path: root/doc/rfc/rfc2338.txt

Network Working Group                                          S. Knight
Request for Comments: 2338                                     D. Weaver
Category: Standards Track                    Ascend Communications, Inc.
                                                              D. Whipple
                                                         Microsoft, Inc.
                                                               R. Hinden
                                                               D. Mitzel
                                                                 P. Hunt
                                                                   Nokia
                                                            P. Higginson
                                                                M. Shand
                                                 Digital Equipment Corp.
                                                               A. Lindem
                                                         IBM Corporation
                                                              April 1998


                   Virtual Router Redundancy Protocol

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 defines the Virtual Router Redundancy Protocol (VRRP).
   VRRP specifies an election protocol that dynamically assigns
   responsibility for a virtual router to one of the VRRP routers on a
   LAN.  The VRRP router controlling the IP address(es) associated with
   a virtual router is called the Master, and forwards packets sent to
   these IP addresses.  The election process provides dynamic fail over
   in the forwarding responsibility should the Master become
   unavailable.  This allows any of the virtual router IP addresses on
   the LAN to be used as the default first hop router by end-hosts.  The
   advantage gained from using VRRP is a higher availability default
   path without requiring configuration of dynamic routing or router
   discovery protocols on every end-host.






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Table of Contents

   1.  Introduction...............................................2
   2.  Required Features..........................................5
   3.  VRRP Overview..............................................6
   4.  Sample Configurations......................................8
   5.  Protocol...................................................9
      5.1  VRRP Packet Format....................................10
      5.2  IP Field Descriptions.................................10
      5.3  VRRP Field Descriptions...............................11
   6.  Protocol State Machine....................................13
      6.1  Parameters............................................13
      6.2  Timers................................................15
      6.3  State Transition Diagram..............................15
      6.4  State Descriptions....................................15
   7.  Sending and Receiving VRRP Packets........................18
      7.1  Receiving VRRP Packets................................18
      7.2  Transmitting Packets..................................19
      7.3  Virtual MAC Address...................................19
   8.  Operational Issues........................................20
      8.1  ICMP Redirects........................................20
      8.2  Host ARP Requests.....................................20
      8.3  Proxy ARP.............................................20
   9.  Operation over FDDI and Token Ring........................21
      9.1  Operation over FDDI...................................21
      9.2  Operation over Token Ring.............................21
   10. Security Considerations...................................23
      10.1  No Authentication....................................23
      10.2  Simple Text Password.................................23
      10.3  IP Authentication Header.............................24
   11. Acknowledgments...........................................24
   12. References................................................24
   13. Authors' Addresses........................................25
   14. Full Copyright Statement..................................27

1.  Introduction

   There are a number of methods that an end-host can use to determine
   its first hop router towards a particular IP destination.  These
   include running (or snooping) a dynamic routing protocol such as
   Routing Information Protocol [RIP] or OSPF version 2 [OSPF], running
   an ICMP router discovery client [DISC] or using a statically
   configured default route.

   Running a dynamic routing protocol on every end-host may be
   infeasible for a number of reasons, including administrative
   overhead, processing overhead, security issues, or lack of a protocol
   implementation for some platforms.  Neighbor or router discovery



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   protocols may require active participation by all hosts on a network,
   leading to large timer values to reduce protocol overhead in the face
   of large numbers of hosts.  This can result in a significant delay in
   the detection of a lost (i.e., dead) neighbor, which may introduce
   unacceptably long "black hole" periods.

   The use of a statically configured default route is quite popular; it
   minimizes configuration and processing overhead on the end-host and
   is supported by virtually every IP implementation.  This mode of
   operation is likely to persist as dynamic host configuration
   protocols [DHCP] are deployed, which typically provide configuration
   for an end-host IP address and default gateway.  However, this
   creates a single point of failure.  Loss of the default router
   results in a catastrophic event, isolating all end-hosts that are
   unable to detect any alternate path that may be available.

   The Virtual Router Redundancy Protocol (VRRP) is designed to
   eliminate the single point of failure inherent in the static default
   routed environment.  VRRP specifies an election protocol that
   dynamically assigns responsibility for a virtual router to one of the
   VRRP routers on a LAN.  The VRRP router controlling the IP
   address(es) associated with a virtual router is called the Master,
   and forwards packets sent to these IP addresses.  The election
   process provides dynamic fail-over in the forwarding responsibility
   should the Master become unavailable.  Any of the virtual router's IP
   addresses on a LAN can then be used as the default first hop router
   by end-hosts.  The advantage gained from using VRRP is a higher
   availability default path without requiring configuration of dynamic
   routing or router discovery protocols on every end-host.

   VRRP provides a function similar to a Cisco Systems, Inc. proprietary
   protocol named Hot Standby Router Protocol (HSRP) [HSRP] and to a
   Digital Equipment Corporation, Inc. proprietary protocol named IP
   Standby Protocol [IPSTB].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC 2119].

   The IESG/IETF take no position regarding the validity or scope of any
   intellectual property right or other rights that might be claimed to
   pertain to the implementation or use of the technology, or the extent
   to which any license under such rights might or might not be
   available.  See the IETF IPR web page at http://www.ietf.org/ipr.html
   for additional information.






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1.1  Scope

   The remainder of this document describes the features, design goals,
   and theory of operation of VRRP.  The message formats, protocol
   processing rules and state machine that guarantee convergence to a
   single Virtual Router Master are presented.  Finally, operational
   issues related to MAC address mapping, handling of ARP requests,
   generation of ICMP redirect messages, and security issues are
   addressed.

   This protocol is intended for use with IPv4 routers only.  A separate
   specification will be produced if it is decided that similar
   functionality is desirable in an IPv6 environment.

1.2  Definitions

   VRRP Router            A router running the Virtual Router Redundancy
                          Protocol.  It may participate in one or more
                          virtual routers.

   Virtual Router         An abstract object managed by VRRP that acts
                          as a default router for hosts on a shared LAN.
                          It consists of a Virtual Router Identifier and
                          a set of associated IP address(es) across a
                          common LAN.  A VRRP Router may backup one or
                          more virtual routers.

   IP Address Owner       The VRRP router that has the virtual router's
                          IP address(es) as real interface address(es).
                          This is the router that, when up, will respond
                          to packets addressed to one of these IP
                          addresses for ICMP pings, TCP connections,
                          etc.

   Primary IP Address     An IP address selected from the set of real
                          interface addresses.  One possible selection
                          algorithm is to always select the first
                          address.  VRRP advertisements are always sent
                          using the primary IP address as the source of
                          the IP packet.

   Virtual Router Master  The VRRP router that is assuming the
                          responsibility of forwarding packets sent to
                          the IP address(es) associated with the virtual
                          router, and answering ARP requests for these
                          IP addresses.  Note that if the IP address
                          owner is available, then it will always become
                          the Master.



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   Virtual Router Backup  The set of VRRP routers available to assume
                          forwarding responsibility for a virtual router
                          should the current Master fail.

2.0 Required Features

   This section outlines the set of features that were considered
   mandatory and that guided the design of VRRP.

2.1 IP Address Backup

   Backup of IP addresses is the primary function of the Virtual Router
   Redundancy Protocol.  While providing election of a Virtual Router
   Master and the additional functionality described below, the protocol
   should strive to:

    - Minimize the duration of black holes.
    - Minimize the steady state bandwidth overhead and processing
      complexity.
    - Function over a wide variety of multiaccess LAN technologies
      capable of supporting IP traffic.
    - Provide for election of multiple virtual routers on a network for
      load balancing
    - Support of multiple logical IP subnets on a single LAN segment.

2.2 Preferred Path Indication

   A simple model of Master election among a set of redundant routers is
   to treat each router with equal preference and claim victory after
   converging to any router as Master.  However, there are likely to be
   many environments where there is a distinct preference (or range of
   preferences) among the set of redundant routers.  For example, this
   preference may be based upon access link cost or speed, router
   performance or reliability, or other policy considerations.  The
   protocol should allow the expression of this relative path preference
   in an intuitive manner, and guarantee Master convergence to the most
   preferential router currently available.

2.3 Minimization of Unnecessary Service Disruptions

   Once Master election has been performed then any unnecessary
   transitions between Master and Backup routers can result in a
   disruption in service.  The protocol should ensure after Master
   election that no state transition is triggered by any Backup router
   of equal or lower preference as long as the Master continues to
   function properly.





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   Some environments may find it beneficial to avoid the state
   transition triggered when a router becomes available that is more
   preferential than the current Master.  It may be useful to support an
   override of the immediate convergence to the preferred path.

2.4 Extensible Security

   The virtual router functionality is applicable to a wide range of
   internetworking environments that may employ different security
   policies.  The protocol should require minimal configuration and
   overhead in the insecure operation, provide for strong authentication
   when increased security is required, and allow integration of new
   security mechanisms without breaking backwards compatible operation.

2.5 Efficient Operation over Extended LANs

   Sending IP packets on a multiaccess LAN requires mapping from an IP
   address to a MAC address.  The use of the virtual router MAC address
   in an extended LAN employing learning bridges can have a significant
   effect on the bandwidth overhead of packets sent to the virtual
   router.  If the virtual router MAC address is never used as the
   source address in a link level frame then the station location is
   never learned, resulting in flooding of all packets sent to the
   virtual router.  To improve the efficiency in this environment the
   protocol should: 1) use the virtual router MAC as the source in a
   packet sent by the Master to trigger station learning; 2) trigger a
   message immediately after transitioning to Master to update the
   station learning; and 3) trigger periodic messages from the Master to
   maintain the station learning cache.

3.0 VRRP Overview

   VRRP specifies an election protocol to provide the virtual router
   function described earlier.  All protocol messaging is performed
   using IP multicast datagrams, thus the protocol can operate over a
   variety of multiaccess LAN technologies supporting IP multicast.
   Each VRRP virtual router has a single well-known MAC address
   allocated to it.  This document currently only details the mapping to
   networks using the IEEE 802 48-bit MAC address.  The virtual router
   MAC address is used as the source in all periodic VRRP messages sent
   by the Master router to enable bridge learning in an extended LAN.

   A virtual router is defined by its virtual router identifier (VRID)
   and a set of IP addresses.  A VRRP router may associate a virtual
   router with its real addresses on an interface, and may also be
   configured with additional virtual router mappings and priority for
   virtual routers it is willing to backup.  The mapping between VRID
   and addresses must be coordinated among all VRRP routers on a LAN.



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   However, there is no restriction against reusing a VRID with a
   different address mapping on different LANs.  The scope of each
   virtual router is restricted to a single LAN.

   To minimize network traffic, only the Master for each virtual router
   sends periodic VRRP Advertisement messages.  A Backup router will not
   attempt to pre-empt the Master unless it has higher priority.  This
   eliminates service disruption unless a more preferred path becomes
   available.  It's also possible to administratively prohibit all pre-
   emption attempts.  The only exception is that a VRRP router will
   always become Master of any virtual router associated with addresses
   it owns.  If the Master becomes unavailable then the highest priority
   Backup will transition to Master after a short delay, providing a
   controlled transition of the virtual router responsibility with
   minimal service interruption.

   VRRP defines three types of authentication providing simple
   deployment in insecure environments, added protection against
   misconfiguration, and strong sender authentication in security
   conscious environments.  Analysis of the protection provided and
   vulnerability of each mechanism is deferred to Section 10.0 Security
   Considerations.  In addition new authentication types and data can be
   defined in the future without affecting the format of the fixed
   portion of the protocol packet, thus preserving backward compatible
   operation.

   The VRRP protocol design provides rapid transition from Backup to
   Master to minimize service interruption, and incorporates
   optimizations that reduce protocol complexity while guaranteeing
   controlled Master transition for typical operational scenarios.  The
   optimizations result in an election protocol with minimal runtime
   state requirements, minimal active protocol states, and a single
   message type and sender.  The typical operational scenarios are
   defined to be two redundant routers and/or distinct path preferences
   among each router.  A side effect when these assumptions are violated
   (i.e., more than two redundant paths all with equal preference) is
   that duplicate packets may be forwarded for a brief period during
   Master election.  However, the typical scenario assumptions are
   likely to cover the vast majority of deployments, loss of the Master
   router is infrequent, and the expected duration in Master election
   convergence is quite small ( << 1 second ).  Thus the VRRP
   optimizations represent significant simplifications in the protocol
   design while incurring an insignificant probability of brief network
   degradation.







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4.  Sample Configurations

4.1  Sample Configuration 1

   The following figure shows a simple network with two VRRP routers
   implementing one virtual router.  Note that this example is provided
   to help understand the protocol, but is not expected to occur in
   actual practice.

                  +-----+      +-----+
                  | MR1 |      | BR1 |
                  |     |      |     |
                  |     |      |     |
     VRID=1       +-----+      +-----+
     IP A ---------->*            *<--------- IP B
                     |            |
                     |            |
                     |            |
   ------------------+------------+-----+--------+--------+--------+--
                                        ^        ^        ^        ^
                                        |        |        |        |
                                      (IP A)   (IP A)   (IP A)   (IP A)
                                        |        |        |        |
                                     +--+--+  +--+--+  +--+--+  +--+--+
                                     |  H1 |  |  H2 |  |  H3 |  |  H4 |
                                     +-----+  +-----+  +--+--+  +--+--+

  Legend:
           ---+---+---+--  =  Ethernet, Token Ring, or FDDI
                        H  =  Host computer
                       MR  =  Master Router
                       BR  =  Backup Router
                        *  =  IP Address
                     (IP)  =  default router for hosts

   The above configuration shows a very simple VRRP scenario.  In this
   configuration, the end-hosts install a default route to the IP
   address of virtual router #1 (IP A) and both routers run VRRP.  The
   router on the left becomes the Master for virtual router #1 (VRID=1)
   and the router on the right is the Backup for virtual router #1.  If
   the router on the left should fail, the other router will take over
   virtual router #1 and its IP addresses, and provide uninterrupted
   service for the hosts.

   Note that in this example, IP B is not backed up by the router on the
   left.  IP B is only used by the router on the right as its interface
   address.  In order to backup IP B, a second virtual router would have
   to be configured.  This is shown in the next section.



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4.2  Sample Configuration 2

   The following figure shows a configuration with two virtual routers
   with the hosts spitting their traffic between them.  This example is
   expected to be very common in actual practice.

                  +-----+      +-----+
                  | MR1 |      | MR2 |
                  |  &  |      |  &  |
                  | BR2 |      | BR1 |
     VRID=1       +-----+      +-----+         VRID=2
     IP A ---------->*            *<---------- IP B
                     |            |
                     |            |
                     |            |
   ------------------+------------+-----+--------+--------+--------+--
                                        ^        ^        ^        ^
                                        |        |        |        |
                                      (IP A)   (IP A)   (IP B)   (IP B)
                                        |        |        |        |
                                     +--+--+  +--+--+  +--+--+  +--+--+
                                     |  H1 |  |  H2 |  |  H3 |  |  H4 |
                                     +-----+  +-----+  +--+--+  +--+--+

  Legend:
           ---+---+---+--  =  Ethernet, Token Ring, or FDDI
                        H  =  Host computer
                       MR  =  Master Router
                       BR  =  Backup Router
                        *  =  IP Address
                     (IP)  =  default router for hosts

   In the above configuration, half of the hosts install a default route
   to virtual router #1's IP address (IP A), and the other half of the
   hosts install a default route to virtual router #2's IP address (IP
   B).  This has the effect of load balancing the outgoing traffic,
   while also providing full redundancy.

5.0  Protocol

   The purpose of the VRRP packet is to communicate to all VRRP routers
   the priority and the state of the Master router associated with the
   Virtual Router ID.

   VRRP packets are sent encapsulated in IP packets.  They are sent to
   the IPv4 multicast address assigned to VRRP.





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5.1  VRRP Packet Format

   This section defines the format of the VRRP packet and the relevant
   fields in the IP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Version| Type  | Virtual Rtr ID|   Priority    | Count IP Addrs|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Auth Type   |   Adver Int   |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IP Address (1)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            .                                  |
      |                            .                                  |
      |                            .                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IP Address (n)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Authentication Data (1)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Authentication Data (2)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.2  IP Field Descriptions

5.2.1  Source Address

   The primary IP address of the interface the packet is being sent
   from.

5.2.2  Destination Address

   The IP multicast address as assigned by the IANA for VRRP is:

       224.0.0.18

   This is a link local scope multicast address.  Routers MUST NOT
   forward a datagram with this destination address regardless of its
   TTL.

5.2.3  TTL

   The TTL MUST be set to 255.  A VRRP router receiving a packet with
   the TTL not equal to 255 MUST discard the packet.





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5.2.4  Protocol

   The IP protocol number assigned by the IANA for VRRP is 112
   (decimal).

5.3 VRRP Field Descriptions

5.3.1  Version

   The version field specifies the VRRP protocol version of this packet.
   This document defines version 2.

5.3.2  Type

   The type field specifies the type of this VRRP packet.  The only
   packet type defined in this version of the protocol is:

       1      ADVERTISEMENT

   A packet with unknown type MUST be discarded.

5.3.3  Virtual Rtr ID (VRID)

   The Virtual Router Identifier (VRID) field identifies the virtual
   router this packet is reporting status for.

5.3.4  Priority

   The priority field specifies the sending VRRP router's priority for
   the virtual router.  Higher values equal higher priority.  This field
   is an 8 bit unsigned integer field.

   The priority value for the VRRP router that owns the IP address(es)
   associated with the virtual router MUST be 255 (decimal).

   VRRP routers backing up a virtual router MUST use priority values
   between 1-254 (decimal).  The default priority value for VRRP routers
   backing up a virtual router is 100 (decimal).

   The priority value zero (0) has special meaning indicating that the
   current Master has stopped participating in VRRP.  This is used to
   trigger Backup routers to quickly transition to Master without having
   to wait for the current Master to timeout.

5.3.5  Count IP Addrs

   The number of IP addresses contained in this VRRP advertisement.




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5.3.6  Authentication Type

   The authentication type field identifies the authentication method
   being utilized.  Authentication type is unique on a per interface
   basis.  The authentication type field is an 8 bit unsigned integer.
   A packet with unknown authentication type or that does not match the
   locally configured authentication method MUST be discarded.

   The authentication methods currently defined are:

       0 - No Authentication
       1 - Simple Text Password
       2 - IP Authentication Header

5.3.6.1 No Authentication

   The use of this authentication type means that VRRP protocol
   exchanges are not authenticated.  The contents of the Authentication
   Data field should be set to zero on transmission and ignored on
   reception.

5.3.6.2 Simple Text Password

   The use of this authentication type means that VRRP protocol
   exchanges are authenticated by a clear text password.  The contents
   of the Authentication Data field should be set to the locally
   configured password on transmission.  There is no default password.
   The receiver MUST check that the Authentication Data in the packet
   matches its configured authentication string.  Packets that do not
   match MUST be discarded.

   Note that there are security implications to using Simple Text
   password authentication, and one should see the Security
   Consideration section of this document.

5.3.6.3 IP Authentication Header

   The use of this authentication type means the VRRP protocol exchanges
   are authenticated using the mechanisms defined by the IP
   Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP
   and AH" [HMAC].  Keys may be either configured manually or via a key
   distribution protocol.

   If a packet is received that does not pass the authentication check
   due to a missing authentication header or incorrect message digest,
   then the packet MUST be discarded.  The contents of the
   Authentication Data field should be set to zero on transmission and
   ignored on reception.



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5.3.7 Advertisement Interval (Adver Int)

   The Advertisement interval indicates the time interval (in seconds)
   between ADVERTISEMENTS.  The default is 1 second.  This field is used
   for troubleshooting misconfigured routers.

5.3.8 Checksum

   The checksum field is used to detect data corruption in the VRRP
   message.

   The checksum is the 16-bit one's complement of the one's complement
   sum of the entire VRRP message starting with the version field.  For
   computing the checksum, the checksum field is set to zero.

5.3.9  IP Address(es)

   One or more IP addresses that are associated with the virtual router.
   The number of addresses included is specified in the "Count IP Addrs"
   field.  These fields are used for troubleshooting misconfigured
   routers.

5.3.10  Authentication Data

   The authentication string is currently only utilized for simple text
   authentication, similar to the simple text authentication found in
   the Open Shortest Path First routing protocol [OSPF].  It is up to 8
   characters of plain text.  If the configured authentication string is
   shorter than 8 bytes, the remaining space MUST be zero-filled.  Any
   VRRP packet received with an authentication string that does not
   match the locally configured authentication string MUST be discarded.
   The authentication string is unique on a per interface basis.

   There is no default value for this field.

6.  Protocol State Machine

6.1 Parameters

6.1.1 Parameters per Interface


   Authentication_Type     Type of authentication being used.  Values
                           are defined in section 5.3.6.

   Authentication_Data     Authentication data specific to the
                           Authentication_Type being used.




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6.1.2 Parameters per Virtual Router

   VRID                    Virtual Router Identifier.  Configured item
                           in the range 1-255 (decimal).  There is no
                           default.

   Priority                Priority value to be used by this VRRP
                           router in Master election for this virtual
                           router.  The value of 255 (decimal) is
                           reserved for the router that owns the IP
                           addresses associated with the virtual
                           router.  The value of 0 (zero) is reserved
                           for Master router to indicate it is
                           releasing responsibility for the virtual
                           router.  The range 1-254 (decimal) is
                           available for VRRP routers backing up the
                           virtual router.  The default value is 100
                           (decimal).

   IP_Addresses            One or more IP addresses associated with
                           this virtual router.  Configured item.  No
                           default.

   Advertisement_Interval  Time interval between ADVERTISEMENTS
                           (seconds).  Default is 1 second.

   Skew_Time               Time to skew Master_Down_Interval in
                           seconds.  Calculated as:

                              ( (256 - Priority) / 256 )

   Master_Down_Interval    Time interval for Backup to declare Master
                           down (seconds).  Calculated as:

                              (3 * Advertisement_Interval) + Skew_time

   Preempt_Mode            Controls whether a higher priority Backup
                           router preempts a lower priority Master.
                           Values are True to allow preemption and
                           False to not prohibit preemption.  Default
                           is True.

                           Note: Exception is that the router that owns
                           the IP address(es) associated with the
                           virtual router always pre-empts independent
                           of the setting of this flag.





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6.2 Timers

   Master_Down_Timer       Timer that fires when ADVERTISEMENT has not
                           been heard for Master_Down_Interval.

   Adver_Timer             Timer that fires to trigger sending of
                           ADVERTISEMENT based on
                           Advertisement_Interval.

6.3  State Transition Diagram

                          +---------------+
               +--------->|               |<-------------+
               |          |  Initialize   |              |
               |   +------|               |----------+   |
               |   |      +---------------+          |   |
               |   |                                 |   |
               |   V                                 V   |
       +---------------+                       +---------------+
       |               |---------------------->|               |
       |    Master     |                       |    Backup     |
       |               |<----------------------|               |
       +---------------+                       +---------------+

6.4  State Descriptions

   In the state descriptions below, the state names are identified by
   {state-name}, and the packets are identified by all upper case
   characters.

   A VRRP router implements an instance of the state machine for each
   virtual router election it is participating in.

6.4.1   Initialize

   The purpose of this state is to wait for a Startup event.  If a
   Startup event is received, then:

    - If the Priority = 255 (i.e., the router owns the IP address(es)
      associated with the virtual router)

       o Send an ADVERTISEMENT
       o Broadcast a gratuitous ARP request containing the virtual
         router MAC address for each IP address associated with the
         virtual router.
       o Set the Adver_Timer to Advertisement_Interval
       o Transition to the {Master} state




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      else

       o Set the Master_Down_Timer to Master_Down_Interval
       o Transition to the {Backup} state

      endif

6.4.2   Backup

   The purpose of the {Backup} state is to monitor the availability and
   state of the Master Router.

   While in this state, a VRRP router MUST do the following:

    - MUST NOT respond to ARP requests for the IP address(s) associated
      with the virtual router.

    - MUST discard packets with a destination link layer MAC address
      equal to the virtual router MAC address.

    - MUST NOT accept packets addressed to the IP address(es) associated
      with the virtual router.

    - If a Shutdown event is received, then:

       o Cancel the Master_Down_Timer
       o Transition to the {Initialize} state

      endif

    - If the Master_Down_Timer fires, then:

       o Send an ADVERTISEMENT
       o Broadcast a gratuitous ARP request containing the virtual
         router MAC address for each IP address associated with the
         virtual router
       o Set the Adver_Timer to Advertisement_Interval
       o Transition to the {Master} state

      endif

    - If an ADVERTISEMENT is received, then:

         If the Priority in the ADVERTISEMENT is Zero, then:

          o Set the Master_Down_Timer to Skew_Time

         else:



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            If Preempt_Mode is False, or If the Priority in the
            ADVERTISEMENT is greater than or equal to the local
            Priority, then:

             o Reset the Master_Down_Timer to Master_Down_Interval

            else:

             o Discard the ADVERTISEMENT

            endif
         endif
      endif

6.4.3   Master

   While in the {Master} state the router functions as the forwarding
   router for the IP address(es) associated with the virtual router.

   While in this state, a VRRP router MUST do the following:

    - MUST respond to ARP requests for the IP address(es) associated
      with the virtual router.

    - MUST forward packets with a destination link layer MAC address
      equal to the virtual router MAC address.

    - MUST NOT accept packets addressed to the IP address(es) associated
      with the virtual router if it is not the IP address owner.

    - MUST accept packets addressed to the IP address(es) associated
      with the virtual router if it is the IP address owner.

    - If a Shutdown event is received, then:

       o Cancel the Adver_Timer
       o Send an ADVERTISEMENT with Priority = 0
       o Transition to the {Initialize} state

      endif

    - If the Adver_Timer fires, then:

       o Send an ADVERTISEMENT
       o Reset the Adver_Timer to Advertisement_Interval

      endif




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    - If an ADVERTISEMENT is received, then:

         If the Priority in the ADVERTISEMENT is Zero, then:

          o Send an ADVERTISEMENT
          o Reset the Adver_Timer to Advertisement_Interval

         else:

            If the Priority in the ADVERTISEMENT is greater than the
            local Priority,
            or
            If the Priority in the ADVERTISEMENT is equal to the local
            Priority and the primary IP Address of the sender is greater
            than the local primary IP Address, then:

             o Cancel Adver_Timer
             o Set Master_Down_Timer to Master_Down_Interval
             o Transition to the {Backup} state

            else:

             o Discard ADVERTISEMENT

            endif
         endif
      endif

7.  Sending and Receiving VRRP Packets

7.1  Receiving VRRP Packets

   Performed the following functions when a VRRP packet is received:

      - MUST verify that the IP TTL is 255.
      - MUST verify the VRRP version
      - MUST verify that the received packet length is greater than or
        equal to the VRRP header
      - MUST verify the VRRP checksum
      - MUST perform authentication specified by Auth Type

   If any one of the above checks fails, the receiver MUST discard the
   packet, SHOULD log the event and MAY indicate via network management
   that an error occurred.

      - MUST verify that the VRID is valid on the receiving interface

   If the above check fails, the receiver MUST discard the packet.



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      - MAY verify that the IP address(es) associated with the VRID are
        valid

   If the above check fails, the receiver SHOULD log the event and MAY
   indicate via network management that a misconfiguration was detected.
   If the packet was not generated by the address owner (Priority does
   not equal 255 (decimal)), the receiver MUST drop the packet,
   otherwise continue processing.

      - MUST verify that the Adver Interval in the packet is the same as
        the locally configured for this virtual router

   If the above check fails, the receiver MUST discard the packet,
   SHOULD log the event and MAY indicate via network management that a
   misconfiguration was detected.

7.2 Transmitting VRRP Packets

   The following operations MUST be performed when transmitting a VRRP
   packet.

      - Fill in the VRRP packet fields with the appropriate virtual
        router configuration state
      - Compute the VRRP checksum
      - Set the source MAC address to Virtual Router MAC Address
      - Set the source IP address to interface primary IP address
      - Set the IP protocol to VRRP
      - Send the VRRP packet to the VRRP IP multicast group

   Note: VRRP packets are transmitted with the virtual router MAC
   address as the source MAC address to ensure that learning bridges
   correctly determine the LAN segment the virtual router is attached
   to.

7.3 Virtual Router MAC Address

   The virtual router MAC address associated with a virtual router is an
   IEEE 802 MAC Address in the following format:

      00-00-5E-00-01-{VRID} (in hex in internet standard bit-order)

   The first three octets are derived from the IANA's OUI.  The next two
   octets (00-01) indicate the address block assigned to the VRRP
   protocol.  {VRID} is the VRRP Virtual Router Identifier.  This
   mapping provides for up to 255 VRRP routers on a network.






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8.  Operational Issues

8.1 ICMP Redirects

   ICMP Redirects may be used normally when VRRP is running between a
   group of routers.  This allows VRRP to be used in environments where
   the topology is not symmetric.

   The IP source address of an ICMP redirect should be the address the
   end host used when making its next hop routing decision.  If a VRRP
   router is acting as Master for virtual router(s) containing addresses
   it does not own, then it must determine which virtual router the
   packet was sent to when selecting the redirect source address.  One
   method to deduce the virtual router used is to examine the
   destination MAC address in the packet that triggered the redirect.

   It may be useful to disable Redirects for specific cases where VRRP
   is being used to load share traffic between a number of routers in a
   symmetric topology.

8.2  Host ARP Requests

   When a host sends an ARP request for one of the virtual router IP
   addresses, the Master virtual router MUST respond to the ARP request
   with the virtual MAC address for the virtual router.  The Master
   virtual router MUST NOT respond with its physical MAC address.  This
   allows the client to always use the same MAC address regardless of
   the current Master router.

   When a VRRP router restarts or boots, it SHOULD not send any ARP
   messages with its physical MAC address for the IP address it owns, it
   should only send ARP messages that include Virtual MAC addresses.
   This may entail:

    - When configuring an interface, VRRP routers should broadcast a
      gratuitous ARP request containing the virtual router MAC address
      for each IP address on that interface.

    - At system boot, when initializing interfaces for VRRP operation;
      delay gratuitous ARP requests and ARP responses until both the IP
      address and the virtual router MAC address are configured.

8.3 Proxy ARP

   If Proxy ARP is to be used on a VRRP router, then the VRRP router
   must advertise the Virtual Router MAC address in the Proxy ARP
   message.  Doing otherwise could cause hosts to learn the real MAC
   address of the VRRP router.



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9.  Operation over FDDI and Token Ring

9.1 Operation over FDDI

   FDDI interfaces remove from the FDDI ring frames that have a source
   MAC address matching the device's hardware address.  Under some
   conditions, such as router isolations, ring failures, protocol
   transitions, etc., VRRP may cause there to be more than one Master
   router.  If a Master router installs the virtual router MAC address
   as the hardware address on a FDDI device, then other Masters'
   ADVERTISEMENTS will be removed from the ring during the Master
   convergence, and convergence will fail.

   To avoid this an implementation SHOULD configure the virtual router
   MAC address by adding a unicast MAC filter in the FDDI device, rather
   than changing its hardware MAC address.  This will prevent a Master
   router from removing any ADVERTISEMENTS it did not originate.

9.2  Operation over Token Ring

   Token ring has several characteristics which make running VRRP
   difficult. These include:

    - In order to switch to a new master located on a different bridge
      token ring segment from the previous master when using source
      route bridges, a mechanism is required to update cached source
      route information.

    - No general multicast mechanism supported across old and new token
      ring adapter implementations. While many newer token ring adapters
      support group addresses, token ring functional address support is
      the only generally available multicast mechanism. Due to the
      limited number of token ring functional addresses these may
      collide with other usage of the same token ring functional
      addresses.

   Due to these difficulties, the preferred mode of operation over token
   ring will be to use a token ring functional address for the VRID
   virtual MAC address. Token ring functional addresses have the two
   high order bits in the first MAC address octet set to B'1'.  They
   range from 03-00-00-00-00-80 to 03-00-02-00-00-00 (canonical format).
   However, unlike multicast addresses, there is only one unique
   functional address per bit position. The functional addresses
   addresses  03-00-00-10-00-00 through 03-00-02-00-00-00 are reserved
   by the Token Ring Architecture [TKARCH] for user-defined
   applications.  However, since there are only 12 user-defined token
   ring functional addresses, there may be other non-IP protocols using
   the same functional address. Since the Novell IPX [IPX] protocol uses



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   the 03-00-00-10-00-00 functional address, operation of VRRP over
   token ring will avoid use of this functional address. In general,
   token ring VRRP users will be responsible for resolution of other
   user-defined token ring functional address conflicts.

   VRIDs are mapped directly to token ring functional addresses. In
   order to decrease the likelihood of functional address conflicts,
   allocation will begin with the largest functional address. Most non-
   IP protocols use the first or first couple user-defined functional
   addresses and it is expected that VRRP users will choose VRIDs
   sequentially starting with 1.

   VRID      Token Ring Functional Address
   ----      -----------------------------
      1             03-00-02-00-00-00
      2             03-00-04-00-00-00
      3             03-00-08-00-00-00
      4             03-00-10-00-00-00
      5             03-00-20-00-00-00
      6             03-00-40-00-00-00
      7             03-00-80-00-00-00
      8             03-00-00-01-00-00
      9             03-00-00-02-00-00
     10             03-00-00-04-00-00
     11             03-00-00-08-00-00

   Or more succinctly, octets 3 and 4 of the functional address are
   equal to (0x4000 >> (VRID - 1)) in non-canonical format.

   Since a functional address cannot be used used as a MAC level source
   address, the real MAC address is used as the MAC source address in
   VRRP advertisements. This is not a problem for bridges since packets
   addressed to functional addresses will be sent on the spanning-tree
   explorer path [802.1D].

   The functional address mode of operation MUST be implemented by
   routers supporting VRRP on token ring.

   Additionally, routers MAY support unicast mode of operation to take
   advantage of newer token ring adapter implementations which support
   non-promiscuous reception for multiple unicast MAC addresses and to
   avoid both the multicast traffic and usage conflicts associated with
   the use of token ring functional addresses. Unicast mode uses the
   same mapping of VRIDs to virtual MAC addresses as Ethernet.  However,
   one important difference exists. ARP request/reply packets contain
   the virtual MAC address as the source MAC address. The reason for
   this is that some token ring driver implementations keep a cache of
   MAC address/source routing information independent of the ARP cache.



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   Hence, these implementations need have to receive a packet with the
   virtual MAC address as the source address in order to transmit to
   that MAC address in a source-route bridged network.

   Unicast mode on token ring has one limitation which should be
   considered.  If there are VRID routers on different source-route
   bridge segments and there are host implementations which keep their
   source-route information in the ARP cache and do not listen to
   gratuitous ARPs, these hosts will not update their ARP source-route
   information correctly when a switch-over occurs. The only possible
   solution is to put all routers with the same VRID on the same source-
   bridge segment and use techniques to prevent that bridge segment from
   being a single point of failure. These techniques are beyond the
   scope this document.

   For both the multicast and unicast mode of operation, VRRP
   advertisements sent to 224.0.0.18 should be encapsulated as described
   in [RFC1469].

10. Security Considerations

   VRRP is designed for a range of internetworking environments that may
   employ different security policies.  The protocol includes several
   authentication methods ranging from no authentication, simple clear
   text passwords, and strong authentication using IP Authentication
   with MD5 HMAC.  The details on each approach including possible
   attacks and recommended environments follows.

   Independent of any authentication type VRRP includes a mechanism
   (setting TTL=255, checking on receipt) that protects against VRRP
   packets being injected from another remote network.  This limits most
   vulnerabilities to local attacks.

10.1 No Authentication

   The use of this authentication type means that VRRP protocol
   exchanges are not authenticated.  This type of authentication SHOULD
   only be used in environments were there is minimal security risk and
   little chance for configuration errors (e.g., two VRRP routers on a
   LAN).

10.2 Simple Text Password

   The use of this authentication type means that VRRP protocol
   exchanges are authenticated by a simple clear text password.






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   This type of authentication is useful to protect against accidental
   misconfiguration of routers on a LAN.  It protects against routers
   inadvertently backing up another router.  A new router must first be
   configured with the correct password before it can run VRRP with
   another router.  This type of authentication does not protect against
   hostile attacks where the password can be learned by a node snooping
   VRRP packets on the LAN.  The Simple Text Authentication combined
   with the TTL check makes it difficult for a VRRP packet to be sent
   from another LAN to disrupt VRRP operation.

   This type of authentication is RECOMMENDED when there is minimal risk
   of nodes on a LAN actively disrupting VRRP operation.  If this type
   of authentication is used the user should be aware that this clear
   text password is sent frequently, and therefore should not be the
   same as any security significant password.

10.3 IP Authentication Header

   The use of this authentication type means the VRRP protocol exchanges
   are authenticated using the mechanisms defined by the IP
   Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP
   and AH", [HMAC].  This provides strong protection against
   configuration errors, replay attacks, and packet
   corruption/modification.

   This type of authentication is RECOMMENDED when there is limited
   control over the administration of nodes on a LAN.  While this type
   of authentication does protect the operation of VRRP, there are other
   types of attacks that may be employed on shared media links (e.g.,
   generation of bogus ARP replies) which are independent from VRRP and
   are not protected.

11. Acknowledgments

   The authors would like to thank Glen Zorn, and Michael Lane, Clark
   Bremer, Hal Peterson, Tony Li, Barbara Denny, Joel Halpern, Steve
   Bellovin, and Thomas Narten for their comments and suggestions.

12.  References

   [802.1D]  International Standard ISO/IEC 10038: 1993, ANSI/IEEE Std
             802.1D, 1993 edition.

   [AUTH]    Kent, S., and R. Atkinson, "IP Authentication Header",
             Work in Progress.

   [DISC]    Deering, S., "ICMP Router Discovery Messages", RFC 1256,
             September 1991.



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   [DHCP]    Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
             March 1997.

   [HMAC]    Madson, C., and R. Glenn, "The Use of HMAC-MD5-96 within
             ESP and AH", Work in Progress.

   [HSRP]    Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot Standby
             Router Protocol (HSRP)", RFC 2281, March 1998.

   [IPSTB]   Higginson, P., M. Shand, "Development of Router Clusters to
             Provide Fast Failover in IP Networks", Digital Technical
             Journal, Volume 9 Number 3, Winter 1997.

   [IPX]     Novell Incorporated., "IPX Router Specification", Version
             1.10, October 1992.

   [OSPF]    Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RIP]     Hedrick, C., "Routing Information Protocol", RFC 1058,
             June 1988.

   [RFC1469] Pusateri, T., "IP over Token Ring LANs", RFC 1469, June
             1993.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [TKARCH]  IBM Token-Ring Network, Architecture Reference, Publication
             SC30-3374-02, Third Edition, (September, 1989).

13. Authors' Addresses

   Steven Knight                        Phone: +1 612 943-8990
   Ascend Communications                EMail: Steven.Knight@ascend.com
   High Performance Network Division
   10250 Valley View Road, Suite 113
   Eden Prairie, MN USA 55344
   USA

   Douglas Weaver                       Phone: +1 612 943-8990
   Ascend Communications                EMail: Doug.Weaver@ascend.com
   High Performance Network Division
   10250 Valley View Road, Suite 113
   Eden Prairie, MN USA 55344
   USA






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   David Whipple                        Phone: +1 206 703-3876
   Microsoft Corporation                EMail: dwhipple@microsoft.com
   One Microsoft Way
   Redmond, WA USA 98052-6399
   USA

   Robert Hinden                        Phone: +1 408 990-2004
   Nokia                                EMail: hinden@iprg.nokia.com
   232 Java Drive
   Sunnyvale, CA 94089
   USA

   Danny Mitzel                         Phone: +1 408 990-2037
   Nokia                                EMail: mitzel@iprg.nokia.com
   232 Java Drive
   Sunnyvale, CA 94089
   USA

   Peter Hunt                           Phone: +1 408 990-2093
   Nokia                                EMail: hunt@iprg.nokia.com
   232 Java Drive
   Sunnyvale, CA 94089
   USA

   P. Higginson                         Phone: +44 118 920 6293
   Digital Equipment Corp.              EMail: higginson@mail.dec.com
   Digital Park
   Imperial Way
   Reading
   Berkshire
   RG2 0TE
   UK

   M. Shand                             Phone: +44 118 920 4424
   Digital Equipment Corp.              EMail: shand@mail.dec.com
   Digital Park
   Imperial Way
   Reading
   Berkshire
   RG2 0TE
   UK

   Acee Lindem                          Phone: 1-919-254-1805
   IBM Corporation                      E-Mail: acee@raleigh.ibm.com
   P.O. Box 12195
   Research Triangle Park, NC  27709
   USA




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14.  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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