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Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6
draft-ietf-rtgwg-vrrp-rfc5798bis-13

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9568.
Authors Acee Lindem , Aditya Dogra
Last updated 2023-12-11 (Latest revision 2023-12-09)
Replaces draft-addogra-rtgwg-vrrp-rfc5798bis
RFC stream Internet Engineering Task Force (IETF)
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Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Yingzhen Qu
Shepherd write-up Show Last changed 2023-11-22
IESG IESG state Became RFC 9568 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Jim Guichard
Send notices to yingzhen.ietf@gmail.com
IANA IANA review state Version Changed - Review Needed
draft-ietf-rtgwg-vrrp-rfc5798bis-13
Network Working Group                                          A. Lindem
Internet-Draft                                   LabN Consulting, L.L.C.
Obsoletes: 5798 (if approved)                                   A. Dogra
Intended status: Standards Track                           Cisco Systems
Expires: 11 June 2024                                    9 December 2023

 Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6
                  draft-ietf-rtgwg-vrrp-rfc5798bis-13

Abstract

   This document defines version 3 of the Virtual Router Redundancy
   Protocol (VRRP) for IPv4 and IPv6.  It is based on VRRP (version 2)
   for IPv4 that is defined in RFC 3768 and in "Virtual Router
   Redundancy Protocol for IPv6", and obsoletes the previous
   specification of this version documented in RFC 5798.  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 Active Router, and it forwards packets sent to
   these IPv4 or IPv6 addresses.  Active Routers are configured with
   virtual IPv4 or IPv6 addresses, and Backup Routers infer the address
   family of the virtual addresses being advertised based on the IP
   protocol version.  Within a VRRP Router, the Virtual Routers in each
   of the IPv4 and IPv6 address families are independent of one another
   and always treated as separate Virtual Router instances.  The
   election process provides dynamic failover in the forwarding
   responsibility should the Active Router 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 switchover to
   Backup Routers than can be obtained with standard IPv6 Neighbor
   Discovery mechanisms.

   The VRRP terminology has been updated to conform to inclusive
   language guidelines for IETF technologies.  The IETF has designated
   National Institute of Standards and Technology (NIST) "Guidance for
   NIST Staff on Using Inclusive Language in Documentary Standards" for
   its inclusive language guidelines.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  RFC 5798 Differences  . . . . . . . . . . . . . . . . . .   4
     1.2.  A Note on Terminology . . . . . . . . . . . . . . . . . .   5
     1.3.  IPv4  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.4.  IPv6  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     1.5.  Requirements Language . . . . . . . . . . . . . . . . . .   7
     1.6.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     1.7.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   8
   2.  Required Features . . . . . . . . . . . . . . . . . . . . . .   9
     2.1.  IPvX Address Backup . . . . . . . . . . . . . . . . . . .   9
     2.2.  Preferred Path Indication . . . . . . . . . . . . . . . .   9
     2.3.  Minimization of Unnecessary Service Disruptions . . . . .  10
     2.4.  Efficient Operation over Extended LANs  . . . . . . . . .  10
     2.5.  Sub-Second Operation for IPv4 and IPv6  . . . . . . . . .  10
   3.  VRRP Overview . . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  Sample Configurations . . . . . . . . . . . . . . . . . . . .  12
     4.1.  Sample Configuration 1  . . . . . . . . . . . . . . . . .  12
     4.2.  Sample Configuration 2  . . . . . . . . . . . . . . . . .  14
   5.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     5.1.  VRRP Packet Format  . . . . . . . . . . . . . . . . . . .  16

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       5.1.1.  IPv4 Field Descriptions . . . . . . . . . . . . . . .  17
         5.1.1.1.  Source Address  . . . . . . . . . . . . . . . . .  17
         5.1.1.2.  Destination Address . . . . . . . . . . . . . . .  17
         5.1.1.3.  TTL . . . . . . . . . . . . . . . . . . . . . . .  17
         5.1.1.4.  Protocol  . . . . . . . . . . . . . . . . . . . .  18
       5.1.2.  IPv6 Field Descriptions . . . . . . . . . . . . . . .  18
         5.1.2.1.  Source Address  . . . . . . . . . . . . . . . . .  18
         5.1.2.2.  Destination Address . . . . . . . . . . . . . . .  18
         5.1.2.3.  Hop Limit . . . . . . . . . . . . . . . . . . . .  18
         5.1.2.4.  Next Header . . . . . . . . . . . . . . . . . . .  18
     5.2.  VRRP Field Descriptions . . . . . . . . . . . . . . . . .  18
       5.2.1.  Version . . . . . . . . . . . . . . . . . . . . . . .  18
       5.2.2.  Type  . . . . . . . . . . . . . . . . . . . . . . . .  18
       5.2.3.  Virtual Rtr ID (VRID) . . . . . . . . . . . . . . . .  19
       5.2.4.  Priority  . . . . . . . . . . . . . . . . . . . . . .  19
       5.2.5.  IPvX Addr Count . . . . . . . . . . . . . . . . . . .  19
       5.2.6.  Reserve . . . . . . . . . . . . . . . . . . . . . . .  19
       5.2.7.  Maximum Advertisement Interval (Max Advertise
               Interval) . . . . . . . . . . . . . . . . . . . . . .  19
       5.2.8.  Checksum  . . . . . . . . . . . . . . . . . . . . . .  20
       5.2.9.  IPvX Address(es)  . . . . . . . . . . . . . . . . . .  20
   6.  Protocol State Machine  . . . . . . . . . . . . . . . . . . .  21
     6.1.  Parameters Per Virtual Router . . . . . . . . . . . . . .  21
     6.2.  Timers  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     6.3.  State Transition Diagram  . . . . . . . . . . . . . . . .  23
     6.4.  State Descriptions  . . . . . . . . . . . . . . . . . . .  23
       6.4.1.  Initialize  . . . . . . . . . . . . . . . . . . . . .  23
       6.4.2.  Backup  . . . . . . . . . . . . . . . . . . . . . . .  25
       6.4.3.  Active  . . . . . . . . . . . . . . . . . . . . . . .  27
   7.  Sending and Receiving VRRP Packets  . . . . . . . . . . . . .  29
     7.1.  Receiving VRRP Packets  . . . . . . . . . . . . . . . . .  29
     7.2.  Transmitting VRRP Packets . . . . . . . . . . . . . . . .  30
     7.3.  Virtual Router MAC Address  . . . . . . . . . . . . . . .  31
     7.4.  IPv6 Interface Identifiers  . . . . . . . . . . . . . . .  32
   8.  Operational Issues  . . . . . . . . . . . . . . . . . . . . .  32
     8.1.  IPv4  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
       8.1.1.  ICMP Redirects  . . . . . . . . . . . . . . . . . . .  32
       8.1.2.  Host ARP Requests . . . . . . . . . . . . . . . . . .  33
       8.1.3.  Proxy ARP . . . . . . . . . . . . . . . . . . . . . .  33
     8.2.  IPv6  . . . . . . . . . . . . . . . . . . . . . . . . . .  33
       8.2.1.  ICMPv6 Redirects  . . . . . . . . . . . . . . . . . .  33
       8.2.2.  ND Neighbor Solicitation  . . . . . . . . . . . . . .  34
       8.2.3.  Router Advertisements . . . . . . . . . . . . . . . .  35
       8.2.4.  Unsolicited Neighbor Advertisements . . . . . . . . .  35
     8.3.  IPvX  . . . . . . . . . . . . . . . . . . . . . . . . . .  35
       8.3.1.  Potential Forwarding Loop . . . . . . . . . . . . . .  35
       8.3.2.  Recommendations Regarding Setting Priority Values . .  36
     8.4.  VRRPv3 and VRRPv2 Interoperation  . . . . . . . . . . . .  36

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       8.4.1.  Assumptions . . . . . . . . . . . . . . . . . . . . .  36
       8.4.2.  VRRPv3 Support of VRRPv2 Interoperation . . . . . . .  36
         8.4.2.1.  Interoperation Considerations . . . . . . . . . .  37
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  37
   10. Contributors and Acknowledgments  . . . . . . . . . . . . . .  39
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  40
   13. Informative References  . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

1.  Introduction

   This document defines version 3 of the Virtual Router Redundancy
   Protocol (VRRP) for IPv4 and IPv6.  It is based on VRRP (version 2)
   for IPv4 that is defined in [RFC3768] and in "Virtual Router
   Redundancy Protocol for IPv6", and obsoletes the previous
   specification of this version documented in [VRRP-IPv6].  VRRP
   specifies an election protocol that dynamically assigns
   responsibility for a Virtual Router (refer to Section 1.7) 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
   Active Router, and it forwards packets sent to these IPv4 or IPv6
   addresses.  VRRP Active Routers are configured with virtual IPv4 or
   IPv6 addresses, and Backup Routers infer the address family of the
   virtual addresses being advertised based on the IP protocol version.
   Within a VRRP Router, the Virtual Routers in each of the IPv4 and
   IPv6 address families are independent of one another and always
   treated as separate Virtual Router instances.  The election process
   provides dynamic failover in the forwarding responsibility should the
   Active Router become unavailable.

   The VRRP terminology has been updated to conform to inclusive
   language guidelines for IETF technologies.  The IETF has designated
   National Institute of Standards and Technology (NIST) "Guidance for
   NIST Staff on Using Inclusive Language in Documentary Standards"
   [NISTIR8366] for its inclusive language guidelines.  This document
   obsoletes VRRP Version 3 [RFC5798].

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

1.1.  RFC 5798 Differences

   The following changes have been made from RFC 5798:

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   1.  The term for the VRRP Router assuming forwarding responsibility
       has been changed to "Active Router" to be consistent with IETF
       inclusive terminology.  Additionally, inconsistencies in RFC 5798
       terminology for both "Active Router" and "Backup Router" were
       corrected.  Additionally, the undesirable term for attracting and
       dropping unreachable packets has been changed.

   2.  Errata pertaining to the state machines in Section 6 were
       corrected.

   3.  The checksum calculation in Section 5.2.8 has been clarified to
       specify precisely what is included and that it does not include
       the pseudo-header for IPv4.

   4.  When a VRRP advertisement is received from a lower priority VRRP
       router, the Active VRRP router will immediately send a VRRP
       advertisement to assure learning bridges will bridge the packets
       to the correct Ethernet segment Section 6.4.3.

   5.  Appendices describing operation over legacy technologies (FDDI,
       Token Ring, and ATM LAN Emulation) were removed.

   6.  A recommendation was added indicating that IPv6 Unsolicited
       Neighbor Advertisements SHOULD be accepted by the Active and
       Backup Routers Section 8.2.4.

   7.  Checking that the Maximum Advertisement Intervals match is
       recommended although this will not result in the VRRP packet
       being dropped Section 7.1.

   8.  Miscellaneous editorial changes were made for readability.

   9.  The IANA considerations section was augmented to include all the
       IPv4/IPv6 multicast address allocations and Ethernet MAC address
       allocations.

1.2.  A Note on Terminology

   This document discusses both IPv4 and IPv6 operations, and with
   respect to the VRRP protocol, many of the descriptions and procedures
   are common.  In this document, it would be less verbose to be able to
   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 this text, where the IP version
   matters, the appropriate term is used, and the use of the term "IP"
   is avoided.

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1.3.  IPv4

   There are a number of methods that an IPv4 end-host can use to
   determine its first-hop router for a particular IPv4 destination.
   These include running (or snooping) a dynamic routing protocol such
   as Routing Information Protocol (RIP) [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 not be
   feasible for a number of reasons, including administrative overhead,
   processing overhead, security issues, or the lack of an
   implementation for a particular platform.  Neighbor or router
   discovery protocols may require active participation by all hosts on
   a network, requiring large timer values to reduce protocol overhead
   associated with the protocol packet processing for each host.  This
   can result in a significant delay in the detection of an unreachable
   router and, such a delay may introduce unacceptably long periods of
   unreachability for the default route.

   The use of a statically configured default route is quite popular
   since 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 an available alternate path.

   The Virtual Router Redundancy Protocol (VRRP) is designed to
   eliminate the single point of failure inherent in a network utilizing
   static default routing.  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 Active
   Router and forwards packets sent to these IPv4 addresses.  The
   election process provides dynamic failover of the forwarding
   responsibility should the Active Router 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 a router discovery protocol on
   every end-host.

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1.4.  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 (ND) protocol [RFC4861].  The Router
   Advertisements are periodically multicast at a rate such that the
   hosts will normally take more than 10 seconds to learn the default
   routers on a LAN.  They are not sent frequently enough to rely on the
   absence of the Router Advertisement to detect router failures.

   The ND protocol 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 router is
   up for a period of time.  Using the default parameters in ND, it can
   take a host more than 10 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 time out.

   While the ND unreachability detection could be made quicker by
   configuring the timer intervals 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 or more routers.

   The Virtual Router Redundancy Protocol for IPv6 provides a much
   faster switchover 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 without any interaction with the
   hosts and a minimum amount of VRRP traffic.

1.5.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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1.6.  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 Active Router are presented.  Finally, operational issues
   related to MAC address mapping, handling of ARP messages, generation
   of ICMP redirect messages, and security issues are addressed.

1.7.  Definitions

   VRRP Router             A router running the Virtual Router
                           Redundancy Protocol.  It may participate as
                           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
                           may back up one or more Virtual Routers.

   Virtual Router Identifier  An integer value (1-255) identifying an
                           instance of a Virtual Router on a LAN.  Also
                           referred by its acronym, VRID.

   Virtual Router MAC Address  The multicast Ethernet MAC address used
                           for VRRP advertisements for a VRID.  Refer to
                           Section 7.3.

   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
                           connection requests, 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, 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.

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

   Backup Router(s)        The set of VRRP Routers available to assume
                           forwarding responsibility for a Virtual
                           Router should the current Active Router fail.

2.  Required Features

   This section describes 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 or addresses is the primary function of
   VRRP.  When providing election of an Active Router and the additional
   functionality described below; the protocol should strive to:

   *  Minimize the duration of unreachability.

   *  Minimize the steady-state bandwidth overhead and processing
      complexity.

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

   *  Allow multiple Virtual Routers on a network for load-balancing.

   *  Support multiple logical IPvX subnets on a single LAN segment.

2.2.  Preferred Path Indication

   A simple model of Active Router election among a set of redundant
   routers is to treat each router with equal preference and claim
   victory after converging to any router as Active Router.  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 Active
   Router convergence to the most preferred Virtual Router currently

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   available.

2.3.  Minimization of Unnecessary Service Disruptions

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

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

2.4.  Efficient Operation over Extended LANs

   Sending IPvX packets, i.e., sending either IPv4 or IPv6, 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 MAC address 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 do
   the following:

   1.  Use the Virtual Router MAC address as the source in a packet sent
       by the Active Router to trigger MAC learning.

   2.  Trigger a message immediately after transitioning to the Active
       Router to update MAC learning.

   3.  Trigger periodic messages from the Active Router to maintain the
       MAC address cache.

2.5.  Sub-Second Operation for IPv4 and IPv6

   Sub-second detection of Active Router failure is needed in both IPv4
   and IPv6 environments.  Earlier work proposed that sub-second
   operation was for IPv6 and this specification leverages that earlier
   approach for both IPv4 and IPv6.

   One possible problematic scenario may occur when using a small
   Advertisement_Interval (refer to Section 6.1) is when a VRRP Router
   is generating more packets than it can transmit, and a queue builds

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   up on the VRRP Router.  When this occurs, it is possible that packets
   being transmitted onto the VRRP-protected LAN could see larger
   queueing delay than the smallest Advertisement_Interval.  In this
   case, the Active_Down_Interval (refer to Section 6.1) may be small
   enough that normal queuing delays might cause a Backup Router to
   conclude that the Active Router is down, and, hence, promote itself
   to Active Router.  Very shortly afterwards, the delayed VRRP packets
   from the original Active Router cause a switch back to Backup Router.
   Furthermore, this process can repeat many times per second, causing a
   significant disruption of traffic.  To mitigate this problem,
   priority forwarding of VRRP packets should be considered.  The Active
   Router SHOULD observe that this situation is occurring and log the
   problem.

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 link of a VRRP Virtual Router has a single
   well-known MAC address allocated to it.  This document currently only
   details the mapping to networks using an IEEE 802 48-bit MAC address.
   The Virtual Router MAC address is used as the source in all periodic
   VRRP messages sent by the Active Router to enable MAC learning by
   layer-2 bridges on 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.
   The scope of each Virtual Router is restricted to a single LAN.  A
   VRRP Router may be configured with additional Virtual Router mappings
   and priority for Virtual Routers it is willing to back up.  The
   mapping between the VRID and its IPvX address(es) must be coordinated
   among all VRRP Routers on a LAN.

   There is no restriction against reusing a VRID with a different
   address mapping on different LANs, nor is there a restriction against
   using the same VRID 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 Active Router for each Virtual
   Router sends periodic VRRP Advertisement messages.  A Backup Router
   will not attempt to preempt the Active Router unless the Backup
   Router has a higher priority.  This eliminates service disruption
   unless a more preferred path becomes available.  It's also possible
   to administratively prohibit Active Router preemption attempts.  The
   only exception is that a VRRP Router will always become the Active

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   Router for any Virtual Router associated with address(es) it owns.
   If the Active Router becomes unavailable, then the highest-priority
   Backup Router will transition to Active Router after a short delay,
   providing a controlled transition of Virtual Router responsibility
   with minimal service interruption.

   The VRRP protocol design provides rapid transition from Backup Router
   to Active Router to minimize service interruption and incorporates
   optimizations that reduce protocol complexity while guaranteeing
   controlled Active Router transition for typical operational
   scenarios.  These 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 for each router.  A side effect when these
   assumptions are violated, i.e., more than two redundant paths with
   equal preference, is that duplicate packets may be forwarded for a
   brief period during Active Router election.  However, the typical
   scenario assumptions are likely to cover the vast majority of
   deployments, loss of the Active Router is infrequent, and the
   expected duration for Active Router election convergence is quite
   small (< 4 seconds when using the default Advertisement_Interval and
   configurable to < 1/25 second).  Thus, the VRRP optimizations
   represent significant simplifications in the protocol design while
   incurring an insignificant probability of brief network disruption.

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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           +-----------+ +-----------+
           | Router-1  | | Router-2  |
           |(AR VRID=1)| |(BR VRID=1)|
           |           | |           |
   VRID=1  +-----------+ +-----------+
   IPvX A------>*            *<---------IPvX B
                |            |
                |            |
   -------------+------------+--+-----------+-----------+-----------+
                                ^           ^           ^           ^
                                |           |           |           |
        Default Router          |           |           |           |
        IPvX addresses ---> (IPvX A)    (IPvX A)    (IPvX A)    (IPvX A)
                                |           |           |           |
                       IPvX H1->*  IPvX H2->*  IPvX H3->*  IPvX H4->*
                             +--+--+     +--+--+     +--+--+     +--+--+
                             |  H1 |     |  H2 |     |  H3 |     |  H4 |
                             +-----+     +-----+     +--+--+     +--+--+
   Legend:
         --+---+---+-- = Ethernet
                     H = Host computer
                    AR = Active 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 case, i.e., IPvX is IPv4 everywhere in the figure, each
   router is permanently assigned an IPv4 address on the LAN interface
   (Router-1 is assigned IPv4 A and Router-2 is assigned IPv4 B), and
   each host installs a static default route through one of the routers
   (in this example, they all use Router-1's IPv4 A).

   In the IPv6 case, i.e., IPvX is IPv6 everywhere in the figure, each
   router has its own Link-Local IPv6 address on the LAN interface, and
   a link-local IPv6 address per VRID that is shared with the other
   routers that serve the same VRID.  Each host learns a default route
   from Router Advertisements through one of the routers (in this
   example, they all use Router-1's IPv6 Link-Local A).

   In an IPv4 VRRP environment, each router supports reception and
   transmission for the exact same IPv4 address.  Router-1 is said to be
   the IPv4 address owner of IPv4 A, and Router-2 is the IPv4 address
   owner of IPv4 B.  A Virtual Router is then defined by associating a
   unique identifier (the VRID) with the address owned by Router-1.

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   In an IPv6 VRRP environment, each router will support transmission
   and reception for the IPv6 addresses associated with the VRID.
   Router-1 is said to be the IPv6 address owner of IPv6 A, and Router-2
   is the IPv6 address owner of IPv6 B.  A Virtual Router is then
   defined by associating a unique identifier (the VRID) with the
   address owned by Router-1.

   Finally, in both the IPv4 and IPv6 cases, the VRRP protocol manages
   Virtual Router failover to a Backup Router.

   The IPvX example above shows a Virtual Router configured to cover the
   IPv4 address owned by Router-1 (VRID=1, IPvX_Address=A).  When VRRP
   is enabled on Router-1 for VRID=1, it will assert itself as Active
   Router, with priority = 255, since it is the IPvX address owner for
   the Virtual Router IPvX address.  When VRRP is enabled on Router-2
   for VRID=1, it will transition to Backup Router, with priority = 100
   (the default priority is 100), since it is not the IPvX address
   owner.  If Router-1 should fail, then the VRRP protocol will
   transition Router-2 to Active Router, temporarily taking over
   forwarding responsibility for IPvX A to provide uninterrupted service
   to the hosts.

   Note that in both cases in this example, IPvX B is not backed up and
   it is only used by Router-2 as its interface address.  In order to
   back up 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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           +-----------+  +-----------+
           |  Router-1 |  | Router-2  |
           |(AR VRID=1)|  |(BR VRID=1)|
           |(BR VRID=2)|  |(AR VRID=2)|
   VRID=1  +-----------+  +-----------+  VRID=2
   IPvX A ----->*             *<---------- IPvX B
                |             |
                |             |
      ----------+-------------+-+-----------+-----------+-----------+
                                ^           ^           ^           ^
                                |           |           |           |
        Default Router          |           |           |           |
        IPvX addresses ---> (IPvX A)    (IPvX A)    (IPvX B)    (IPvX B)
                                |           |           |           |
                       IPvX H1->*  IPvX H2->*  IPvX H3->*  IPvX H4->*
                             +--+--+     +--+--+     +--+--+     +--+--+
                             |  H1 |     |  H2 |     |  H3 |     |  H4 |
                             +-----+     +-----+     +--+--+     +--+--+

    Legend:
         ---+---+---+--  =  Ethernet
                      H  =  Host computer
                     AR  =  Active 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, i.e., IPvX is IPv4 everywhere in the
   figure, half of the hosts have configured a static default route
   through Router-1's IPv4 A, and half are using Router-2'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 Router-2 (VRID=2,
   IPv4_Address=B).  In this case, Router-2 will assert itself as Active
   Router for VRID=2 while Router-1 will act as a Backup Router.  This
   scenario demonstrates a deployment providing load-splitting when both
   routers are available, while providing full redundancy for
   robustness.

   In the IPv6 example above, i.e., IPvX is IPv6 everywhere in the
   figure, half of the hosts have learned a default route through
   Router-1's IPv6 A, and half are using Router-2'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 Router-2 (VRID=2,

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   IPv6_Address=B).  In this case, Router-2 will assert itself as Active
   Router for VRID=2 while Router-1 will act as a Backup Router.  This
   scenario demonstrates a deployment providing load-splitting when both
   routers are available, while providing full redundancy for
   robustness.

   Note that the details of load-balancing are out of scope of this
   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 traffic between the routers.

5.  Protocol

   The purpose of the VRRP Advertisement is to communicate to all VRRP
   Routers the priority, Max Advertisement Interval, and IPvX addresses
   of the Active Router associated with the VRID.

   When VRRP is protecting an IPv4 address, VRRP packets are sent
   encapsulated in IPv4 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 IPvX header (in conjunction with the address family).

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    IPv4 Fields or IPv6 Fields                 |
    ...                                                             ...
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Version| Type  | Virtual Rtr ID|   Priority    |IPvX Addr Count|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Reserve| Max Advertise Interval|          Checksum             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                       IPvX Address(es)                        |
    +                                                               +
    +                                                               +
    +                                                               +
    +                                                               +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.1.1.  IPv4 Field Descriptions

5.1.1.1.  Source Address

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

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.

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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

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

5.1.2.2.  Destination Address

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

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

   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.

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.

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5.2.3.  Virtual Rtr ID (VRID)

   The Virtual Rtr ID field identifies the Virtual Router for which this
   packet is reporting status.

5.2.4.  Priority

   The priority field specifies the sending VRRP Router's priority for
   the Virtual Router.  Higher values indicate 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).  Refer to Section 8.3.2
   for recommendations on setting the priority.

   The priority value zero (0) has special meaning, indicating that the
   current Active Router has stopped participating in VRRP.  This is
   used to trigger Backup Routers to quickly transition to Active Router
   without having to wait for the current Active_Down_Interval (refer to
   Section 6.1).

5.2.5.  IPvX Addr Count

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

5.2.6.  Reserve

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

5.2.7.  Maximum Advertisement Interval (Max Advertise Interval)

   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).

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   Note that higher-priority Active Routers with slower transmission
   rates than their Backup Routers are unstable.  This is because lower-
   priority Backup Routers configured to faster rates could join the LAN
   and decide they should be Active Routers before they have heard
   anything from the higher-priority Active Router with a slower rate.
   When this happens, it is temporary, once the lower-priority node does
   hear from the higher-priority Active Router, it will relinquish
   Active Router status.

5.2.8.  Checksum

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

   For both the IPv4 and IPv6 address families, 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.  See RFC1071 for more
   detail [RFC1071].

   For the IPv4 address family, the checksum calculation only includes
   the VRRP message starting with the Version field and ending after the
   last IPv4 address (refer to Section 5.2).

   For the IPv6 address family, the checksum calculation also includes a
   prepended "pseudo-header" as defined in Section 8.1 of [RFC8200].
   The next header field in the "pseudo-header" should be set to 112
   (decimal) for VRRP.

5.2.9.  IPvX Address(es)

   This refers to one or more IPvX addresses associated with the Virtual
   Router.  The number of addresses included is specified in the "IPvX
   Addr Count" 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 simplify comparisons.

   For IPv4 addresses, this refers to one or more IPv4 addresses that
   are backed up by 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.  The addresses, IPv4 or IPv6 but not both, MUST be
   the same as the VRRP packet's IPvX header address family.

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6.  Protocol State Machine

6.1.  Parameters Per Virtual Router

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

   Priority                    Priority value to be used by this VRRP
                               router in Active Router 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 the Active Router to
                               indicate it is relinquishing
                               responsibility for the Virtual Router.
                               The range 1-254 (decimal) is available
                               for VRRP Routers backing up the Virtual
                               Router.  Higher values indicate higher
                               priorities.  The default value is 100
                               (decimal).

   IPv4_Addresses              One or more IPv4 addresses associated
                               with this Virtual Router.  Configured
                               list of addresses with no default.

   IPv6_Addresses              One or more IPv6 addresses associated
                               with this Virtual Router.  Configured
                               list of addresses with no default.  The
                               first address must be the Link-Local
                               address associated with the Virtual
                               Router.

   IPvX_Addresses              Refer to either the IPv4 or IPv6 address
                               associated with this Virtual Router (see
                               IPv4_Addresses and IPv6_Addresses above).

   Advertisement_Interval      Time interval between ADVERTISEMENTS
                               (centiseconds) sent by this Virtual
                               Router.  Default is 100 centiseconds (1
                               second).

   Active_Adver_Interval       Advertisement interval contained in

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                               ADVERTISEMENTS received from the Active
                               Router (centiseconds).  This value is
                               saved by Virtual Routers in the Backup
                               state and used to compute Skew_Time and
                               Active_Down_Interval.  The initial value
                               is the same as Advertisement_Interval.

   Skew_Time                   Time to skew Active_Down_Interval in
                               centiseconds.  Calculated as:

                                   (((256 - Priority) *
                                   Active_Adver_Interval) / 256)

   Active_Down_Interval        Time interval for the Backup Router to
                               declare the Active Router down
                               (centiseconds).  Calculated as:

                                   (3 * Active_Adver_Interval) +
                                   Skew_Time

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

                               Note: The 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
                               Active state will accept packets
                               addressed to the address owner's IPvX
                               address as its own even if it is not the
                               IPvX address owner.  The default is
                               False.  Deployments that rely on, for
                               example, pinging the address owner's IPvX
                               address may wish to configure Accept_Mode
                               to True.

                               Note: IPv6 Neighbor Solicitations and
                               Neighbor Advertisements MUST NOT be
                               dropped when Accept_Mode is False.

   Virtual_Router_MAC_Address  The MAC address used for the source MAC

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                               address in VRRP advertisements and
                               advertised in ARP/ND messages as the MAC
                               address to use for IPvX_Addresses.

6.2.  Timers

   Active_Down_Timer        Timer that fires when a VRRP Advertisement
                            has not been received for
                            Active_Down_Interval (Backup Routers only).

   Adver_Timer              Timer that fires to trigger transmission of
                            a VRRP Advertisement based on the
                            Advertisement_Interval (Active Routers
                            only).

6.3.  State Transition Diagram

                      +---------------+
           +--------->|               |<-------------+
           |          |  Initialize   |              |
           |   +------|               |----------+   |
           |   |      +---------------+          |   |
           |   |                                 |   |
           |   V                                 V   |
      +---------------+                       +---------------+
      |               |---------------------->|               |
      |    Active     |                       |    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-uppercase
   characters.

   A VRRP Router implements an instance of the state machine for each
   Virtual Router in which it is participating.

6.4.1.  Initialize

   The purpose of this state is to wait for a Startup event, that is, an
   implementation-defined mechanism that initiates the protocol once it
   has been configured.  The configuration mechanism is out of scope for
   this specification.

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    (100) If a Startup event is received, then:

         (105) - If the Priority = 255, i.e., the router owns the IPvX
                 address(es) associated with the Virtual Router, then:

            (110) + Send an ADVERTISEMENT

            (115) + If the protected IPvX address is an IPv4 address,
                    then:

               (120) * For each IPv4 address associated with the Virtual
                       Router, broadcast a gratuitous ARP message
                       containing the Virtual Router MAC address and
                       with the target link-layer address set to the
                       Virtual Router MAC address.

            (125) + else // IPv6

               (130) * 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) clear, the Override flag (O)
                       set, the target address set to the IPv6 address
                       of the Virtual Router, and the target link-layer
                       address set to the Virtual Router MAC address.

            (135) +endif // was protected address IPv4?

            (140) + Set the Adver_Timer to Advertisement_Interval

            (145) + Transition to the {Active} state

         (150) - else // Router is not the address owner

            (155) + Set Active_Adver_Interval to Advertisement_Interval

            (160) + Set the Active_Down_Timer to Active_Down_Interval

            (165) + Transition to the {Backup} state

         (170) -endif // was priority 255?

     (175) endif // startup event was received

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6.4.2.  Backup

   The purpose of the {Backup} state is to monitor the availability and
   state of the Active Router.  The Solicited-Node multicast address
   [RFC4291] is referenced in the pseudo-code below.

   (300) While in Backup state, a VRRP Router MUST do the following:

         (305) - If the protected IPvX address is an IPv4 address,
                 then:

            (310) + MUST NOT respond to ARP requests for the IPv4
                    address(es) associated with the Virtual Router.

         (315) - else // protected address is IPv6

            (320) + MUST NOT respond to ND Neighbor Solicitation
                    messages for the IPv6 address(es) associated with
                    the Virtual Router.

            (325) + MUST NOT send ND Router Advertisement messages
                    for the Virtual Router.

         (330) -endif // was protected address IPv4?

         (335) - MUST discard packets with a destination link-layer
                 MAC address equal to the Virtual Router MAC address.

         (340) - MUST NOT accept packets addressed to the IPvX
                 address(es) associated with the Virtual Router.

         (345) - If a Shutdown event is received, then:

            (350) + Cancel the Active_Down_Timer

            (355) + Transition to the {Initialize} state

         (360) -endif // shutdown received

         (365) - If the Active_Down_Timer fires, then:

            (370) + Send an ADVERTISEMENT

            (375) + If the protected IPvX address is an IPv4 address,
                    then:

               (380) * For each IPv4 address associated with the Virtual
                       Router, broadcast a gratuitous ARP message

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                       containing the Virtual Router MAC address and
                       with the target link-layer address set to the
                       Virtual Router MAC address.

            (385) + else // IPv6

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

               (395) * 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) clear, the Override flag (O)
                       set, the target address set to the IPv6 address
                       of the Virtual Router, and the target link-layer
                       address set to the Virtual Router MAC address.

            (400) +endif // was protected address IPv4?

            (405) + Set the Adver_Timer to Advertisement_Interval

            (410) + Transition to the {Active} state

         (415) -endif // Active_Down_Timer fired

         (420) - If an ADVERTISEMENT is received, then:

            (425) + If the Priority in the ADVERTISEMENT is 0, then:

               (430) * Set the Active_Down_Timer to Skew_Time

            (440) + else // priority non-zero

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

                  (450) @ Set Active_Adver_Interval to Max Advertise
                        Interval contained in the ADVERTISEMENT

                  (452) @ Recompute the Skew_Time

                  (455) @ Recompute the Active_Down_Interval,

                  (460) @ Set the Active_Down_Timer to
                          Active_Down_Interval

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                  (465) * else // preempt was true and priority was less
                                  than the local priority

                  (470) @ Discard the ADVERTISEMENT

               (475) *endif // preempt test

            (480) +endif // was priority 0?

         (485) -endif // was advertisement received?

      (490) endwhile // Backup state

6.4.3.  Active

   While in the {Active} state, the router functions as the forwarding
   router for the IPvX address(es) associated with the Virtual Router.

   Note that in the Active state, the Preempt_Mode Flag is not
   considered.

   (600) While in Active state, a VRRP Router MUST do the following:

         (605) - If the protected IPvX address is an IPv4 address, then:

            (610) + MUST respond to ARP requests for the IPv4
                    address(es) associated with the Virtual Router.

         (615) - else // IPv6

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

            (625) + MUST respond to ND Neighbor Solicitation message for
                    the IPv6 address(es) associated with the Virtual
                    Router.

            (630) + MUST send ND Router Advertisements for the Virtual
                    Router.

            (635) + If Accept_Mode is False:  MUST NOT drop IPv6
                    Neighbor Solicitations and Neighbor Advertisements.

         (640) -endif // IPv4?

         (645) - MUST forward packets with a destination link-layer MAC
                 address equal to the Virtual Router MAC address.

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         (650) - 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.

         (655) - If a Shutdown event is received, then:

            (660) + Cancel the Adver_Timer

            (665) + Send an ADVERTISEMENT with Priority = 0

            (670) + Transition to the {Initialize} state

         (675) -endif // shutdown received

         (680) - If the Adver_Timer fires, then:

            (685) + Send an ADVERTISEMENT

            (690) + Reset the Adver_Timer to Advertisement_Interval

         (695) -endif // advertisement timer fired

         (700) - If an ADVERTISEMENT is received, then:

            (705) + If the Priority in the ADVERTISEMENT is 0, then:

               (710) * Send an ADVERTISEMENT

               (715) * Reset the Adver_Timer to Advertisement_Interval

            (720) + else // priority was non-zero

               (725) * If the Priority in the ADVERTISEMENT is greater
                       than the local Priority,

               (730) * or

               (735) * 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:

                  (740) @ Cancel Adver_Timer

                  (745) @ Set Active_Adver_Interval to Max Advertise
                          Interval contained in the ADVERTISEMENT

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                  (750) @ Recompute the Skew_Time

                  (755) @ Recompute the Active_Down_Interval

                  (760) @ Set Active_Down_Timer to Active_Down_Interval

                  (765) @ Transition to the {Backup} state

               (770) * else // new Active Router logic

                  (775) @ Discard ADVERTISEMENT

                  (778) @ Send an ADVERTISEMENT immediately to assert
                        Active state to the sending VRRP Router and
                        to update any learning bridges with the correct
                        Active VRRP Router path.

               (780) *endif // new Active Router detected

            (785) +endif // was priority zero?

         (790) -endif // advert received

      (795) endwhile // in Active state

   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 to which the Virtual Router is
   attached.

7.  Sending and Receiving VRRP Packets

7.1.  Receiving VRRP Packets

   The following functions must be performed when a VRRP packet is
   received:

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   - If the received packet is an IPv4 packet, then:

      + MUST verify that the IPv4 TTL is 255.

   - else // IPv6 VRRP packet received

     + MUST verify that the IPv6 Hop Limit is 255.

   -endif

   - MUST verify that the VRRP Version is 3.

   - MUST verify that the VRRP Packet Type is 1 (ADVERTISEMENT).

   - 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 = 255 (decimal)).

   If any one of the above checks fails, the receiver MUST discard
   the packet, SHOULD log the event (subject to rate-limiting), and
   MAY indicate via network management that an error occurred.

   A receiver SHOULD also verify that the Max Advertise Interval in the
   received VRRP packet matches the Advertisement_Interval configured
   for the VRID.  Instability can occur with differing intervals (refer
   to Section 5.2.7).  If this check fails, the receiver SHOULD log the
   event (subject to rate-limiting) and MAY indicate via network
   management that a misconfiguration was detected.

   A receiver MAY also verify that "IPvX Addr Count" and the list of
   IPvX address(es) match the IPvX Address(es) configured for the VRID.
   If this check fails, the receiver SHOULD log (subject to rate-
   limiting) 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:

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   - Fill in the VRRP packet fields with the appropriate Virtual
     Router configuration state

   - Compute the VRRP checksum

   - Set the source MAC address to the Virtual Router MAC Address

   - If the protected address is an IPv4 address, then:

      + Set the source IPv4 address to the interface primary IPv4
        address

   - else // IPv6

      + Set the source IPv6 address to the 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 to which the Virtual Router is
   attached.

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 Organizational
   Unique Identifier (OUI).  The next two octets (00-01) indicate the
   address block assigned to the VRRP protocol for the IPv4 protocol.
   {VRID} is the Virtual Router Identifier.  This mapping provides for
   up to 255 IPv4 VRRP Routers on a LAN.

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

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   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
   protocol for the IPv6 protocol. {VRID} is the Virtual Router
   Identifier.  This mapping provides for up to 255 IPv6 VRRP Routers on
   a LAN.

7.4.  IPv6 Interface Identifiers

   [RFC8064] specifies that [RFC7217] be used as the default scheme for
   generating stable address in IPv6 Stateless Address Autoconfiguration
   (SLAAC) [RFC4862].  The Virtual Router MAC MUST NOT be used for the
   Net_Iface parameter used in the Interface Identifier (IID) derivation
   algorithms in [RFC7217] and [RFC8981].

   Similarly, the Virtual Router MAC MUST NOT be used for the Net_Iface
   parameter used for the Interface Identifier (IID) derivation
   algorithms in [RFC7217] and [RFC8981].

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

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
   that the end-host used when making its next-hop routing decision.  If
   a VRRP Router is acting as Active Router for Virtual Router(s)
   containing address(es) it does not own, then it must determine to
   which Virtual Router the packet was sent 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.

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8.1.2.  Host ARP Requests

   When a host sends an ARP request for one of the Virtual Router IPv4
   addresses, the Active Router MUST respond to the ARP request with an
   ARP response that indicates the Virtual Router MAC address for the
   Virtual Router.  Note that the source address of the Ethernet frame
   of this ARP response is the physical MAC address of the physical
   router.  The Active Router MUST NOT respond with its physical MAC
   address in the ARP response.  This allows the host to always use the
   same MAC address regardless of the current Active Router.

   When a VRRP Router restarts or boots, it SHOULD NOT send any ARP
   messages using its physical MAC address for an IPv4 address for which
   it is the IPv4 Address Owner (as defined in Section 1.7), and it
   should only send ARP messages that include Virtual Router MAC
   addresses.

   This may entail the following:

   *  When configuring an interface, Active Routers should broadcast a
      gratuitous ARP message containing the Virtual Router MAC address
      for each IPv4 address on that interface.

   *  At system boot, when initializing interfaces for VRRP operation,
      gratuitous ARP messages must be delayed until both the IPv4
      address and the Virtual Router MAC address are configured.

   *  When, for example, SSH access to a particular VRRP Router is
      required, an IPv4 address known to belong to that router must be
      used.

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.

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   The IPv6 source address of an ICMPv6 redirect should be the address
   that the end-host used when making its next-hop routing decision.  If
   a VRRP Router is acting as Active Router for Virtual Router(s)
   containing address(es) it does not own, then it must determine to
   which Virtual Router the packet was sent 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 a Virtual
   Router IPv6 address, the Active Router MUST respond to the ND
   Neighbor Solicitation message with the Virtual Router MAC address for
   the Virtual Router.  The Active Router MUST NOT respond with its
   physical MAC address.  This allows the host to always use the same
   MAC address regardless of the current Active Router.

   When an Active Router sends an ND Neighbor Solicitation message for a
   host's IPv6 address, the Active Router MUST include the Virtual
   Router 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
   and it should only send ND messages that include Virtual Router MAC
   addresses.

   This may entail the following:

   *  When configuring an interface, Active Routers should send an
      unsolicited ND Neighbor Advertisement message containing the
      Virtual Router MAC address for the IPv6 address on that interface.

   *  At system boot, when initializing interfaces for VRRP operation,
      all ND Router and Neighbor Advertisements and Solicitation
      messages must be delayed until both the IPv6 address and the
      Virtual Router MAC address are configured.

   Note that on a restarting Active Router where the VRRP protected
   address is an interface address, i.e., the address owner, 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.

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8.2.3.  Router Advertisements

   When a Backup VRRP Router has become Active Router 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.

8.2.4.  Unsolicited Neighbor Advertisements

   A VRRP Router acting as either an IPv6 Active Router or Backup
   Router, SHOULD accept Unsolicited Neighbor Advertisements and update
   the corresponding neighbor cache [RFC4861].  Since these are sent to
   the IPv6 all-nodes multicast address (FF02::1) [RFC4861] or the IPv6
   all-routers multicast address (FF02::2), they will be received.
   Unsolicited Neighbor Advertisements are sent both in the case where
   the link-level addresses change [RFC4861] and for gratuitous neighbor
   discovery by first hop routers [RFC9131].  Additional configuration
   MAY be required in order for Unsolicited Neighbor Advertisements to
   update the corresponding neighbor cache.

8.3.  IPvX

8.3.1.  Potential Forwarding Loop

   If it is not the address owner, a VRRP Router SHOULD NOT forward
   packets addressed to the IPvX address for which it becomes Active
   Router.  Forwarding these packets would result in unnecessary
   traffic.  Also, in the case of LANs that receive packets they
   transmit, this can result in a forwarding loop that is only
   terminated when the IPvX TTL expires.

   One mechanism for VRRP Routers to these forwarding loops is to add/
   delete a host reject route for each non-owned IPvX address when
   transitioning to/from Active 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" for the VRID.  VRRP Routers with priority 255 will, as
   soon as they start up, preempt all lower-priority routers.  For a
   VRID, only a single VRRP Router on the link SHOULD be configured with
   priority 255.  If multiple VRRP Routers advertising priority 255 are
   detected, the condition SHOULD be logged (subject to rate-limiting).
   If no VRRP Router has this priority, and preemption is disabled, then
   no preemption will occur.

   In order to avoid two or more Backup Routers simultaneously becoming
   Active Routers after the previous Active Router fails or is shut
   down, all Virtual Routers SHOULD be configured with different
   priorities, and with sufficient differences in priority so that lower
   priority Backup Routers do not transition to Active state before
   receiving an advertisement from the highest priority Backup Router
   following it transitioning to Active Router.

   Since the Skew_Time is is inversely proportional to the priority,
   faster convergence can be obtained by using a higher priority for the
   preferred Backup Router.  However, with multiple Backup Routers, the
   priorities should have sufficient differences as previously
   recommended.

8.4.  VRRPv3 and VRRPv2 Interoperation

8.4.1.  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.

8.4.2.  VRRPv3 Support of VRRPv2 Interoperation

   As mentioned above, this support is intended for upgrade scenarios
   and is 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 a Virtual Router is configured this way and is the Active
   Router, it MUST send both types at the configured rate, even if sub-
   second.

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   When a Virtual Router is configured this way and is the Backup
   Router, it should time out based on the rate advertised by the Active
   Router.  In the case of a VRRPv2 Active Router, this means it must
   translate the timeout value it receives (in seconds) into
   centiseconds.  Also, a Backup Router should ignore VRRPv2
   advertisements from the current Active Router if it is also receiving
   VRRPv3 packets from it.  It MAY report when a VRRPv3 Active Router is
   not sending VRRPv2 packets as this suggests they don't agree on
   whether they're supporting VRRPv2 interoperation.

8.4.2.1.  Interoperation Considerations

8.4.2.1.1.  Slow, High-Priority Active Routers

   See also Section 5.2.7, "Maximum Advertisement Interval (Max
   Advertise Interval)".

   The VRRPv2 Active 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 with which it is interoperating if the
   VRRPv2/VRRPv3 router's advertisement rate is sub-second.

8.4.2.1.2.  Overwhelming VRRPv2 Backups

   It seems possible that a VRRPv3 Active Router sending at centisecond
   rates could potentially overwhelm a VRRPv2 Backup Router with
   potentially non-deterministic results.

   In this upgrade case, a deployment should initially run the VRRPv3
   Active Routers with lower frequencies, e.g., 100 centiseconds, until
   the VRRPv2 routers are upgraded.  Then, once the deployment has
   verified that VRRPv3 is working properly, the VRRPv2 support may be
   disabled and the desired sub-second rates may be configured.

9.  Security Considerations

   VRRP for IPvX does not currently include any type of authentication.
   Earlier versions of the VRRP specification included several types of
   authentication ranging from no authentication to strong
   authentication.  Operational experience and further analysis
   determined that these did not provide sufficient security to overcome
   the vulnerability of misconfigured secrets, causing multiple Active
   Routers to be elected.  Due to the nature of the VRRP protocol, even
   if VRRP messages are cryptographically protected, it does not prevent
   hostile nodes from behaving as if they are an Active Router, creating
   multiple Active Routers.  Authentication of VRRP messages could have

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   prevented a hostile node from causing all properly functioning
   routers from going into Backup state.  However, having multiple
   Active Routers can cause as much disruption as no routers, which
   authentication cannot prevent.  Also, even if a hostile node could
   not disrupt VRRP, it can disrupt ARP and create the same effect as
   having all routers go into Backup state.

   Some L2 switches provide the capability to filter out, for example,
   ARP and/or ND messages from end-hosts on a switch-port basis.  This
   mechanism could also filter VRRP messages from switch ports
   associated with end-hosts and can be considered for deployments with
   untrusted hosts.

   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 perform
   include:

   *  Promiscuously receiving packets for any router's MAC address.

   *  Sending packets with the router's MAC address as the source MAC
      address in the L2 header to tell the L2 switches to send packets
      addressed to the router to the malicious node instead of the
      router.

   *  Sending redirects to tell hosts to send their traffic somewhere
      else.

   *  Sending unsolicited ND replies.

   *  Answering ND requests, etc.

   All of these can be done independently of implementing VRRP.  VRRP
   does not add to these vulnerabilities.

   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 attacks on the local
   network.

   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)
   is deployed, VRRP is compatible with the "trust anchor" and "trust
   anchor or CGA" modes of SEND [RFC3971].  The SEND configuration needs

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   to give the Active and Backup Routers the same prefix delegation in
   the certificates so that Active and Backup Routers advertise the same
   set of subnet prefixes.  However, the Active and Backup Routers
   should have their own key pairs to avoid private key sharing.

10.  Contributors and Acknowledgments

   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 [VRRP-IPv6] 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, 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.

   Thanks to Steve Nadas for his work merging/editing [RFC3768] and
   [VRRP-IPv6] into the draft that eventually became RFC 5798 [RFC5798].

   Thanks to Stewart Bryant, Sasha Vainshtein, Pascal Thubert, Alexander
   Okonnikov, Ben Niven-Jenkins, Tim Chown, and Malisa Vucinic for
   comments on the current document (RFC 5798 BIS).  Thanks to Gyan
   Mishra, Paul Congdon, and Jon Rosen for discussions related to the
   removal of legacy technology appendices.  Thanks to Dhruv Dhody and
   Donald Eastlake for comments and suggestions for improving the IANA
   section.  Thanks to Sasha Vainshtein for recommending "Maximum
   Advertisement Interval" validation.  Thanks to Tim Chown and Fernando
   Gont for discussions and updates related to IPv6 SLAAC.

   Special thanks to Quentin Armitage for a detailed review and
   extensive comments on the current document (RFC 5798 BIS).

11.  IANA Considerations

   The value 112 is assigned to VRRP in the Assigned Internet Protocol
   Numbers Registry.

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   In the "Local Network Control Block (224.0.0.0 - 224.0.0.255
   (224.0.0/24))" of the "IPv4 Multicast Address Space Registry"
   [RFC5771], IANA has assigned the IPv4 multicast address 224.0.0.18
   for VRRP.

   In the "Link-Local Scope Multicast Addresses" block of the "IPv6
   Multicast Address Space Registry" [RFC3307], IANA has assigned the
   IPv6 link-local scope multicast address FF02:0:0:0:0:0:0:12 for VRRP
   for IPv6.

   In the "IANA MAC ADDRESS BLOCK" registry [RFC7042], IANA has assigned
   blocks of Ethernet unicast addresses as follows (in hexadecimal):

         00-01-00 to 00-01-FF VRRP
         00-02-00 to 00-02-FF VRRP IPv6

12.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3307]  Haberman, B., "Allocation Guidelines for IPv6 Multicast
              Addresses", RFC 3307, DOI 10.17487/RFC3307, August 2002,
              <https://www.rfc-editor.org/info/rfc3307>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
              DOI 10.17487/RFC5771, March 2010,
              <https://www.rfc-editor.org/info/rfc5771>.

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   [RFC7042]  Eastlake 3rd, D. and J. Abley, "IANA Considerations and
              IETF Protocol and Documentation Usage for IEEE 802
              Parameters", BCP 141, RFC 7042, DOI 10.17487/RFC7042,
              October 2013, <https://www.rfc-editor.org/info/rfc7042>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

13.  Informative References

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

   [NISTIR8366]
              "Guidance for NIST Staff on Using Inclusive Language in
              Documentary Standards, National Institute of Standards and
              Technology (NIST) Interagency or Internal Report 8366",
              NISTIR 8366, April 2021,
              <https://doi.org/10.6028/NIST.IR.8366>.

   [RFC1071]  Braden, R., Borman, D., and C. Partridge, "Computing the
              Internet checksum", RFC 1071, DOI 10.17487/RFC1071,
              September 1988, <https://www.rfc-editor.org/info/rfc1071>.

   [RFC1256]  Deering, S., Ed., "ICMP Router Discovery Messages",
              RFC 1256, DOI 10.17487/RFC1256, September 1991,
              <https://www.rfc-editor.org/info/rfc1256>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2281]  Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot
              Standby Router Protocol (HSRP)", RFC 2281,
              DOI 10.17487/RFC2281, March 1998,
              <https://www.rfc-editor.org/info/rfc2281>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

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   [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,
              DOI 10.17487/RFC2338, April 1998,
              <https://www.rfc-editor.org/info/rfc2338>.

   [RFC2453]  Malkin, G., "RIP Version 2", STD 56, RFC 2453,
              DOI 10.17487/RFC2453, November 1998,
              <https://www.rfc-editor.org/info/rfc2453>.

   [RFC3768]  Hinden, R., Ed., "Virtual Router Redundancy Protocol
              (VRRP)", RFC 3768, DOI 10.17487/RFC3768, April 2004,
              <https://www.rfc-editor.org/info/rfc3768>.

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <https://www.rfc-editor.org/info/rfc3971>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC5798]  Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
              Version 3 for IPv4 and IPv6", RFC 5798,
              DOI 10.17487/RFC5798, March 2010,
              <https://www.rfc-editor.org/info/rfc5798>.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,
              <https://www.rfc-editor.org/info/rfc7217>.

   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,
              <https://www.rfc-editor.org/info/rfc8064>.

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,
              <https://www.rfc-editor.org/info/rfc8981>.

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   [RFC9131]  Linkova, J., "Gratuitous Neighbor Discovery: Creating
              Neighbor Cache Entries on First-Hop Routers", RFC 9131,
              DOI 10.17487/RFC9131, October 2021,
              <https://www.rfc-editor.org/info/rfc9131>.

   [VRRP-IPv6]
              Hinden, R. and J. Cruz, "Virtual Router Redundancy
              Protocol for IPv6", Work in Progress, March 2007.

Authors' Addresses

   Acee Lindem
   LabN Consulting, L.L.C.
   301 Midenhall Way
   Cary, NC 27513
   United States of America
   Email: acee.ietf@gmail.com

   Aditya Dogra
   Cisco Systems
   Sarjapur Outer Ring Road
   Bangalore 560103
   Karnataka
   India
   Email: addogra@cisco.com

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