path: root/doc/rfc/rfc1587.txt

Network Working Group                                          R. Coltun
Request for Comments: 1587                  RainbowBridge Communications
Category: Standards Track                                      V. Fuller
                                                     Stanford University
                                                              March 1994


                          The OSPF NSSA Option

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.

Table Of Contents

   1.0 Abstract .................................................  1
   2.0 Overview .................................................  2
   2.1 Motivation ...............................................  2
   2.2 Proposed Solution ........................................  3
   3.0 Implementation Details ...................................  5
   3.1 The N-bit ................................................  5
   3.2 Type-7 Address Ranges ....................................  5
   3.3 Type-7 LSAs ..............................................  5
   3.4 Originating Type-7 LSAs ..................................  7
   3.5 Calculating Type-7 AS External Routes ....................  8
   3.6 Incremental Updates ...................................... 10
   4.0 Originating Type-5 LSAs .................................. 10
   4.1 Translating Type-7 LSAs .................................. 10
   4.2 Flushing Translated Type-7 LSAs .......................... 13
   5.0 Acknowledgements ......................................... 13
   6.0 References ............................................... 13
   7.0 Security Considerations .................................. 13
   8.0 Authors' Addresses ....................................... 14
   Appendix A: Type-7 LSA Packet Format ......................... 15
   Appendix B: The Options Field ................................ 16
   Appendix C: Configuration Parameters ......................... 17

1.0  Abstract

   This document describes a new optional type of OSPF area, somewhat
   humorously referred to as a "not-so-stubby" area (or NSSA).  NSSAs
   are similar to the existing OSPF stub area configuration option but
   have the additional capability of importing AS external routes in a
   limited fashion.



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2.0  Overview

2.1  Motivation

   Wide-area transit networks (such as the NSFNET regionals) often have
   connections to moderately-complex "leaf" sites.  A leaf site may have
   multiple IP network numbers assigned to it.

   Typically, one of the leaf site's networks is directly connected to a
   router provided and administered by the transit network while the
   others are distributed throughout and administered by the site.  From
   the transit network's perspective, all of the network numbers
   associated with the site make up a single "stub" entity.  For
   example, BARRNet has one site composed of a class-B network,
   130.57.0.0, and a class-C network, 192.31.114.0.  From BARRNet's
   perspective, this configuration looks something like this:

                    192.31.114
                        |
                      (cloud)
                  -------------- 130.57.4
                        |
                        |
                     ------ 131.119.13 ------
                     |BR18|------------|BR10|
                     ------            ------
                                          |
                                          V
                                  to BARRNet "core" OSPF system


   where the "cloud" consists of the subnets of 130.57 and network
   192.31.114, all of which are learned by RIP on router BR18.
   Topologically, this cloud looks very much like an OSPF stub area.
   The advantages of running the cloud as an OSPF stub area are:

             1. Type-5 routes (OSPF external link-state advertisements
                (LSAs)) are not advertised beyond the router
                labeled "BR10". This is advantageous because the
                link between BR10 and BR18 may be a low-speed link
                or the router BR18 may have limited resources.

             2. The transit network is abstracted to the "leaf"
                router BR18 by advertising only a default route
                across the link between BR10 and BR18.

             3. The cloud becomes a single, manageable "leaf" with
                respect to the transit network.



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             4. The cloud can become, logically, a part of the transit
                network's OSPF routing system.

             5. Translated type-5 LSAs that are sent into the
                backbone from the cloud (which is a separate
                stub area) may be considered "leaf" nodes
                when performing the Dijkstra calculation.

   However, the current definition of the OSPF protocol [1] imposes
   topological limitations which restrict simple cloud topologies from
   becoming OSPF stub areas.  In particular, it is illegal for a stub
   area to import routes external to OSPF; it is not possible for
   routers BR18 and BR10 to both be members of the stub area and to
   import the routes learned from RIP or other IP routing protocols as
   type-5 (OSPF external LSAs) into the OSPF system.  In order to run
   OSPF out to BR18, BR18 must be a member of a non-stub area or the
   OSPF backbone to import routes other than its directly-connected
   network(s).  Since it is not acceptable for BR18 to maintain all of
   BARRNet's external (type-5) routes, BARRNet is forced by OSPF's
   topological limitations to run OSPF out to BR10 and to run RIP
   between BR18 and BR10.

2.2 Proposed Solution

   This document describes a new optional type of OSPF area, somewhat
   humorously referred to as a "not-so-stubby" area (or NSSA) which has
   the capability of importing external routes in a limited fashion.

   The OSPF specification defines two general classes of area
   configuration.  The first allows type-5 LSAs to be flooded throughout
   the area.  In this configuration, type-5 LSAs may be originated by
   routers internal to the area or flooded into the area by area border
   routers.  These areas, referred to herein as type-5 capable areas (or
   just plain areas in the OSPF spec), are distinguished by the fact
   that they can carry transit traffic.  The backbone is always a type-5
   capable area.  The second type of area configuration, called stub,
   allows no type-5 LSAs to be propagated into/throughout the area and
   instead depends on default routing to external destinations.

   NSSAs are defined in much the same manner as existing stub areas.  To
   support NSSAs, a new option bit (the "N" bit) and a new type of LSA
   (type-7) area defined.  The "N" bit ensures that routers belonging to
   an NSSA agree on its configuration.  Similar to the stub area's use
   of the "E" bit, both NSSA neighbors must agree on the setting of the
   "N" bit or the OSPF neighbor adjacency will not form.

   Type-7 LSAs provide for carrying external route information within an
   NSSA.  Type-7 AS External LSAs have virtually the same syntax as the



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   Type-5 AS External LSAs with the obvious exception of the link-state
   type (see section 3.2 for more details). There are two major semantic
   differences between type-5 and type-7 LSAs.

          o  Type-7 LSAs may be originated by and advertised
             throughout an NSSA; as with stub areas, NSSA's do not
             receive or originate type-5 LSAs.

          o  Type-7 LSAs are advertised only within a single NSSA;
             they are not flooded into the backbone area or any
             other area by border routers, though the information
             which they contain can be propagated into the backbone
             area (see section 3.6).

   In order to allow limited exchange of external information across an
   NSSA area border, NSSA border routers will translate selected type-7
   LSAs received from the NSSA into type-5 LSAs.  These type-5 LSAs will
   be flooded to all type-5 capable areas.  NSSA area border routers may
   be configured with address ranges so that several type-7 LSAs may be
   represented by a single type-5 LSA.

   In addition, an NSSA area border router can originate a default
   type-7 LSA (IP address of 0.0.0.0) into the NSSA.  Default routes are
   necessary because NSSAs do not receive full routing information and
   must have a default route to route to AS-external destinations.  Like
   stub areas, NSSAs may be connected to the backbone at more than one
   area border router, but may not be used as a transit area.  Note that
   the default route originated by an NSSA area border router is never
   translated into a type-5 LSA, however, a default route originated by
   an NSSA internal AS boundary router (one that is not also an area
   border router) may be translated into a type-5 LSA.

   It should also be noted that unlike stub areas, all OSPF summary
   routes (type-3 LSAs) must be imported into NSSAs.  This is to ensure
   that OSPF internal routes are always chosen over OSPF external
   (type-7) routes.

   In our example topology the subnets of 130.57 and network 192.31.114,
   will still be learned by RIP on router BR18 but now both BR10 and
   BR18 can be in an NSSA and all of BARRNets external routes are hidden
   from BR18; BR10 becomes an NSSA area border router and BR18 becomes
   an AS boundary router internal to the NSSA.  BR18 will import the
   subnets of 130.57 and network 192.31.114 as type-7 LSAs into the
   NSSA.  BR10 then translates these routes into type-5 LSAs and floods
   them into BARRNet's backbone.






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3.0  Implementation Details

3.1  The N-bit

   The N-bit ensures that all members of a NSSA agree on the area's
   configuration.  Together, the N-bit and E-bit reflect an interface's
   (and consequently the interface's associated area's) external LSA
   flooding capability.  As explained in section 10.5 of the OSPF
   specification, if type-5 LSAs are not flooded into/throughout the
   area, the E-bit must be clear in the option field of the received
   Hello packets. Interfaces associated with an NSSA will not send or
   receive type-5 LSAs on that interface but may send and receive type-7
   LSAs.  Therefore, if the N-bit is set in the options field, the E-bit
   must be cleared.

   To support the NSSA option an additional check must be made in the
   function that handles receiving Hello packet to verify that both the
   N-bit and the E-bit found in the Hello packet's option field match
   the value of the options that have been configured in the receiving
   interface.  A mismatch in the options causes processing of the
   received Hello packet to stop and the packet to be dropped.

3.2  Type-7 Address Ranges

   NSSA area border routers may be configured with type-7 address
   ranges.  Each address range is defined as an [address,mask] pair.
   Many separate type-7 networks may then be represented by in a single
   address range (as advertised by a type-5 LSA), just as a subnetted
   network is composed of many separate subnets.  Area border routers
   may then summarize type-7 routes by advertising a single type-5 route
   for each type-7 address range.  The type-5 route, resulting from a
   type-7 address range match will be distributed to all type-5 capable
   areas.  Section 4.1 gives the details of generating type-5 routes
   from type-7 address ranges.

   A type-7 address range includes the following configurable items.

               o An [address,mask] pair.

               o A status indication of either Advertise or
                 DoNotAdvertise.

               o External route tag.

3.3  Type-7 LSAs: NSSA External Link-State Advertisements

   External routes are imported into NSSAs as type-7 LSAs by the NSSA's
   AS boundary routers.  An NSSA AS boundary routers is a router which



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   has an interface associated with the NSSA and is exchanging routing
   information with routers belonging to another AS.  As with type-5
   LSAs a separate type-7 LSA is originated for each destination
   network.  To support NSSA areas, the link-state database must
   therefore be expanded to contain a type-7 LSA.

   Type 7-LSAs are identical to type-5 LSAs except for the following
   (see  section  12.3.4  "AS external links" in the OSPF
   specification).

      1. The type field in the LSA header is 7.

      2. Type-7 LSAs are only flooded within the NSSA.
         The flooding of type-7 LSAs follow the same rules
         as the flooding of type 1-4 LSAs.

      3. Type-7 LSAs are kept within the NSSA's LSDB (are
         area specific) whereas because type-5 LSAs are
         flooded to all type-5 capable areas, type-5 LSAs
         global scope in the router's LSDB.

      4. At the area border router, selected type-7 LSAs are
         translated into type 5-LSAs and flooded into the
         backbone.

      5. Type 7 LSAs have a  propagate (P) bit which is
         used to flag the area border router to translate the
         type-7 LSA into a type-5 LSA. Examples of how the P-bit
         is used for loop avoidance are in the following sections.

      6. Those type-7 LSAs that are to be translated into type-5
         LSAs must have their forwarding address set.
         Type-5 LSAs that have been translated from type-7 LSAs
         for the most part must contain a forwarding address.
         The execption to this is if the translation to a type-5
         LSA is the result of an address range match, in which
         case the type-5 LSA will not contain a forwarding address
         (see section 4.1 for details).
         The forwarding address contained in type-5 LSAs will
         result in more efficient routing to the AS external
         networks when there are multiple NSSA area
         border routers. Having the forwarding address in the
         type-7 LSAs will ease the translation of type-7 into
         type-5 LSAs as the NSSA area border router will
         not be required to compute the forwarding address.

         If the network between the NSSA AS boundary router and the
         adjacent AS is advertised into OSPF as an internal OSPF



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         route, the forwarding address should be the next hop
         address as is currently done in type-5 LSAs, but unlike
         type-5 LSAs if the intervening network is not advertised
         into OSPF as an internal OSPF route, the forwarding
         address should be any one of the router's active OSPF
         interface addresses.

   Type-5 and type-7 metrics and path types are directly comparable.

3.4  Originating Type-7 LSAs

   NSSA AS boundary routers may originate type-7 LSAs.  All NSSA area
   border routers must also be AS boundary routers since they all must
   have the capability of translating a type-7 LSAs into a type-5 LSAs
   (see section 3.6 routes for the translation algorithm).  NSSA area
   border routers must set the E-bit (external bit) as well as the B-bit
   (border bit) in its router (type-1) LSAs (both in the backbone and in
   the NSSA area).

   When an NSSA internal AS boundary router originates a type-7 LSA that
   it wants to be translated into a type-5 LSA by the NSSA area border
   router (and subsequently flooded into the backbone), it must set the
   P-bit in the LS header's option field and add a valid forwarding
   address in the type-7 LSA.

   If a router is attached to another AS and is also an NSSA area border
   router, it may originate a both a type-5 and a type-7 LSA for the
   same network.  The type-5 LSA will be flooded to the backbone (and
   all attached type-5 capable areas) and the type-7 will be flooded
   into the NSSA.  If this is the case, the P-bit must be reset in the
   type-7 NSSA so the type-7 LSA isn't again translated into a type-5
   LSA by another NSSA area border router.

   A type-7 default route (network 0.0.0.0) may be originated into the
   NSSA by an NSSA area border router or by an NSSA AS boundary router
   which is internal to the NSSA.  The type-7 default route originated
   by the NSSA area border router must have the P-bit reset so that the
   default route originated by the NSSA area border router will not find
   its way out of the NSSA into the rest of the AS system via another
   NSSA area border router.  The type-7 default route originated by an
   NSSA AS boundary router which is not an NSSA area border router may
   have the P-bit set.  Type-7 routes which are originated by the NSSA
   area border router will not get added to other NSSA area border
   router's routing table.

   A default route must not be injected into the NSSA as a summary
   (type-3) LSA as in the stub area case.  The reason for this is that
   the preferred summary default route would be chosen over all more



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   specific type-7 routes.  Because summary routes are preferred to
   external routes and to ensure that summary routes are chosen over
   external within the NSSA, all summary routes (unlike stub areas in
   which this is optional) must be imported into an NSSA.

3.5 Calculating Type-7 AS External Routes

   This section is very similar to section 16.4 (Calculating AS external
   routes) in the OSPF specification.  An NSSA area border router should
   examine both type-5 LSAs and type-7 LSAs if either type-5 or type-7
   routes need to be updated.  Type-7 LSAs should be examined after
   type-5 LSAs.  An NSSA internal router should examine type-7 LSAs when
   type-7 routes need to be recalculated.

   In relation to the steps to calculate the routing table as presented
   in the OSPF specification (chapter 16, "Calculation of the Routing
   Table"), type-7 LSAs should be examined after step 5 where the routes
   to external destinations are calculated.

   Type-7 routes are calculated by examining type-7 LSAs.  Each of LSAs
   are considered in turn. Most type-7 LSAs describe routes to specific
   IP destinations.  A type-7 LSA can also describe a default route for
   the NSSA (destination = DefaultDestination).  For each type-7 LSA:

      1. If the metric specified by the LSA is LSInfinity, the
         age of the LSA equals MaxAge or the advertising router
         field is equal to this router's router ID, examine the
         next advertisement.

      2. Call the destination described by the LSA N. Look up the
         routing table entry for the AS boundary router (ASBR) that
         originated the LSA. If no entry exists for the ASBR
         (i.e., ASBR is unreachable), do nothing with this LSA and
         consider the next in the list.

         If the destination is the default route (destination =
         DefaultDestination) and if the originator of the LSA and
         the calculating router are both NSSA area border routers
         do nothing with this LSA and consider the next in the list.

         Else, this LSA describes an AS external path to destination
         N. If the forwarding address (as specified in the forwarding
         address field of the LSA) is 0.0.0.0, the packets routed
         to the external destination N will be routed to the
         originating ASBR. If the forwarding address is not 0.0.0.0,
         look up the forwarding address in the routing table. Packets
         routed to the external destination N will be routed within
         the NSSA to this forwarding address. An intra-area path



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         must therefore exist to the forwarding address. If no such
         path exists, do nothing with the LSA and consider the next
         in the list.

         Call the routing table distance to the forwarding address
         (or the distance to the originating ASBR if the forwarding
         address is 0.0.0.0) X, and the cost specified in the type-7
         LSA Y. X is in terms of the link-state metric, and Y is a
         Type-1 or Type-2 external metric.

      3. Now, look up the routing table entry for the destination
         N. If no entries exist for N, install the AS external path
         to N, with the next hop equal to the list of next hops to
         the forwarding address/ASBR, and the 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.

      4. Else, if the paths present in the table are not Type-1 or
         Type-2 external paths, do nothing (AS external paths have
         the lowest priority).

      5. Otherwise, compare the cost of this new AS external path
         to the ones present in the table. Note that type-5 and
         type-7 routes are directly comparable. Type-1 external
         paths are always shorter than Type-2 external paths.
         Type-1 external paths are compared by looking at the sum
         of the distance to the forwarding address/ASBR and the
         advertised Type-1 paths (X+Y). Type-2 external paths are
         compared by looking at the advertised Type-2 metrics,
         and then if necessary, the distance to the forwarding
         address/ASBR.

         When a type-5 LSA and a type-7 LSA are found to have the
         same type and an equal distance, the following priorities
         apply (listed from highest to lowest) for breaking the tie.

                 a. Any type 5 LSA.
                 b. A type-7 LSA with the P-bit set and the forwarding
                    address non-zero.
                 c. Any other type-7 LSA.

         If the new path is shorter, it replaces the present paths
         in the routing table entry. If the new path is the same
         cost, it is added to the routing table entry's list of
         paths.



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3.6 Incremental Updates

   Incremental updates for type-7 LSAs should be treated the same as
   incremental updates for type-5 LSAs (see section 16.6 of the OSPF
   specification).  That is, if a new instance of a type-7 LSA is
   received it is not necessary to recalculate the entire routing table.
   If there is already an OSPF internal route to the destination
   represented by the type-7 LSA, no recalculation is necessary.
   Otherwise, the procedure in the proceeding section will have to be
   performed but only for the external routes (type-5 and type-7) whose
   networks describe the same networks as the newly received LSA.

4.0 Originating Type-5 LSAs

4.1 Translating Type-7 LSAs Into Type-5 LSAs

   This step is performed as part of the NSSA's Dijkstra calculation
   after type-5 and type-7 routes have been calculated.  If the
   calculating router is not an area border router this translation
   algorithm should be skipped.  All reachable area border routers in
   the NSSA should now be examined noting the one with the highest
   router ID.  If this router has the highest router ID, it will be the
   one translating type-7 LSAs into type-5 LSAs for the NSSA, otherwise
   the translation algorithm should not be performed.

   All type-7 routes that have been added to the routing table should be
   examined.  If the type-7 LSA (associated with the route being
   examined) has the P-bit set and a non-zero forwarding address, the
   following steps should be taken.

      The translation procedure must first check for a configured type-7
      address range.  Recall that an type-7 address range consists of an
      [address,mask] pair and a status indication of either Advertise or
      DoNotAdvertise.  At most a single type-5 LSA is made for each
      range.  If the route being examined falls within the type-7
      address range, (the [address,mask] pair of the route equal to or a
      more specific instance of the [address,mask] pair of the type-7
      address range), one of following three actions may take place.

         1. When the range's status indicates Advertise and the
            route's address and mask are equal to the address
            and mask of the type-7 range a type-5 LSA should be
            originated if:

            o there currently is no type-5 LSA originated from
              this router corresponding to the type-7 LSA,





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            o the path type or the metric in the corresponding
              type-5 LSA is different from the type-7 LSA or

            o The forwarding address in the corresponding
              type-5 LSA is different from the type-7 LSA.

              The newly originated type-5 LSA will describe
              the same network and have the same network mask,
              metrics, forwarding address, external route tag
              and path type as the type-7 LSA, however, the
              advertising router field will be the router ID
              of this area border router.

         2. When the range's status indicates Advertise and the
            route's address or mask indicates a more specific
            route (i.e., the route's address is subsumed by the
            range or the route has a longer mask), a type-5 LSA
            is generated with link-state ID equal to the range's
            address (if necessary, the link-state ID can also have
            one or more of the range's "host" bits set; see
            Appendix F of the OSPF specification for details),
            the network mask, external route tag and
            path type will be set to the configured type-7 range
            values. The advertising router field will be the
            router ID of this area border router.
            The forwarding address will not be set.
            The path type should always be set to the highest
            path type that is subsumed by the net range.
            The metric for the type-5 LSA will be set as follows:

            o if the path type is externl type 2, the type-5
              metric should be set to the largest type-7 metric
              subsumed by this net range + 1.

            o if the path type is external type 1, the type-5
              metric should be set to the largest metric.

            For example, given a net range of [10.0.0.0, 255.0.0.0]
            for an area that has type-7 routes of:

                    10.1.0.0 path type 1, metric 10
                    10.2.0.0 path type 1, metric 11
                    10.3.0.0 path type 2, metric 5

             a type-5 LSA will be generated with a path type of 2
             and a metric of 6.





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             As another example, given a net range of
             [10.0.0.0, 255.0.0.0] for an area that has
             type-7 routes of:

                    10.1.0.0 path type 1, metric 10
                    10.2.0.0 path type 1, metric 11
                    10.3.0.0 path type 1, metric 5

             a type-5 LSA will be generated with a path type of 1
             and a metric of 11.

             These metric and path type rules will avoid routing
             loops in the event that path type 1 and 2 are both
             used within the area.

         3. When the range's status indicates DoNotAdvertise,
            the type-5 LSA is suppressed and the component networks
            remain hidden from the rest of the AS.

            By default (given that the P-bit is set and the LSA has a
            non-zero forwarding address) if a network is not contained
            in any explicitly configured address range, a type-7 to
            type-5 LSA translation will occur.

            A new instance of a type-5 LSA should be originated and
            flooded to all attached type-5 capable areas if one of the
            following is true.

            1. There currently is no type-5 LSA originated from this
               router corresponding to the type-7 LSA.

            2. The path type or the metric in the corresponding
               type-5 LSA is different from the type-7 LSA.

            3. The forwarding address in the corresponding
               type-5 LSA is different from the type-7 LSA.

            The newly originated type-5 LSAs will describe the same
            network and have the same network mask, metrics, forwarding
            address, external route tag and path type as the type-7 LSA.
            The advertising router field will be the router ID of this
            area border router.

            As with all newly originated type-5 LSAs, a type-5 LSA that
            is the result of a type-7 to type-5 translation (type-7 range
            or default case) is flooded to all attached type-5 capable
            areas.




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4.2 Flushing Translated Type-7 LSAs

   If an NSSA area border router has translated a type-7 LSA to a type-5
   LSA that should no longer be translated, the type-5 LSA should be
   flushed (set to MaxAge and flooded).  The translated type-5 LSA
   should be flushed whenever the routing table entry that caused the
   translation changes so that either the routing table entry is
   unreachable or the entry's associated LSA is not a type-7 with the
   P-bit set and a non-zero forwarding address.

5.0 Acknowledgements

   This document was produced by the OSPF Working Group, chaired by John
   Moy.

   In addition, the comments of the following individuals are also
   acknowledged:

                  Phani Jajjarvarpu  cisco
                  Dino Farinacci     cisco
                  Jeff Honig         Cornell University
                  John Moy           Proteon, Inc.
                  Doug Williams      IBM

6.0 References

   [1] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc., March 1994.

   [2] Moy, J., "Multicast Extensions to OSPF", RFC 1584, Proteon, Inc.,
       Proteon, Inc., March 1994.

7.0 Security Considerations

   Security issues are not discussed in this memo.

















Coltun & Fuller                                                [Page 13]
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RFC 1587                    OSPF NSSA Option                  March 1994


8.0 Authors' Addresses

   Rob Coltun
   RainbowBridge Communications

   Phone: (301) 340-9416
   EMail: rcoltun@rainbow-bridge.com


   Vince Fuller
   BARRNet
   Stanford University
   Pine Hall 115
   Stanford, CA, 94305-4122

   Phone: (415) 723-6860
   EMail: vaf@Valinor.Stanford.EDU


































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RFC 1587                    OSPF NSSA Option                  March 1994


Appendix A: Type-7 Packet Format

          0                                32
          -----------------------------------
          |                | OPTS   |   7   |
          |                ------------------
          |        Link-State Header        |
          |                                 |
          -----------------------------------
          | Network Mask                    |
          -----------------------------------  ______
          |E| Tos  |        metric          |  .
          -----------------------------------  .  repeated for each TOS
          | Forwarding Address              |  .
          -----------------------------------  .
          | External Route Tag              |  ______
          -----------------------------------

   The definitions of the link-state ID, network mask, metrics and
   external route tag are the same as the definitions for the type-5
   LSAs (see A.4.5 in the OSPF specification) except for:

               The Forwarding Address

   If the network between the NSSA AS boundary router and the adjacent
   AS is advertised into OSPF as an internal OSPF route, the forwarding
   address should be the next hop address but if the intervening network
   is not advertised into OSPF as an internal OSPF route, the forwarding
   address should be any one of the router's active OSPF interface
   addresses.





















Coltun & Fuller                                                [Page 15]
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RFC 1587                    OSPF NSSA Option                  March 1994


Appendix B: The Options Field

   The OSPF options field is present in OSPF Hello packets, Database
   Description packets and all link-state advertisements. See appendix
   A.2 in the OSPF specification for a description of option field.

                   ------------------------------------
                   | * | * | * | * | N/P | MC | E | T |
                   ------------------------------------

                       The Type-7 LSA options field


             T-bit:  The T-bit describes the router's TOS capability.

             E-bit:  Type-5 AS external link advertisements are not
                     flooded into/through OSPF stub and NSSA areas.
                     The E-bit ensures that all members of a stub area
                     agree on that area configuration. The E-bit is
                     meaningful only in OSPF Hello packets. When the
                     E-bit is reset in the Hello packet sent out a
                     particular interface, it means that the router
                     will neither send nor receive type-5 AS external
                     link state advertisements on that interface (in
                     other words, the interface connects to a stub
                     area). Two routers will not become neighbors
                     unless they agree on the state of the E-bit.

             MC-bit: The MC-bit describes the multicast capability of
                     the various pieces of the OSPF routing domain
                     [2].

             N-bit:  The N-bit describes the the router's NSSA
                     capability.  The N-bit is used only in Hello
                     packets and ensures that all members of an NSSA
                     agree on that area's configuration. When the
                     N-bit is reset in the Hello packet sent out a
                     particular interface, it means that the router
                     will neither send nor receive type-7 LSAs on that
                     interface. Two routers will not form an adjacency
                     unless they agree on the state of the N-bit. If
                     the N-bit is set in the options field, the E-bit
                     must be reset.

             P-bit:  The P-bit is used only in the type-7 LSA header.
                     It flags the NSSA area border router to translate
                     the type-7 LSA into a type-5 LSA.




Coltun & Fuller                                                [Page 16]
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RFC 1587                    OSPF NSSA Option                  March 1994


Appendix C:  Configuration Parameters

   Appendix C.2 in the OSPF specification lists the area parameters.
   The area ID, list of address ranges for type-3 summary routes and
   authentication type remain unchanged.  Section 3.2 of this document
   lists the configuration parameters for type-7 address ranges.

   For NSSAs the external capabilities of the area must be set to accept
   type-7 external routes.  Additionally there must be a way of
   configuring the NSSA area border router to send a default route into
   the NSSA using a specific metric (type-1 or type-2 and the actual
   cost).







































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