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Versions: (draft-nadas-vrrp-unified-spec) 00 01 02 03 04 05 RFC 5798

VRRP                                                       S. Nadas, Ed.
Internet-Draft                                                  Ericsson
Intended status: Standards Track                          March 19, 2008
Expires: September 20, 2008


     Virtual Router Redundancy Protocol Version 3 for IPv4 and IPv6
                    draft-ietf-vrrp-unified-spec-01

Status of this Memo

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   This Internet-Draft will expire on September 20, 2008.

Abstract

   This memo defines the Virtual Router Redundancy Protocol (VRRP) for
   IPv4 and IPv6.  It is version three (3) of the protocol and it is
   based on VRRP (version 2) for IPv4 that is defined in RFC 3768 and on
   draft-ieft-vrrp-ipv6-spec-08.txt.  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
   IPv4 or IPv6 address(es) associated with a virtual router is called
   the Master, and forwards packets sent to these IPv4 or IPv6
   addresses.  VRRP Master routers are configured with virtual IPv4 or
   IPv6 addresses and VRRP Backup routers infer the address family of
   the virtual addresses being carried based on the transport protocol.
   Within a VRRP router the virtual routers in each of the IPv4 and IPv6



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   address families are a domain unto themselves and do not overlap.
   The election process provides dynamic fail over in the forwarding
   responsibility should the Master become unavailable.  For IPv4, 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.  For IPv6, the advantage
   gained from using VRRP for IPv6 is a quicker switch over to back up
   routers than can be obtained with standard IPv6 Neighbor Discover
   (RFC 4861) mechanisms.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  A Note on Terminology  . . . . . . . . . . . . . . . . . .  5
     1.2.  IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Requirements Language  . . . . . . . . . . . . . . . . . .  7
     1.5.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     1.6.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  7
   2.  Required Features  . . . . . . . . . . . . . . . . . . . . . .  8
     2.1.  IPvX Address Backup  . . . . . . . . . . . . . . . . . . .  8
     2.2.  Preferred Path Indication  . . . . . . . . . . . . . . . .  9
     2.3.  Minimization of Unnecessary Service Disruptions  . . . . .  9
     2.4.  Efficient Operation over Extended LANs . . . . . . . . . .  9
     2.5.  Sub-second Operation for IPv4 and IPv6 . . . . . . . . . . 10
   3.  VRRP Overview  . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Sample Configurations  . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Sample Configuration 1 . . . . . . . . . . . . . . . . . . 11
     4.2.  Sample Configuration 2 . . . . . . . . . . . . . . . . . . 13
   5.  Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  VRRP Packet Format . . . . . . . . . . . . . . . . . . . . 15
       5.1.1.  IPv4 Field Descriptions  . . . . . . . . . . . . . . . 15
         5.1.1.1.  Source Address . . . . . . . . . . . . . . . . . . 16
         5.1.1.2.  Destination Address  . . . . . . . . . . . . . . . 16
         5.1.1.3.  TTL  . . . . . . . . . . . . . . . . . . . . . . . 16
         5.1.1.4.  Protocol . . . . . . . . . . . . . . . . . . . . . 16
       5.1.2.  IPv6 Field Descriptions  . . . . . . . . . . . . . . . 16
         5.1.2.1.  Source Address . . . . . . . . . . . . . . . . . . 16
         5.1.2.2.  Destination Address  . . . . . . . . . . . . . . . 16
         5.1.2.3.  Hop Limit  . . . . . . . . . . . . . . . . . . . . 16
         5.1.2.4.  Next Header  . . . . . . . . . . . . . . . . . . . 17
     5.2.  VRRP Field Descriptions  . . . . . . . . . . . . . . . . . 17
       5.2.1.  Version  . . . . . . . . . . . . . . . . . . . . . . . 17
       5.2.2.  Type . . . . . . . . . . . . . . . . . . . . . . . . . 17
       5.2.3.  Virtual Rtr ID (VRID)  . . . . . . . . . . . . . . . . 17
       5.2.4.  Priority . . . . . . . . . . . . . . . . . . . . . . . 17
       5.2.5.  Count IPvX Addr  . . . . . . . . . . . . . . . . . . . 17



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       5.2.6.  Rsvd . . . . . . . . . . . . . . . . . . . . . . . . . 18
       5.2.7.  Maximum Advertisement Interval (Max Adver Int) . . . . 18
       5.2.8.  Checksum . . . . . . . . . . . . . . . . . . . . . . . 18
       5.2.9.  IPvX Address(es) . . . . . . . . . . . . . . . . . . . 18
   6.  Protocol State Machine . . . . . . . . . . . . . . . . . . . . 19
     6.1.  Parameters per Virtual Router  . . . . . . . . . . . . . . 19
     6.2.  Timers . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.3.  State Transition Diagram . . . . . . . . . . . . . . . . . 21
     6.4.  State Descriptions . . . . . . . . . . . . . . . . . . . . 21
       6.4.1.  Initialize . . . . . . . . . . . . . . . . . . . . . . 21
       6.4.2.  Backup . . . . . . . . . . . . . . . . . . . . . . . . 22
       6.4.3.  Master . . . . . . . . . . . . . . . . . . . . . . . . 25
   7.  Sending and Receiving VRRP Packets . . . . . . . . . . . . . . 27
     7.1.  Receiving VRRP Packets . . . . . . . . . . . . . . . . . . 27
     7.2.  Transmitting VRRP Packets  . . . . . . . . . . . . . . . . 28
     7.3.  Virtual Router MAC Address . . . . . . . . . . . . . . . . 29
     7.4.  IPv6 Interface Identifiers . . . . . . . . . . . . . . . . 29
   8.  Operational Issues . . . . . . . . . . . . . . . . . . . . . . 30
     8.1.  IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
       8.1.1.  ICMP Redirects . . . . . . . . . . . . . . . . . . . . 30
       8.1.2.  Host ARP Requests  . . . . . . . . . . . . . . . . . . 30
       8.1.3.  Proxy ARP  . . . . . . . . . . . . . . . . . . . . . . 31
     8.2.  IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
       8.2.1.  ICMPv6 Redirects . . . . . . . . . . . . . . . . . . . 31
       8.2.2.  ND Neighbor Solicitation . . . . . . . . . . . . . . . 31
       8.2.3.  Router Advertisements  . . . . . . . . . . . . . . . . 32
     8.3.  IPvX . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
       8.3.1.  Potential Forwarding Loop  . . . . . . . . . . . . . . 32
       8.3.2.  Recommendations regarding setting priority values  . . 33
   9.  Operation over FDDI, Token Ring, and ATM LANE  . . . . . . . . 33
     9.1.  Operation over FDDI  . . . . . . . . . . . . . . . . . . . 33
     9.2.  Operation over Token Ring  . . . . . . . . . . . . . . . . 33
     9.3.  Operation over ATM LANE  . . . . . . . . . . . . . . . . . 35
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 35
   11. Intellectual Property Rights Claimed . . . . . . . . . . . . . 37
   12. Contributors & Acknowledgments . . . . . . . . . . . . . . . . 37
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 38
     14.2. Informative References . . . . . . . . . . . . . . . . . . 39
   Appendix A.  VRRPv3 and VRRPv2 Interoperation  . . . . . . . . . . 40
     A.1.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . 40
     A.2.  VRRPv3 support of VRRPv2 . . . . . . . . . . . . . . . . . 40
     A.3.  VRRPv3 support of VRRPv2 Considerations  . . . . . . . . . 40
       A.3.1.  Slow, High-Priority Masters  . . . . . . . . . . . . . 40
       A.3.2.  Overwhelming VRRPv2 Backups  . . . . . . . . . . . . . 40
   Appendix B.  Changes from draft-ietf-vrrp-unified-spec-00  . . . . 41
   Appendix C.  Changes from [ietf-vrrp-unified-spec] and



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                [RFC3768] . . . . . . . . . . . . . . . . . . . . . . 41
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 42
   Intellectual Property and Copyright Statements . . . . . . . . . . 44
















































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1.  Introduction

   This memo defines the Virtual Router Redundancy Protocol (VRRP) for
   IPv4 and IPv6.  It is version three (3) of the protocol.  It is based
   on VRRP (version 2) for IPv4 that is defined in [RFC3768] and on
   [I-D.ietf-vrrp-ipv6-spec].  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 IPv4 or IPv6
   address(es) associated with a virtual router is called the Master,
   and forwards packets sent to these IPv4 or IPv6 addresses.  VRRP
   Master routers are configured with virtual IPv4 or IPv6 addresses and
   VRRP Backup routers infer the address family of the virtual addresses
   being carried based on the transport protocol.  Within a VRRP router
   the virtual routers in each of the IPv4 and IPv6 address families are
   a domain unto themselves and do not overlap.  The election process
   provides dynamic fail over in the forwarding responsibility should
   the Master become unavailable.

   Comments are solicited and should be addressed to the working group's
   mailing list at vrrp@ietf.org and/or the editor.

   VRRP provides a function similar to the proprietary protocols "Hot
   Standby Router Protocol (HSRP)" [RFC2281] and "IP Standby Protocol"
   [IPSTB].

1.1.  A Note on Terminology

   This draft discusses both IPv4 and IPv6 operation and with respect to
   the VRRP protocol, many of the descriptions and procedures are
   common.  In this draft, it would be less verbose to be able refer to
   "IP" to mean either "IPv4 or IPv6".  However, historically, the term
   "IP" usually refers to IPv4.  For this reason, in this specification,
   the term "IPvX" (where X is 4 or 6) is introduced to mean either
   "IPv4 or IPv6", in text where the IP version matters, the appropriate
   term is used and the use of the term "IP" is avoided.

1.2.  IPv4

   There are a number of methods that an IPv4 end-host can use to
   determine its first hop router towards a particular IPv4 destination.
   These include running (or snooping) a dynamic routing protocol such
   as Routing Information Protocol [RFC2453] or OSPF version 2
   [RFC2328], running an ICMP router discovery client [RFC1256] 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



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   implementation for some platforms.  Neighbor or router discovery
   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, that 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 IPv4 implementation.  This mode of
   operation is likely to persist as dynamic host configuration
   protocols [RFC2131] are deployed, which typically provide
   configuration for an end-host IPv4 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 IPv4
   address(es) associated with a virtual router is called the Master,
   and forwards packets sent to these IPv4 addresses.  The election
   process provides dynamic fail-over in the forwarding responsibility
   should the Master become unavailable.  Any of the virtual router's
   IPv4 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.

1.3.  IPv6

   IPv6 hosts on a LAN will usually learn about one or more default
   routers by receiving Router Advertisements sent using the IPv6
   Neighbor Discovery protocol [RFC4861].  The Router Advertisements are
   multicast periodically at a rate that the hosts will learn about the
   default routers in a few minutes.  They are not sent frequently
   enough to rely on the absence of the router advertisement to detect
   router failures.

   Neighbor Discovery (ND) includes a mechanism called Neighbor
   Unreachability Detection to detect the failure of a neighbor node
   (router or host) or the forwarding path to a neighbor.  This is done
   by sending unicast ND Neighbor Solicitation messages to the neighbor
   node.  To reduce the overhead of sending Neighbor Solicitations, they
   are only sent to neighbors to which the node is actively sending
   traffic and only after there has been no positive indication that the



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   router is up for a period of time.  Using the default parameters in
   ND, it will take a host about 38 seconds to learn that a router is
   unreachable before it will switch to another default router.  This
   delay would be very noticeable to users and cause some transport
   protocol implementations to timeout.

   While the ND unreachability detection could be speeded up by changing
   the parameters to be more aggressive (note that the current lower
   limit for this is 5 seconds), this would have the downside of
   significantly increasing the overhead of ND traffic.  Especially when
   there are many hosts all trying to determine the reachability of one
   of more routers.

   The Virtual Router Redundancy Protocol for IPv6 provides a much
   faster switch over to an alternate default router than can be
   obtained using standard ND procedures.  Using VRRP a backup router
   can take over for a failed default router in around three seconds
   (using VRRP default parameters).  This is done with out any
   interaction with the hosts and a minimum amount of VRRP traffic.

1.4.  Requirements Language

   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 [RFC2119].

1.5.  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.

1.6.  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 either a set of associated
                           IPv4 addresses or a set of associated IPv6
                           addresses across a common LAN.  A VRRP Router



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                           may backup one or more virtual routers.

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

   Primary IP Address      In IPv4, an IPv4 address selected from the
                           set of real interface addresses.  One
                           possible selection algorithm is to always
                           select the first address.  In IPv4 mode, VRRP
                           advertisements are always sent using the
                           primary IPv4 address as the source of the
                           IPv4 packet.  In IPv6, the link-local address
                           of the interface over which the packet is
                           transmitted is used.

   Virtual Router Master   The VRRP router that is assuming the
                           responsibility of forwarding packets sent to
                           the IPvX address(es) associated with the
                           virtual router, and answering ARP requests
                           for these IPv4 address(es) or and answering
                           ND requests for these IPv6 address(es).  Note
                           that if the IPvX address owner is available,
                           then it will always become the Master.

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


2.  Required Features

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

2.1.  IPvX Address Backup

   Backup of an IPvX address(es) 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:

   o  Minimize the duration of black holes.





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   o  Minimize the steady state bandwidth overhead and processing
      complexity.

   o  Function over a wide variety of multiaccess LAN technologies
      capable of supporting IPvX traffic.

   o  Provide for election of multiple virtual routers on a network for
      load balancing.

   o  Support of multiple logical IPvX 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.

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

2.4.  Efficient Operation over Extended LANs

   Sending either IPvX packets on a multiaccess LAN requires mapping
   from an IPvX 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



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   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.

2.5.  Sub-second Operation for IPv4 and IPv6

   Sub-second detection of Master VRRP router failure is needed in both
   IPv4 and IPv6 environments.  In [I-D.ietf-vrrp-ipv6-spec], sub-second
   operation was defined for IPv6; this specification extends that
   support for IPv4.


3.  VRRP Overview

   VRRP specifies an election protocol to provide the virtual router
   function described earlier.  All protocol messaging is performed
   using either IPv4 or IPv6 multicast datagrams, thus the protocol can
   operate over a variety of multiaccess LAN technologies supporting
   IPvX 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 either IPv4 or IPv6 address(es).  A VRRP router may
   associate a virtual router with its real address 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 its IPvX address(es) must be coordinated among all
   VRRP routers on a LAN.  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.  Note
   that there is no restriction against using the same virtual router
   identifier number for a set of IPv4 addresses and a set of IPv6
   addresses; however, these are two different virtual routers.

   To minimize network traffic, only the Master for each virtual router
   sends periodic VRRP Advertisement messages.  A Backup router will not
   attempt to preempt 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
   preemption 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



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   Backup will transition to Master after a short delay, providing a
   controlled transition of the virtual router responsibility with
   minimal service interruption.

   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.


4.  Sample Configurations

4.1.  Sample Configuration 1

   The following figure shows a simple network with two VRRP routers
   implementing one virtual router.





















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            +-----------+ +-----------+
            |   Rtr1    | |   Rtr2    |
            |(MR VRID=1)| |(BR VRID=1)|
            |           | |           |
    VRID=1  +-----------+ +-----------+
    IPvX A--------->*            *<---------IPvX B
                    |            |
                    |            |
  ------------------+------------+-----+--------+--------+--------+--
                                       ^        ^        ^        ^
                                       |        |        |        |
                                    (IPvX A) (IPvX A) (IPvX A) (IPvX A)
                                       |        |        |        |
                                    +--+--+  +--+--+  +--+--+  +--+--+
                                    |  H1 |  |  H2 |  |  H3 |  |  H4 |
                                    +-----+  +-----+  +--+--+  +--+--+
     Legend:
           --+---+---+-- = Ethernet, Token Ring, or FDDI
                       H = Host computer
                      MR = Master Router
                      BR = Backup Router
                      *  =  IPvX Address, X is 4 everywhere in IPv4 case
                                          X is 6 everywhere in IPv6 case
                      (IPvX) = default router for hosts

   Eliminating all mention of VRRP (VRID=1) from the figure above leaves
   it as a typical deployment.

   In the IPv4 case (that is, IPvX is IPv4 everywhere in the figure),
   each router is permanently assigned an IPv4 address on the LAN
   interface (Rtr1 is assigned IPv4 A and Rtr2 is assigned IPv4 B), and
   each host installs a static default route through one of the routers
   (in this example they all use Rtr1's IPv4 A).

   In the IPv6 case,(that is, IPvX is IPv6 everywhere in the figure),
   each router has a link-local IPv6 address on the LAN interface (Rtr1
   is assigned IPv6 Link-Local A and Rtr2 is assigned IPv6 Link-Local
   B), and each host learns a default route from Router Advertisements
   through one of the routers (in this example they all use Rtr1's IPv6
   Link-Local A).

   Moving to an IPv4 VRRP environment, each router has the exact same
   permanently assigned IPv4 address.  Rtr1 is said to be the IPv4
   address owner of IPv4 A, and Rtr2 is the IP address owner of IPv4 B.
   A virtual router is then defined by associating a unique identifier
   (the virtual router ID) with the address owned by a router.

   Moving to an IPv6 VRRP environment, each router has the exact same



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   Link-Local IPv6 address.  Rtr1 is said to be the IPv6 address owner
   of IPv6 A, and Rtr2 is the IPv6 address owner of IPv6 B. A virtual
   router is then defined by associating a unique identifier (the
   virtual router ID) with the address owned by a router.

   Finally, in both the IPv4 and IPv6 cases, the VRRP protocol manages
   virtual router fail over to a backup router.

   The IPv4 example above shows a virtual router configured to cover the
   IPv4 address owned by Rtr1 (VRID=1,IPv4_Address=A).  When VRRP is
   enabled on Rtr1 for VRID=1 it will assert itself as Master, with
   priority=255, since it is the IP address owner for the virtual router
   IP address.  When VRRP is enabled on Rtr2 for VRID=1 it will
   transition to Backup, with priority=100 (the default priority is 100)
   since it is not the IPv4 address owner.  If Rtr1 should fail then the
   VRRP protocol will transition Rtr2 to Master, temporarily taking over
   forwarding responsibility for IPv4 A to provide uninterrupted service
   to the hosts.

   The IPv6 example above shows a virtual router configured to cover the
   IPv6 address owned by Rtr1 (VRID=1,IPv6_Address=A).  When VRRP is
   enabled on Rtr1 for VRID=1 it will assert itself as Master, with
   priority=255, since it is the IPv6 address owner for the virtual
   router IPv6 address.  When VRRP is enabled on Rtr2 for VRID=1 it will
   transition to Backup, with priority=100 (the default priority is 100)
   since it is not the IPv6 address owner.  If Rtr1 should fail then the
   VRRP protocol will transition Rtr2 to Master, temporarily taking over
   forwarding responsibility for IPv6 A to provide uninterrupted service
   to the hosts.

   Note that in both cases, in this example IPvX B is not backed up, it
   is only used by Rtr2 as its interface address.  In order to backup
   IPvX B, a second virtual router must be configured.  This is shown in
   the next section.

4.2.  Sample Configuration 2

   The following figure shows a configuration with two virtual routers
   with the hosts splitting their traffic between them.












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           +-----------+      +-----------+
           |   Rtr1    |      |   Rtr2    |
           |(MR VRID=1)|      |(BR VRID=1)|
           |(BR VRID=2)|      |(MR VRID=2)|
   VRID=1  +-----------+      +-----------+  VRID=2
   IPvX A -------->*            *<---------- IPvX B
                   |            |
                   |            |
 ------------------+------------+-----+--------+--------+--------+--
                                      ^        ^        ^        ^
                                      |        |        |        |
                                   (IPvX A) (IPvX A) (IPvX B) (IPvX B)
                                      |        |        |        |
                                   +--+--+  +--+--+  +--+--+  +--+--+
                                   |  H1 |  |  H2 |  |  H3 |  |  H4 |
                                   +-----+  +-----+  +--+--+  +--+--+
    Legend:
         ---+---+---+--  =  Ethernet, Token Ring, or FDDI
                      H  =  Host computer
                     MR  =  Master Router
                     BR  =  Backup Router
                      *  =  IPvX Address, X is 4 everywhere in IPv4 case
                                          X is 6 everywhere in IPv6 case
                 (IPvX)  =  default router for hosts

   In the IPv4 example above (that is, IPvX is IPv4 everywhere in the
   figure), half of the hosts have configured a static route through
   Rtr1's IPv4 A and half are using Rtr2's IPv4 B. The configuration of
   virtual router VRID=1 is exactly the same as in the first example
   (see section 4.1), and a second virtual router has been added to
   cover the IPv4 address owned by Rtr2 (VRID=2, IPv4_Address=B).  In
   this case Rtr2 will assert itself as Master for VRID=2 while Rtr1
   will act as a backup.  This scenario demonstrates a deployment
   providing load splitting when both routers are available while
   providing full redundancy for robustness.

   In the IPv6 example above (that is, IPvX is IPv6 everywhere in the
   figure),, half of the hosts have learned a default route through
   Rtr1's IPv6 A and half are using Rtr2's IPv6 B. The configuration of
   virtual router VRID=1 is exactly the same as in the first example
   (see section 4.1), and a second virtual router has been added to
   cover the IPv6 address owned by Rtr2 (VRID=2, IPv6_Address=B).  In
   this case Rtr2 will assert itself as Master for VRID=2 while Rtr1
   will act as a backup.  This scenario demonstrates a deployment
   providing load splitting when both routers are available while
   providing full redundancy for robustness.

   Note that the details of a load balancing are out of scope of this



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   document.  However, in a case where the servers need different
   weights, it may not make sense to rely on router advertisements alone
   to balance the host load between the routers.


5.  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.

   When VRRP is protecting an IPv4 address, VRRP packets are sent
   encapsulated in IP packets.  They are sent to the IPv4 multicast
   address assigned to VRRP.

   When VRRP is protecting an IPv6 address, VRRP packets are sent
   encapsulated in IPv6 packets.  They are sent to the IPv6 multicast
   address assigned to VRRP.

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 IPvX Addr|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |(rsvd) |     Max Adver Int     |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                       IPvX Address(es)                        |
      +                                                               +
      +                                                               +
      +                                                               +
      +                                                               +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.1.1.  IPv4 Field Descriptions







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5.1.1.1.  Source Address

   The primary IPv4 address of the interface the packet is being sent
   from.

5.1.1.2.  Destination Address

   The IPv4 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.

5.1.1.3.  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.

5.1.1.4.  Protocol

   The IPv4 protocol number assigned by the IANA for VRRP is 112
   (decimal).

5.1.2.  IPv6 Field Descriptions

5.1.2.1.  Source Address

   The IPv6 link-local address of the interface the packet is being sent
   from.

5.1.2.2.  Destination Address

   The IPv6 multicast address as assigned by the IANA for VRRP is:

   FF02:0:0:0:0:0:XXXX:XXXX

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

5.1.2.3.  Hop Limit

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





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5.1.2.4.  Next Header

   The IPv6 Next Header protocol assigned by the IANA for VRRP is 112
   (decimal).

5.2.  VRRP Field Descriptions

5.2.1.  Version

   The version field specifies the VRRP protocol version of this packet.
   This document defines version 3.

5.2.2.  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.

5.2.3.  Virtual Rtr ID (VRID)

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

5.2.4.  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 IPvX address
   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.

5.2.5.  Count IPvX Addr

   The number of either IPv4 addresses or IPv6 addresses contained in
   this VRRP advertisement.  The minimum value is 1.



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5.2.6.  Rsvd

   This field MUST be set to zero on transmission and ignored on
   reception.

5.2.7.  Maximum Advertisement Interval (Max Adver Int)

   The Maximum Advertisement Interval is a 12-bit field that indicates
   the time interval (in centiseconds) between ADVERTISEMENTS.  The
   default is 100 centiseconds (1 second).

   Note that higher priority Master routers with slower transmission
   rates than their Backup routers are unstable.  This is because low
   priority nodes configured to faster rates could come online and
   decide they should be masters before they have heard anything from
   the higher priority master with a slower rate.

5.2.8.  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 and a
   "pseudo-header" as defined in section 8.1 of [RFC2460].  The next
   header field in the "pseudo-header" should be set to 112 (decimal)
   for VRRP.  For computing the checksum, the checksum field is set to
   zero.  See RFC1071 for more detail [RFC1071].

5.2.9.  IPvX Address(es)

   One or more IPvX addresses associated with the virtual router.  The
   number of addresses included is specified in the "Count IP Addr"
   field.  These fields are used for troubleshooting misconfigured
   routers.  If more than one address is sent it is recommended that all
   routers be configured to send these addresses in the same order to
   make it easier to do this comparison.

   For IPv4 addresses, one or more IPv4 addresses that are associated
   with the virtual router.

   For IPv6, the first address must be the IPv6 link-local address
   associated with the virtual router.

   This field contains either one or more IPv4 addresses or one or more
   IPv6 addresses, that is, IPv4 and IPv6 MUST NOT both be carried in
   one IPvX Address field.




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6.  Protocol State Machine

6.1.  Parameters per Virtual Router

   VRID                    Virtual Router Identifier.  Configurable 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 IPvX address
                           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).

   IPv4_Addresses          One or more IPv4 addresses associated with
                           this virtual router.  Configured item.  No
                           default

   IPv6_Addresses          One or more IPv6 addresses associated with
                           this virtual router.  Configured item.  No
                           default.  The first address must be the Link-
                           Local address associated with the virtual
                           router.

   Advertisement_Interval  Time interval between ADVERTISEMENTS
                           (centiseconds).  Default is 100 centiseconds
                           (1 second).

   Master_Adver_Interval   Advertisement interval contained in
                           ADVERTISEMENTS received from the Master
                           (centiseconds).  This value is saved by
                           virtual routers in Backup state and used to
                           compute Skew_Time and Master_Down_Interval.
                           The initial value is same as
                           Advertisement_Interval.

   Skew_Time               Time to skew Master_Down_Interval in
                           centiseconds.  Calculated as:







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                           (((256 - priority) * Master_Adver_Interval) /
                           256).

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

                           (3 * Master_Adver_Interval) + Skew_time

   Preempt_Mode            Controls whether a (starting or restarting)
                           higher priority Backup router preempts a
                           lower priority Master router.  Values are
                           True to allow preemption and False to
                           prohibit preemption.  Default is True.

                           Note: Exception is that the router that owns
                           the IPvX address associated with the virtual
                           router always preempts independent of the
                           setting of this flag.

   Accept_Mode             Controls whether a virtual router in Master
                           state will accept packets addressed to the
                           address owner's IPvX address as its own if it
                           is not the IPvX address owner.  Default is
                           False.

                           Note: IPv6 Neighbor Solicitations and
                           Neighbor Advertisements should not be dropped
                           when Accept_Mode is False.

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.














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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 IPvX address
      associated with the virtual router) then:



         + Send an ADVERTISEMENT

         + If the protected IPvX address is an IPv4 address:



            * Broadcast a gratuitous ARP request containing the virtual
            router MAC address for each IP address associated with the
            virtual router.






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         + else // IPv6



            * For each IPv6 address associated with the Virtual Router,
            send an unsolicited ND Neighbor Advertisement with the
            Router Flag (R) set, the Solicited Flag (S) unset, the
            Override flag (O) set, the Target Address set to the IPv6
            link-local address of the Virtual Router, and the Target
            Link Layer address set to the virtual router MAC address.

         +endif // was prot addr IPv4?

         + Set the Adver_Timer to Advertisement_Interval

         + Transition to the {Master} state

      - else // rtr does not own virt addr



         + Set Master_Adver_Interval to Advertisement_Interval

         + Set the Master_Down_Timer to Master_Down_Interval

         + Transition to the {Backup} state

      -endif // pri was not 255

      endif // startup event was recv

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:

      - If the protected IPvX address is an IPv4 address:



         + MUST NOT respond to ARP requests for the IPv4 address(s)
         associated with the virtual router.

      - else // prot addr is v6





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         + MUST NOT respond to ND Neighbor Solicitation messages for the
         IPv6 address(es) associated with the virtual router.

         + MUST NOT send ND Router Advertisement messages for the
         virtual router.

      -endif // was prot v4?

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

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

      - If a Shutdown event is received, then:



         + Cancel the Master_Down_Timer

         + Transition to the {Initialize} state

      -endif // shutdown recv

      - If the Master_Down_Timer fires, then:



         + Send an ADVERTISEMENT

         + If the protected IPvX address is an IPv4 address:



            * Broadcast a gratuitous ARP request containing the virtual
            router MAC address for each IPv4 address associated with the
            virtual router

         + else // ipv6



            * Compute and join the Solicited-Node multicast address
            [RFC4291] for the IPv6 address(es) addresses associated with
            the Virtual Router.




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            * Send an unsolicited ND Neighbor Advertisement with the
            Router Flag (R) set, the Solicited Flag (S) unset, the
            Override flag (O) set, the Target Address set to the IPv6
            link-local address of the Virtual Router, and the Target
            Link Layer address set to the virtual router MAC address.

         +endif // was prot addr ipv4?

         + Set the Adver_Timer to Advertisement_Interval

         + Transition to the {Master} state

      -endif // master down fired

      - If an ADVERTISEMENT is received, then:



         + If the Priority in the ADVERTISEMENT is Zero, then:



            * Set the Master_Down_Timer to Skew_Time

         + else // pri non-zero



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



               @ Set Master_Adver_Interval to Adver Interval contained
               in the ADVERTISEMENT.

               @ Recompute the Master_Down_Interval

               @ Reset the Master_Down_Timer to Master_Down_Interval

            * else // preempt was true or pri was less



               @ Discard the ADVERTISEMENT





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            *endif // preempt test

         +endif // was pri zero?

      -endif // was adv recv?

   endwhile // backup state

6.4.3.  Master

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

   Note that in the Master state the Preempt_Mode Flag is not
   considered.

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

      - If the protected IPvX address is an IPv4 address:



         + MUST respond to ARP requests for the IPv4 address(es)
         associated with the virtual router.

      - else // ipv6



         + MUST be a member of the Solicited-Node multicast address for
         the IPv6 address(es) associated with the virtual router.

         + MUST respond to ND Neighbor Solicitation message for the IPv6
         address(es) associated with the virtual router.

         + MUST send ND Router Advertisements for the virtual router.

      -endif // ipv4?

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

      - MUST accept packets addressed to the IPvX address(es) associated
      with the virtual router if it is the IPvX address owner or if
      Accept_Mode is True.  Otherwise, MUST NOT accept these packets.

      Note: IPv6 Neighbor Solicitations and Neighbor Advertisements
      should not be dropped when Accept_Mode is False.



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      - If a Shutdown event is received, then:



         + Cancel the Adver_Timer

         + Send an ADVERTISEMENT with Priority = 0

         + Transition to the {Initialize} state

      -endif // shutdown recv

      - If the Adver_Timer fires, then:



         + Send an ADVERTISEMENT

         + Reset the Adver_Timer to Advertisement_Interval

      -endif // advert timer fired

      - If an ADVERTISEMENT is received, then:



         + If the Priority in the ADVERTISEMENT is Zero, then:



            * Send an ADVERTISEMENT

            * Reset the Adver_Timer to Advertisement_Interval

         + else // pri was nonzero



            * 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 IPvX Address of the sender is
            greater than the local primary IPvX Address, then:





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               @ Cancel Adver_Timer

               @ Recompute the Master_Down_Interval

               @ Set Master_Adver_Interval to Adver Interval contained
               in the ADVERTISEMENT

               @ Set Master_Down_Timer to Master_Down_Interval

               @ Transition to the {Backup} state

            * else // new master logic



               @ Discard ADVERTISEMENT

            *endif // new master detected

         +endif // was pri zero?

      -endif // advert recvd

   endwhile // in master


7.  Sending and Receiving VRRP Packets

7.1.  Receiving VRRP Packets

   Performed the following functions when a VRRP packet is received:

      - If the received packet is an IPv4 packet:



         + MUST verify that the IPv4 TTL is 255.

      - else // ipv6 recv



         + MUST verify that the IPv6 Hop Limit is 255.






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      - endif

      - MUST verify the VRRP version is 3

      - MUST verify that the received packet contains the complete VRRP
      packet (including fixed fields, and IPvX Address.

      - MUST verify the VRRP checksum

      - MUST verify that the VRID is configured on the receiving
      interface and the local router is not the IPvX Address owner
      (Priority equals 255 (decimal)).

   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.

      - MAY verify that "Count IPvX Addrs" and the list of IPvX Address
      matches the IPvX Address(es) configured for the VRID

   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.

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

      - If the protected address is an IPv4 address:



         + Set the source MAC address to Virtual Router MAC Address

         + Set the source IPv4 address to interface primary IPv4 address

      - else // ipv6






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         + Set the source MAC address to Virtual Router MAC Address

         + Set the source IPv6 address to interface link-local IPv6
         address

      - endif

      - Set the IPvX protocol to VRRP

      - Send the VRRP packet to the VRRP IPvX 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:

   IPv4 case: 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 for
   IPv4 protocol. {VRID} is the VRRP Virtual Router Identifier.  This
   mapping provides for up to 255 IPv4 VRRP routers on a network.

   IPv6 case: 00-00-5E-00-02-{VRID} (in hex in internet standard bit-
   order)

   The first three octets are derived from the IANA's OUI.  The next two
   octets (00-02) indicate the address block assigned to the VRRP for
   IPv6 protocol. {VRID} is the VRRP Virtual Router Identifier.  This
   mapping provides for up to 255 IPv6 VRRP routers on a network.

7.4.  IPv6 Interface Identifiers

   IPv6 Routers running VRRP MUST create their Interface Identifiers in
   the normal manner (e.g., RFC2464 "Transmission of IPv6 Packets over
   Ethernet").  They MUST NOT use the Virtual Router MAC address to
   create the Modified EUI-64 identifiers.

   This VRRP specification describes how to advertise and resolve the
   VRRP routers IPv6 link local address and other associated IPv6
   addresses into the Virtual Router MAC address.




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   For IPv6, each VRRP virtual router requires a link-local address.  If
   there are several VRRP routers, it is cumbersome for the operator to
   configure the same VRRP protected link-local address on all of them.
   An implementation might choose simplify this for the operator by
   using the VRRP MAC in the formation of these link local addresses.


8.  Operational Issues

8.1.  IPv4

8.1.1.  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 IPv4 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.

8.1.2.  Host ARP Requests

   When a host sends an ARP request for one of the virtual router IPv4
   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 IPv4 address it owns,
   it should only send ARP messages that include Virtual MAC addresses.
   This may entail:

   o  When configuring an interface, VRRP routers should broadcast a
      gratuitous ARP request containing the virtual router MAC address
      for each IPv4 address on that interface.





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   o  At system boot, when initializing interfaces for VRRP operation;
      delay gratuitous ARP requests and ARP responses until both the
      IPv4 address and the virtual router MAC address are configured.

8.1.3.  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.

8.2.  IPv6

8.2.1.  ICMPv6 Redirects

   ICMPv6 Redirects may be used normally when VRRP is running between a
   group of routers [RFC4443].  This allows VRRP to be used in
   environments where the topology is not symmetric (e.g., the VRRP
   routers do not connect to the same destinations).

   The IPv6 source address of an ICMPv6 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.  A method to deduce the virtual router used is to examine
   the destination MAC address in the packet that triggered the
   redirect.

8.2.2.  ND Neighbor Solicitation

   When a host sends an ND Neighbor Solicitation message for the virtual
   router IPv6 address, the Master virtual router MUST respond to the ND
   Neighbor Solicitation message 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 Master virtual router sends an ND Neighbor Solicitation
   message for a host's IPv6 address, the Master virtual router MUST
   include the virtual MAC address for the virtual router if it sends a
   source link-layer address option in the neighbor solicitation
   message.  It MUST NOT use its physical MAC address in the source
   link-layer address option.

   When a VRRP router restarts or boots, it SHOULD not send any ND
   messages with its physical MAC address for the IPv6 address it owns,
   it should only send ND messages that include Virtual MAC addresses.



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   This may entail:

   o  When configuring an interface, VRRP routers should send an
      unsolicited ND Neighbor Advertisement message containing the
      virtual router MAC address for the IPv6 address on that interface.

   o  At system boot, when initializing interfaces for VRRP operation;
      delay all ND Router and Neighbor Advertisements and Solicitation
      messages until both the IPv6 address and the virtual router MAC
      address are configured.

   Note that on a restarting Master router where the VRRP protected
   address is the interface address, (that is, priority 255) duplicate
   address detection (DAD) may fail, as the Backup router may answer
   that it owns the address.  One solution is to not run DAD in this
   case.

8.2.3.  Router Advertisements

   When a backup VRRP router has become Master for a virtual router, it
   is responsible for sending Router Advertisements for the virtual
   router as specified in section 6.4.3.  The backup routers must be
   configured to send the same Router Advertisement options as the
   address owner.

   Router Advertisement options that advertise special services (e.g.,
   Home Agent Information Option) that are present in the address owner,
   should not be sent by the address owner unless the backup routers are
   prepared to assume these services in full and have a complete and
   synchronized database for this service.

8.3.  IPvX

8.3.1.  Potential Forwarding Loop

   A VRRP router SHOULD not forward packets addressed to the IPvX
   Address it becomes Master for if it is not the owner.  Forwarding
   these packets would result in unnecessary traffic.  Also in the case
   of LANs that receive packets they transmit (e.g., token ring) this
   can result in a forwarding loop that is only terminated when the IPvX
   TTL expires.

   One such mechanism for VRRP routers is to add/delete a reject host
   route for each adopted IPvX address when transitioning to/from MASTER
   state.






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8.3.2.  Recommendations regarding setting priority values

   A priority value of 255 designates a particular router as the "IPvX
   address owner".  Care must be taken not to configure more than one
   router on the link in this way for a single VRID.

   Routers with priority 255 will, as soon as they start up, preempt all
   lower priority routers.  Configure no more than one router on the
   link with priority 255, especially if preemption is set.  If no
   router has this priority, and preemption is disabled, then no
   preemption will occur.

   When there are multiple Backup routers, their priority values should
   be uniformly distributed.  For example, if one Backup routers has the
   default priority of 100 and another BR is added, a priority of 50
   would be a better choice for it than 99 or 100 to facilitate faster
   convergence.


9.  Operation over FDDI, Token Ring, and ATM LANE

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 that make running VRRP
   difficult.  These include:

   o  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.





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   o  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 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-IPvX protocols using the same functional address.
   Since the Novell IPX [IPX] protocol uses 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-
   IPvX 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.



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   Since a functional address cannot be 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 [ISO.10038.1993].

   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 that 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.  ND 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 ND cache.
   Hence, these implementations 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 that should be
   considered.  If there are VRID routers on different source-route
   bridge segments and there are host implementations that keep their
   source-route information in the ND cache and do not listen to
   gratuitous NDs, these hosts will not update their ND 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].

9.3.  Operation over ATM LANE

   Operation of VRRP over ATM LANE on routers with ATM LANE interfaces
   and/or routers behind proxy LEC's are beyond the scope of this
   document.


10.  Security Considerations

   VRRP for IPvX does not currently include any type of authentication.



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   Earlier versions of the VRRP (for IPv4) specification included
   several types of authentication ranging from none to strong.
   Operational experience and further analysis determined that these did
   not provide sufficient security to overcome the vulnerability of
   misconfigured secrets causing multiple masters to be elected.  Due to
   the nature of the VRRP protocol, even if VRRP messages are
   cryptographically protected, it does not prevent hostile routers from
   behaving as if they are a VRRP master, creating multiple masters.
   Authentication of VRRP messages could have prevented a hostile router
   from causing all properly functioning routers from going into backup
   state.  However, having multiple masters can cause as much disruption
   as no routers, which authentication cannot prevent.  Also, even if a
   hostile router could not disrupt VRRP, it can disrupt ARP and create
   the same effect as having all routers go into backup.

   It should be noted that these attacks are not worse and are a subset
   of the attacks that any node attached to a LAN can do independently
   of VRRP.  The kind of attacks a malicious node on a LAN can do
   include promiscuously receiving packets for any router's MAC address,
   sending packets with the router's MAC address as the source MAC
   addresses in the L2 header to tell the L2 switches to send packets
   addressed to the router to the malicious node instead of the router,
   send redirects to tell the hosts to send their traffic somewhere
   else, send unsolicited ND replies, answer ND requests, etc., etc.
   All of this can be done independently of implementing VRRP.  VRRP
   does not add to these vulnerabilities.

   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.

   VRRP does not provide any confidentiality.  Confidentiality is not
   necessary for the correct operation of VRRP and there is no
   information in the VRRP messages that must be kept secret from other
   nodes on the LAN.

   In the context of IPv6 operation, if SEcure Neighbor Discovery (SEND)
   [RFC3791] is deployed, VRRP authentication could be usefully added,
   because misconfiguration of secrets will not be an issue.  Routers
   with different secrets will have different IPv6 addresses, and
   therefore there will be no issue with multiple masters with the same
   IPv6 (and MAC) addresses.  Also, SEND will prevent malicious routers
   from sending misleading ND messages.







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11.  Intellectual Property Rights Claimed

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed
   rights.


12.  Contributors & Acknowledgments

   The editor would like to thank V. Ullanatt for his review of an early
   version.  This draft consists of very little new material (there is
   some new text in appendix A) and was created by merging and "xml-
   izing" the [I-D.ietf-vrrp-ipv6-spec] and [RFC3768] and then adding in
   the changes discussed recently on the mailing list.  R. Hinden is the
   author and J. Cruz is the editor of the former.  The contributors for
   the latter appear below.

   The IPv6 text in this specification is based on [RFC2338].  The
   authors of RFC2338 are S. Knight, D. Weaver, D. Whipple, R. Hinden,
   D. Mitzel, P. Hunt, P. Higginson, M. Shand, and A. Lindem.

   The author of [I-D.ietf-vrrp-ipv6-spec] would also like to thank Erik
   Nordmark, Thomas Narten, Steve Deering, Radia Perlman, Danny Mitzel,
   Mukesh Gupta, Don Provan, Mark Hollinger, John Cruz, and Melissa
   Johnson for their helpful suggestions.

   The IPv4 text in this specification is based on RFC3768.  The authors
   of that specification would like to thank Glen Zorn, and Michael
   Lane, Clark Bremer, Hal Peterson, Tony Li, Barbara Denny, Joel
   Halpern, Steve Bellovin, Thomas Narten, Rob Montgomery, Rob Coltun,
   Radia Perlman, Russ Housley, Harald Alvestrand, Steve Bellovin, Ned
   Freed, Ted Hardie, Russ Housley, Bert Wijnen, Bill Fenner, and Alex
   Zinin for their comments and suggestions.


13.  IANA Considerations

   VRRP for IPv6 needs an IPv6 link-local scope multicast address
   assigned by the IANA for this specification.  The IPv6 multicast
   address should be of the following form:

      FF02:0:0:0:0:0:XXXX:XXXX

   The values assigned address should be entered into section 5.1.2.2.

   A convenient assignment of this link-local scope multicast would be:




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      FF02:0:0:0:0:0:0:12

   as this would be consistent with the IPv4 assignment for VRRP.

   The IANA should also reserve a block of IANA Ethernet unicast
   addresses from:

      00-00-5E-00-02-00 to 00-00-5E-00-02-FF in hex

   for VRRP for IPv6.  Similar assignments are documented in:

   http://www.iana.org/assignments/ethernet-numbers


14.  References

14.1.  Normative References

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

   [ISO.10038.1993]
              International Organization for Standardization,
              "Information technology - Telecommunications and
              information exchange between systems - Local area networks
              - Media access control (MAC) bridges", ISO Standard 10038,
              1993.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC3768]  Hinden, R., "Virtual Router Redundancy Protocol (VRRP)",
              RFC 3768, April 2004.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.




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   [TKARCH]   IBM Incorporated, "IBM Token-Ring Network, Architecture
              Specificaton, Publication SC30-3374-02, Third Edition",
              September 1989.

14.2.  Informative References

   [I-D.ietf-vrrp-ipv6-spec]
              Hinden, R. and J. Cruz, "Virtual Router Redundancy
              Protocol for IPv6", draft-ietf-vrrp-ipv6-spec-08 (work in
              progress), March 2007.

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

   [RFC1071]  Braden, R., Borman, D., Partridge, C., and W. Plummer,
              "Computing the Internet checksum", RFC 1071,
              September 1988.

   [RFC1256]  Deering, S., "ICMP Router Discovery Messages", RFC 1256,
              September 1991.

   [RFC1469]  Pusateri, T., "IP Multicast over Token-Ring Local Area
              Networks", RFC 1469, June 1993.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC2281]  Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot
              Standby Router Protocol (HSRP)", RFC 2281, March 1998.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC2338]  Knight, S., Weaver, D., Whipple, D., Hinden, R., Mitzel,
              D., Hunt, P., Higginson, P., Shand, M., and A. Lindem,
              "Virtual Router Redundancy Protocol", RFC 2338,
              April 1998.

   [RFC2453]  Malkin, G., "RIP Version 2", STD 56, RFC 2453,
              November 1998.

   [RFC3791]  Olvera, C. and P. Nesser, "Survey of IPv4 Addresses in
              Currently Deployed IETF Routing Area Standards Track and
              Experimental Documents", RFC 3791, June 2004.







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Appendix A.  VRRPv3 and VRRPv2 Interoperation

A.1.  Assumptions

   1.  VRRPv2 and VRRPv3 interoperation is optional.

   2.  Mixing VRRPv2 and VRRPv3 should only be done when transitioning
       from VRRPv2 to VRRPv3.  Mixing the two versions should not be
       considered a permanent solution.

A.2.  VRRPv3 support of VRRPv2

   As mentioned above, this support is intended for upgrade scenarions
   and NOT recommended for permanent deployments.

   An implementation MAY implement a configuration flag that tells it to
   listen for and send both VRRPv2 and VRRPv3 advertisements.

   When configured this way and the Master, it MUST send both types at
   the configured rate, even if sub-second.

   When configured this way and the Backup, it should time out based on
   the rate advertised by the master; in the case of a VRRPv2 master
   this means it must translate the timeout value it receives (in
   seconds) into centi-seconds.  Also, a backup should ignore VRRPv2
   advertisements from the current master if it is also receiving VRRPv3
   packets from it.  It MAY report when a v3 master is *not* sending v2
   packets: that suggests they don't agree on whether they're supporting
   v2 routers.

A.3.  VRRPv3 support of VRRPv2 Considerations

A.3.1.  Slow, High-Priority Masters

   See also discussion at "Maximum Advertisement Interval (Max Adver
   Int)"

   The VRRPv2 Master router interacting with a sub-second VRRPv3 Backup
   router is the most important example of this.

   A VRRPv2 implementation should not be given a higher priority than a
   VRRPv2/VRRPv3 implementation it is interacting with if the VRRPv2/
   VRRPv3 rate is subsecond.

A.3.2.  Overwhelming VRRPv2 Backups

   It seems possible that an VRRPv3 Master router sending at centi-sec
   rates could potentially overwhelm a VRRPv2 Backup router with



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   potentially unclear results.

   In this upgrade case, a deployment should initially run the VRRPv3
   Master routers with lower frequencies (e.g., 100 centi-sec) until the
   VRRPv2 rtrs are upgraded.  Then, once the deployment has convinced
   itself that VRRPv3 is working properly, the VRRPv2 support may be
   unconfigured then the desired sub-second rates configured.


Appendix B.  Changes from draft-ietf-vrrp-unified-spec-00

   o  Remove typo in section 1.3 para 4.

   o  Reword reference to vrrp group in 3 para 2 to remove word group
      here.

   o  In section 6.4.3, when a master transitions to backup based on the
      receipt of a higher priority advertisement, add a bullet between
      "Cancel the Adver_Timer" and "Recompute the Master_Down_Interval"
      saying "Set Master_Adver_Interval to Adver Interval contained in
      the ADVERTISEMENT".

   o  At the very end of section 8.2.2 remove superfluous "(".

   o  In section 9.2 in the paragraph following the two bullet items fix
      the sentence "The functional addresses addresses......"

   o  In the second to last paragraph of section 9.2 fix mangled syntax
      "...these implementations need have to receive...."

   o  clarify that IPv6 accept-mode = off behaviour should not apply to
      NS/NA messages in sections 6.1 and 6.4.3.)

   o  remove typo in section 1.6 in defn of Primary IP address

   o  remove extra "associated" typo in section 5.2.9

   o  in section 6.4.1 for IPv6 clarify that ND NA message should be
      sent for all the IPv6 addresses associated with the VR

   o  change refs to 2461 to 4861 (nits)


Appendix C.  Changes from [ietf-vrrp-unified-spec] and [RFC3768]

   o  Draft-ietf-vrrp-unified-spec-00 was based on
      [I-D.ietf-vrrp-ipv6-spec] with material from [RFC3768] added in
      for the IPv4 parts.  In this process it was also moved to xml.



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   o  Rework Abstract, Intro paragraphs to describe unified approach and
      add editor's note.

   o  Use "IPvX" when talking about both IPv4 and IPv6 (all over the
      doc).

   o  Pull IPv4 Intro material into intro IPv4 section and make IPv6
      section

   o  Add sub-second operation in required features

   o  Modify sample configurations to refer to "IPvX"

   o  Rename "Adver Int" to "Min Adver Int" in VRRP Packet format and
      use "IPvX" term

   o  Add separate IPv4 and IPv6 field description sections

   o  In state descriptions and sending and receiving VRRP packets
      sections, add pseudo code test for IPv4, then add actions from
      [RFC3768] and arrange the else branch to be the IPv6 actions.

   o  Include both IPv4 and IPv6 VR MAC addrs

   o  In operational issues, create IPv4, IPv6 and IPvX sections

   o  rework contributors and acks section.

   o  add appendix A to discuss optional VRRPv2 and VRRpv3 interop

   o  add appendix B to track changes made

   o  add a paragraph to the IPv6 operational issues to mention DAD when
      master owns IPv6 address (this was issue 1 in the some comments on
      -08 thread)

   o  in various places, change IPv6 language to mention not only link-
      locals but any other IPv6 protected addresses as well. (this was
      issue 4 in the some comments on -08 thread)

   o  add appendix C to track remaining todos










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Author's Address

   Stephen Nadas (editor)
   Ericsson
   920 Main Campus Dr., Suite 500
   Raleigh, NC  27606
   USA

   Phone: +1 919 472 9935
   Email: stephen.nadas@ericsson.com









































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