop accepting packets that
            have been created with the given key.



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        In order to achieve smooth key transition, KeyStartAccept should
        be less than KeyStartGenerate and KeyStopGenerate should be less
        than KeyStopAccept. If KeyStopGenerate and KeyStopAccept are
        left unspecified, the key's lifetime is infinite. When a new key
        replaces an old, the KeyStartGenerate time for the new key must
        be less than or equal to the KeyStopGenerate time of the old
        key.

        Key storage should persist across a system restart, warm or
        cold, to avoid operational issues. In the event that the last
        key associated with an interface expires, it is unacceptable to
        revert to an unauthenticated condition, and not advisable to
        disrupt routing.  Therefore, the router should send a "last
        authentication key expiration" notification to the network
        manager and treat the key as having an infinite lifetime until
        the lifetime is extended, the key is deleted by network
        management, or a new key is configured.

    D.4 Message generation

        After building the contents of an OSPF packet, the
        authentication procedure indicated by the sending interface's
        Autype value is called before the packet is sent. The
        authentication procedure modifies the OSPF packet as follows.

        D.4.1 Generating Null authentication

            When using Null authentication, the packet is modified as
            follows:

            (1) The Autype field in the standard OSPF header is set to
                0.

            (2) The checksum field in the standard OSPF header is set to
                the standard IP checksum of the entire contents of the
                packet, starting with the OSPF packet header but
                excluding the 64-bit authentication field.  This
                checksum is calculated as the 16-bit one's complement of
                the one's complement sum of all the 16-bit words in the
                packet, excepting the authentication field.  If the





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                packet's length is not an integral number of 16-bit
                words, the packet is padded with a byte of zero before
                checksumming.

        D.4.2 Generating Simple password authentication

            When using Simple password authentication, the packet is
            modified as follows:

            (1) The Autype field in the standard OSPF header is set to
                1.

            (2) The checksum field in the standard OSPF header is set to
                the standard IP checksum of the entire contents of the
                packet, starting with the OSPF packet header but
                excluding the 64-bit authentication field.  This
                checksum is calculated as the 16-bit one's complement of
                the one's complement sum of all the 16-bit words in the
                packet, excepting the authentication field.  If the
                packet's length is not an integral number of 16-bit
                words, the packet is padded with a byte of zero before
                checksumming.

            (3) The 64-bit authentication field in the OSPF packet
                header is set to the 64-bit password (i.e.,
                authentication key) that has been configured for the
                interface.

        D.4.3 Generating Cryptographic authentication

            When using Cryptographic authentication, there may be
            multiple keys configured for the interface. In this case,
            among the keys that are valid for message generation (i.e,
            that have KeyStartGenerate <= current time <
            KeyStopGenerate) choose the one with the most recent
            KeyStartGenerate time. Using this key, modify the packet as
            follows:

            (1) The Autype field in the standard OSPF header is set to
                2.





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            (2) The checksum field in the standard OSPF header is not
                calculated, but is instead set to 0.

            (3) The Key ID (see Figure 18) is set to the chosen key's
                Key ID.

            (4) The Auth Data Len field is set to the length in bytes of
                the message digest that will be appended to the OSPF
                packet. When using MD5 as the authentication algorithm,
                Auth Data Len will be 16.

            (5) The 32-bit Cryptographic sequence number (see Figure 18)
                is set to a non-decreasing value (i.e., a value at least
                as large as the last value sent out the interface). The
                precise values to use in the cryptographic sequence
                number field are implementation-specific. For example,
                it may be based on a simple counter, or be based on the
                system's clock.

            (6) The message digest is then calculated and appended to
                the OSPF packet.  The authentication algorithm to be
                used in calculating the digest is indicated by the key
                itself.  Input to the authentication algorithm consists
                of the OSPF packet and the secret key. When using MD5 as
                the authentication algorithm, the message digest
                calculation proceeds as follows:

                (a) The 16 byte MD5 key is appended to the OSPF packet.

                (b) Trailing pad and length fields are added, as
                    specified in [Ref17].

                (c) The MD5 authentication algorithm is run over the
                    concatenation of the OSPF packet, secret key, pad
                    and length fields, producing a 16 byte message
                    digest (see [Ref17]).

                (d) The MD5 digest is written over the OSPF key (i.e.,
                    appended to the original OSPF packet). The digest is
                    not counted in the OSPF packet's length field, but





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                    is included in the packet's IP length field. Any
                    trailing pad or length fields beyond the digest are
                    not counted or transmitted.

    D.5 Message verification

        When an OSPF packet has been received on an interface, it must
        be authenticated. The authentication procedure is indicated by
        the setting of Autype in the standard OSPF packet header, which
        matches the setting of Autype for the receiving OSPF interface.

        If an OSPF protocol packet is accepted as authentic, processing
        of the packet continues as specified in Section 8.2. Packets
        which fail authentication are discarded.

        D.5.1 Verifying Null authentication

            When using Null authentication, the checksum field in the
            OSPF header must be verified. It must be set to the 16-bit
            one's complement of the one's complement sum of all the 16-
            bit words in the packet, excepting the authentication field.
            (If the packet's length is not an integral number of 16-bit
            words, the packet is padded with a byte of zero before
            checksumming.)

        D.5.2 Verifying Simple password authentication

            When using Simple password authentication, the received OSPF
            packet is authenticated as follows:

            (1) The checksum field in the OSPF header must be verified.
                It must be set to the 16-bit one's complement of the
                one's complement sum of all the 16-bit words in the
                packet, excepting the authentication field.  (If the
                packet's length is not an integral number of 16-bit
                words, the packet is padded with a byte of zero before
                checksumming.)

            (2) The 64-bit authentication field in the OSPF packet
                header must be equal to the 64-bit password (i.e.,
                authentication key) that has been configured for the
                interface.



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        D.5.3 Verifying Cryptographic authentication

            When using Cryptographic authentication, the received OSPF
            packet is authenticated as follows:

            (1) Locate the receiving interface's configured key having
                Key ID equal to that specified in the received OSPF
                packet (see Figure 18). If the key is not found, or if
                the key is not valid for reception (i.e., current time <
                KeyStartAccept or current time >= KeyStopAccept), the
                OSPF packet is discarded.

            (2) If the cryptographic sequence number found in the OSPF
                header (see Figure 18) is less than the cryptographic
                sequence number recorded in the sending neighbor's data
                structure, the OSPF packet is discarded.

            (3) Verify the appended message digest in the following
                steps:

                (a) The received digest is set aside.

                (b) A new digest is calculated, as specified in Step 6
                    of Section D.4.3.

                (c) The calculated and received digests are compared. If
                    they do not match, the OSPF packet is discarded. If
                    they do match, the OSPF protocol packet is accepted
                    as authentic, and the "cryptographic sequence
                    number" in the neighbor's data structure is set to
                    the sequence number found in the packet's OSPF
                    header.













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E. An algorithm for assigning Link State IDs

    The Link State ID in AS-external-LSAs and summary-LSAs is usually
    set to the described network's IP address. However, if necessary one
    or more of the network's host bits may be set in the Link State ID.
    This allows the router to originate separate LSAs for networks
    having the same address, yet different masks. Such networks can
    occur in the presence of supernetting and subnet 0s (see [Ref10]).

    This appendix gives one possible algorithm for setting the host bits
    in Link State IDs.  The choice of such an algorithm is a local
    decision. Separate routers are free to use different algorithms,
    since the only LSAs affected are the ones that the router itself
    originates. The only requirement on the algorithms used is that the
    network's IP address should be used as the Link State ID whenever
    possible; this maximizes interoperability with OSPF implementations
    predating RFC 1583.

    The algorithm below is stated for AS-external-LSAs.  This is only
    for clarity; the exact same algorithm can be used for summary-LSAs.
    Suppose that the router wishes to originate an AS-external-LSA for a
    network having address NA and mask NM1. The following steps are then
    used to determine the LSA's Link State ID:

    (1) Determine whether the router is already originating an AS-
        external-LSA with Link State ID equal to NA (in such an LSA the
        router itself will be listed as the LSA's Advertising Router).
        If not, the Link State ID is set equal to NA and the algorithm
        terminates. Otherwise,

    (2) Obtain the network mask from the body of the already existing
        AS-external-LSA. Call this mask NM2. There are then two cases:

        o   NM1 is longer (i.e., more specific) than NM2. In this case,
            set the Link State ID in the new LSA to be the network
            [NA,NM1] with all the host bits set (i.e., equal to NA or'ed
            together with all the bits that are not set in NM1, which is
            network [NA,NM1]'s broadcast address).

        o   NM2 is longer than NM1. In this case, change the existing
            LSA (having Link State ID of NA) to reference the new
            network [NA,NM1] by incrementing the sequence number,



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            changing the mask in the body to NM1 and inserting the cost
            of the new network. Then originate a new LSA for the old
            network [NA,NM2], with Link State ID equal to NA or'ed
            together with the bits that are not set in NM2 (i.e.,
            network [NA,NM2]'s broadcast address).

    The above algorithm assumes that all masks are contiguous; this
    ensures that when two networks have the same address, one mask is
    more specific than the other. The algorithm also assumes that no
    network exists having an address equal to another network's
    broadcast address. Given these two assumptions, the above algorithm
    always produces unique Link State IDs. The above algorithm can also
    be reworded as follows:  When originating an AS-external-LSA, try to
    use the network number as the Link State ID.  If that produces a
    conflict, examine the two networks in conflict. One will be a subset
    of the other. For the less specific network, use the network number
    as the Link State ID and for the more specific use the network's
    broadcast address instead (i.e., flip all the "host" bits to 1).  If
    the most specific network was originated first, this will cause you
    to originate two LSAs at once.

    As an example of the algorithm, consider its operation when the
    following sequence of events occurs in a single router (Router A).


    (1) Router A wants to originate an AS-external-LSA for
        [10.0.0.0,255.255.255.0]:

        (a) A Link State ID of 10.0.0.0 is used.

    (2) Router A then wants to originate an AS-external-LSA for
        [10.0.0.0,255.255.0.0]:

        (a) The LSA for [10.0.0,0,255.255.255.0] is reoriginated using a
            new Link State ID of 10.0.0.255.

        (b) A Link State ID of 10.0.0.0 is used for
            [10.0.0.0,255.255.0.0].

    (3) Router A then wants to originate an AS-external-LSA for
        [10.0.0.0,255.0.0.0]:




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        (a) The LSA for [10.0.0.0,255.255.0.0] is reoriginated using a
            new Link State ID of 10.0.255.255.

        (b) A Link State ID of 10.0.0.0 is used for
            [10.0.0.0,255.0.0.0].

        (c) The network [10.0.0.0,255.255.255.0] keeps its Link State ID
            of 10.0.0.255.





































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F. Multiple interfaces to the same network/subnet

    There are at least two ways to support multiple physical interfaces
    to the same IP subnet. Both methods will interoperate with
    implementations of RFC 1583 (and of course this memo). The two
    methods are sketched briefly below. An assumption has been made that
    each interface has been assigned a separate IP address (otherwise,
    support for multiple interfaces is more of a link-level or ARP issue
    than an OSPF issue).

    Method 1:
        Run the entire OSPF functionality over both interfaces, sending
        and receiving hellos, flooding, supporting separate interface
        and neighbor FSMs for each interface, etc. When doing this all
        other routers on the subnet will treat the two interfaces as
        separate neighbors, since neighbors are identified (on broadcast
        and NBMA networks) by their IP address.

        Method 1 has the following disadvantages:

        (1) You increase the total number of neighbors and adjacencies.

        (2) You lose the bidirectionality test on both interfaces, since
            bidirectionality is based on Router ID.

        (3) You have to consider both interfaces together during the
            Designated Router election, since if you declare both to be
            DR simultaneously you can confuse the tie-breaker (which is
            Router ID).

    Method 2:
        Run OSPF over only one interface (call it the primary
        interface), but include both the primary and secondary
        interfaces in your Router-LSA.

        Method 2 has the following disadvantages:

        (1) You lose the bidirectionality test on the secondary
            interface.

        (2) When the primary interface fails, you need to promote the
            secondary interface to primary status.



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G. Differences from RFC 2178

    This section documents the differences between this memo and RFC
    2178.  All differences are backward-compatible. Implementations of
    this memo and of RFCs 2178, 1583, and 1247 will interoperate.

    G.1 Flooding modifications

        Three changes have been made to the flooding procedure in
        Section 13.

        The first change is to step 4 in Section 13. Now MaxAge LSAs are
        acknowledged and then discarded only when both a) there is no
        database copy of the LSA and b) none of router's neighbors are
        in states Exchange or Loading. In all other cases, the MaxAge
        LSA is processed like any other LSA, installing the LSA in the
        database and flooding it out the appropriate interfaces when the
        LSA is more recent than the database copy (Step 5 of Section
        13). This change also affects the contents of Table 19.

        The second change is to step 5a in Section 13. The MinLSArrival
        check is meant only for LSAs received during flooding, and
        should not be performed on those LSAs that the router itself
        originates.

        The third change is to step 8 in Section 13. Confusion between
        routers as to which LSA instance is more recent can cause a
        disastrous amount of flooding in a link-state protocol (see
        [Ref26]). OSPF guards against this problem in two ways: a) the
        LS age field is used like a TTL field in flooding, to eventually
        remove looping LSAs from the network (see Section 13.3), and b)
        routers refuse to accept LSA updates more frequently than once
        every MinLSArrival seconds (see Section 13).  However, there is
        still one case in RFC 2178 where disagreements regarding which
        LSA is more recent can cause a lot of flooding traffic:
        responding to old LSAs by reflooding the database copy.  For
        this reason, Step 8 of Section 13 has been amended to only
        respond with the database copy when that copy has not been sent
        in any Link State Update within the last MinLSArrival seconds.






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    G.2 Changes to external path preferences

        There is still the possibility of a routing loop in RFC 2178
        when both a) virtual links are in use and b) the same external
        route is being imported by multiple ASBRs, each of which is in a
        separate area. To fix this problem, Section 16.4.1 has been
        revised. To choose the correct ASBR/forwarding address, intra-
        area paths through non-backbone areas are always preferred.
        However, intra-area paths through the backbone area (Area 0) and
        inter-area paths are now of equal preference, and must be
        compared solely based on cost.

        The reasoning behind this change is as follows. When virtual
        links are in use, an intra-area backbone path for one router can
        turn into an inter-area path in a router several hops closer to
        the destination. Hence, intra-area backbone paths and inter-area
        paths must be of equal preference. We can safely compare their
        costs, preferring the path with the smallest cost, due to the
        calculations in Section 16.3.

        Thanks to Michael Briggs and Jeremy McCooey of the UNH
        InterOperability Lab for pointing out this problem.

    G.3 Incomplete resolution of virtual next hops

        One of the functions of the calculation in Section 16.3 is to
        determine the actual next hop(s) for those destinations whose
        next hop was calculated as a virtual link in Sections 16.1 and
        16.2.  After completion of the calculation in Section 16.3, any
        paths calculated in Sections 16.1 and 16.2 that still have
        unresolved virtual next hops should be discarded.

    G.4 Routing table lookup

        The routing table lookup algorithm in Section 11.1 has been
        modified to reflect current practice. The "best match" routing
        table entry is now always selected to be the one providing the
        most specific (longest) match. Suppose for example a router is
        forwarding packets to the destination 192.9.1.1. A routing table
        entry for 192.9.1/24 will always be a better match than the
        routing table entry for 192.9/16, regardless of the routing
        table entries' path-types. Note however that when multiple paths



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RFC 2328                     OSPF Version 2                   April 1998


        are available for a given routing table entry, the calculations
        in Sections 16.1, 16.2, and 16.4 always yield the paths having
        the most preferential path-type. (Intra-area paths are the most
        preferred, followed in order by inter-area, type 1 external and
        type 2 external paths; see Section 11).








































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RFC 2328                     OSPF Version 2                   April 1998


Security Considerations

    All OSPF protocol exchanges are authenticated. OSPF supports
    multiple types of authentication; the type of authentication in use
    can be configured on a per network segment basis. One of OSPF's
    authentication types, namely the Cryptographic authentication
    option, is believed to be secure against passive attacks and provide
    significant protection against active attacks. When using the
    Cryptographic authentication option, each router appends a "message
    digest" to its transmitted OSPF packets. Receivers then use the
    shared secret key and received digest to verify that each received
    OSPF packet is authentic.

    The quality of the security provided by the Cryptographic
    authentication option depends completely on the strength of the
    message digest algorithm (MD5 is currently the only message digest
    algorithm specified), the strength of the key being used, and the
    correct implementation of the security mechanism in all
    communicating OSPF implementations.  It also requires that all
    parties maintain the secrecy of the shared secret key.

    None of the OSPF authentication types provide confidentiality. Nor
    do they protect against traffic analysis. Key management is also not
    addressed by this memo.

    For more information, see Sections 8.1, 8.2, and Appendix D.

Author's Address

    John Moy
    Ascend Communications, Inc.
    1 Robbins Road
    Westford, MA 01886

    Phone: 978-952-1367
    Fax:   978-392-2075
    EMail: jmoy@casc.com








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RFC 2328                     OSPF Version 2                   April 1998


Full Copyright Statement

    Copyright (C) The Internet Society (1998).  All Rights Reserved.

    This document and translations of it may be copied and furnished to
    others, and derivative works that comment on or otherwise explain it
    or assist in its implementation may be prepared, copied, published
    and distributed, in whole or in part, without restriction of any
    kind, provided that the above copyright notice and this paragraph
    are included on all such copies and derivative works.  However, this
    document itself may not be modified in any way, such as by removing
    the copyright notice or references to the Internet Society or other
    Internet organizations, except as needed for the purpose of
    developing Internet standards in which case the procedures for
    copyrights defined in the Internet Standards process must be
    followed, or as required to translate it into languages other than
    English.

    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assigns.

    This document and the information contained herein is provided on an
    "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
    TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
    BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
    HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
    MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


















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