Internet Engineering Task Force (IETF) A. Lindem

Internet Engineering Task Force (IETF) A. Lindem
Request for Comments: 9568 LabN Consulting, L.L.C.
Obsoletes: 5798 A. Dogra
Category: Standards Track Cisco Systems
ISSN: 2070-1721 April 2024

Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6

Abstract

This document defines version 3 of the Virtual Router Redundancy
Protocol (VRRP) for IPv4 and IPv6. It obsoletes RFC 5798, which
previously specified VRRP (version 3). RFC 5798 obsoleted RFC 3768,
which specified VRRP (version 2) for IPv4. 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 routed 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.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9568.

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Differences from RFC 5798
1.2. A Note on Terminology
1.3. IPv4
1.4. IPv6
1.5. Requirements Language
1.6. Scope
1.7. Definitions
2. Required Features
2.1. IPvX Address Backup
2.2. Preferred Path Indication
2.3. Minimization of Unnecessary Service Disruptions
2.4. Efficient Operation over Extended LANs
2.5. Sub-second Operation for IPv4 and IPv6
3. VRRP Overview
4. Sample VRRP Networks
4.1. Sample VRRP Network 1
4.2. Sample VRRP Network 2
5. Protocol
5.1. VRRP Packet Format
5.1.1. IPv4 Field Descriptions
5.1.1.1. Source Address
5.1.1.2. Destination Address
5.1.1.3. TTL
5.1.1.4. Protocol
5.1.2. IPv6 Field Descriptions
5.1.2.1. Source Address
5.1.2.2. Destination Address
5.1.2.3. Hop Limit
5.1.2.4. Next Header
5.2. VRRP Field Descriptions
5.2.1. Version
5.2.2. Type
5.2.3. Virtual Rtr ID (VRID)
5.2.4. Priority
5.2.5. IPvX Addr Count
5.2.6. Reserve
5.2.7. Maximum Advertisement Interval (Max Advertise Interval)
5.2.8. Checksum
5.2.9. IPvX Address(es)
6. Protocol State Machine
6.1. Parameters per Virtual Router
6.2. Timers
6.3. State Transition Diagram
6.4. State Descriptions
6.4.1. Initialize
6.4.2. Backup
6.4.3. Active
7. Sending and Receiving VRRP Packets
7.1. Receiving VRRP Packets
7.2. Transmitting VRRP Packets
7.3. Virtual Router MAC Address
7.4. IPv6 Interface Identifiers
8. Operational Issues
8.1. IPv4
8.1.1. ICMP Redirects
8.1.2. Host ARP Requests
8.1.3. Proxy ARP
8.2. IPv6
8.2.1. ICMPv6 Redirects
8.2.2. ND Neighbor Solicitation
8.2.3. Router Advertisements
8.2.4. Unsolicited Neighbor Advertisements
8.3. IPvX
8.3.1. Potential Forwarding Loop
8.3.2. Recommendations Regarding Setting Priority Values
8.4. VRRPv3 and VRRPv2 Interoperation
8.4.1. Assumptions
8.4.2. VRRPv3 Support of VRRPv2 Interoperation
8.4.2.1. Interoperation Considerations
9. Security Considerations
10. IANA Considerations
11. References
11.1. Normative References
11.2. Informative References
Acknowledgments
Authors' Addresses

1. Introduction

This document defines version 3 of the Virtual Router Redundancy
Protocol (VRRP) for IPv4 and IPv6. It obsoletes [RFC5798], which
previously specified VRRP (version 3). [RFC5798] obsoleted
[RFC3768], which specified VRRP (version 2) for IPv4. 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 routed to these IPv4 or IPv6 addresses (except for
packets addressed to these addresses as described in Section 8.3.1).
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.

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

1.1. Differences from RFC 5798

The following changes have been made from [RFC5798]:

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

2. 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 the
terminology of [RFC5798] for both "Active Router" and "Backup
Router" were corrected. Additionally, the undesirable term for
attracting and dropping unreachable packets has been changed.

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

4. 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.

5. 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 (refer to Section 6.4.3).

6. Appendices describing operation over legacy technologies (Fiber
Distributed Data Interface (FDDI), Token Ring, and ATM LAN
Emulation) were removed.

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

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

9. Miscellaneous editorial changes were made for readability.

10. The IANA Considerations section was augmented to include all the
IPv4/IPv6 multicast address allocations and Ethernet Media
Access Control (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" often 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.

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], running
DHCPv4 [RFC2131], 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 manually configured default route (either via a static
route or DHCPv4) is quite popular since it minimizes configuration
and processing overhead on the end-host and is supported by virtually
every IPv4 implementation. 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
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.

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 can 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 Neighbor 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.

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
can serve as a Backup Router for 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.

Forwarding Responsibility The responsibility for forwarding packets
sent to the IPvX address(es) associated with
the Virtual Router. This includes receiving
packets sent to the Virtual Router MAC
address, forwarding these packets based on
the local Routing Information Base (RIB) /
Forwarding Information Base (FIB), answering
ARP requests for the IPv4 address(es), and
answering ND requests for the IPv6
address(es).

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.

Drop Route A route installed in the Routing Information
Base (RIB) that will result in traffic with a
destination address that matches the route to
be dropped.

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,
and

* 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 the 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 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 of 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 that 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
up on the VRRP Router. When this occurs, it is possible that packets
being transmitted onto the VRRP-protected LAN could see a 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 the VRRP Router to switch back
to Backup Router. Furthermore, this process can repeat many times
per second, causing a significant disruption of traffic. To mitigate
this problem, giving VRRP packets priority on egress interface queues
should be considered. If the Active Router observes that this is
occurring, it SHOULD log the problem (subject to rate-limiting).

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 (L2) 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
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 the 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 the Backup
Router to the 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 VRRP Networks

4.1. Sample VRRP Network 1

The following figure shows a simple network with two VRRP Routers
implementing one Virtual Router.

+-----------+ +-----------+
| 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

Figure 1: Sample VRRP Network 1

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 default route (learned through DHCPv4 or via a
configured static 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.

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
IPvX 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 the
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 the 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 the 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 VRRP Network 2

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

+-----------+ +-----------+
| 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

Figure 2: Sample VRRP Network 2

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 the
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 are using 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, IPv6_Address=B).
In this case, Router-2 will assert itself as the 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 [RFC4311].

5. Protocol

The purpose of the VRRP Advertisement is to communicate to all VRRP
Routers the priority, Maximum 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).

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) |
+ +
+ +
+ +
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: IPv4/IPv6 VRRP Advertisement Packet Format

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 [RFC5082].

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
[RFC5082].

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

5.2.4. Priority

The Priority field specifies sending the 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 the Active
Router without having to wait for the current Active_Down_Interval
(refer to Section 6.1).

5.2.5. IPvX Addr Count

The IPvX Addr Count field is the number of either IPv4 addresses or
IPv6 addresses contained in this VRRP advertisement. The minimum
value is 1. If the received count is 0, the VRRP advertisement MUST
be ignored.

5.2.6. Reserve

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

5.2.7. Maximum Advertisement Interval (Max Advertise Interval)

The Max Advertise 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 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, i.e., 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 VRRP
message. For computing the checksum, the Checksum field is set to
zero. See [RFC1071] for more details.

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 address family of the addresses, IPv4 or IPv6
but not both, MUST be the same as the VRRP packet's IPvX header
address family.

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 VRRP Advertisements
(centiseconds) sent by this Virtual
Router. Default is 100 centiseconds (1
second).

Active_Adver_Interval Advertisement interval contained in VRRP
Advertisements received from the Active
Router (in centiseconds). This value is
saved by Virtual Routers in the Backup
state and used to compute Skew_Time (as
specified in Section 8.3.2) 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
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 |
| |<----------------------| |
+---------------+ +---------------+

Figure 4: State Transition Diagram

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.

If a Startup event is received, then:

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

- Send an ADVERTISEMENT

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

o 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.

- else // IPv6

o 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.

- endif // was protected address IPv4?

- Set the Adver_Timer to Advertisement_Interval

- Transition to the {Active} state

* else // Router is not the address owner

- Set the Active_Adver_Interval to Advertisement_Interval

- Set the Active_Down_Timer to Active_Down_Interval

- Transition to the {Backup} state

* endif // was priority 255?

endif // Startup event was received

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 pseudocode below.

While in the {Backup} state, a VRRP Router MUST do the following:

* If the protected IPvX address is an IPv4 address, then:

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

* else // protected address is IPv6

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

- It MUST NOT send ND Router Advertisement messages for the
Virtual Router.

* endif // was protected address IPv4?

* It MUST discard packets with a destination link-layer MAC address
equal to the Virtual Router MAC address.

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

* If a Shutdown event is received, then:

- Cancel the Active_Down_Timer

- Transition to the {Initialize} state

* endif // Shutdown event received

* If the Active_Down_Timer fires, then:

- Send an ADVERTISEMENT

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

o 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.

- else // IPv6

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

o 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.

- endif // was protected address IPv4?

- Set the Adver_Timer to Advertisement_Interval

- Transition to the {Active} state

* endif // Active_Down_Timer fired

* If an ADVERTISEMENT is received, then:

- If the Priority in the ADVERTISEMENT is 0, then:

o Set the Active_Down_Timer to Skew_Time

- else // priority non-zero

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

+ Set the Active_Adver_Interval to the Max Advertise
Interval contained in the ADVERTISEMENT

+ Recompute the Skew_Time

+ Recompute the Active_Down_Interval

+ Set the Active_Down_Timer to Active_Down_Interval

o else // preempt was true and priority was less than the
local priority

+ Discard the ADVERTISEMENT

o endif // preempt test

- endif // was priority 0?

* endif // was advertisement received?

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.

While in the {Active} state, a VRRP Router MUST do the following:

* If the protected IPvX address is an IPv4 address, then:

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

* else // IPv6

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

- It MUST respond to ND Neighbor Solicitation messages (with the
Router Flag (R) set) for the IPv6 address(es) associated with
the Virtual Router.

- It MUST send ND Router Advertisements for the Virtual Router.

- If Accept_Mode is False:

o It MUST NOT drop IPv6 Neighbor Solicitations and Neighbor
Advertisements.

* endif // IPv4?

* It MUST forward packets with a destination link-layer MAC address
equal to the Virtual Router MAC address.

* It 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, it MUST NOT accept these
packets.

* If a Shutdown event is received, then:

- Cancel the Adver_Timer

- Send an ADVERTISEMENT with Priority = 0

- Transition to the {Initialize} state

* endif // shutdown received

* If the Adver_Timer fires, then:

- Send an ADVERTISEMENT

- Reset the Adver_Timer to Advertisement_Interval

* endif // advertisement timer fired

* If an ADVERTISEMENT is received, then:

- If the Priority in the ADVERTISEMENT is 0, then:

o Send an ADVERTISEMENT

o Reset the Adver_Timer to Advertisement_Interval

- else // priority was non-zero

o If the Priority in the ADVERTISEMENT is greater than the
local Priority or 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 (based
on an unsigned integer comparison of the IPvX addresses in
network byte order), then:

+ Cancel Adver_Timer

+ Set the Active_Adver_Interval to the Max Advertise
Interval contained in the ADVERTISEMENT

+ Recompute the Skew_Time

+ Recompute the Active_Down_Interval

+ Set the Active_Down_Timer to Active_Down_Interval

+ Transition to the {Backup} state

o else // new Active Router logic

+ Discard the ADVERTISEMENT

+ Send an ADVERTISEMENT immediately to assert the {Active}
state to the sending VRRP Router and to update any
learning bridges with the correct Active VRRP Router
path.

o endif // new Active Router detected

- endif // was priority zero?

* endif // advert received

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:

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

- It MUST verify that the IPv4 TTL is 255.

* else // IPv6 VRRP packet received

- It MUST verify that the IPv6 Hop Limit is 255.

* endif

* It MUST verify that the VRRP version is 3.

* It MUST verify that the VRRP packet type is 1 (ADVERTISEMENT).

* It MUST verify that the received packet contains the complete VRRP
packet (including fixed fields and the IPvX address).

* It MUST verify the VRRP checksum.

* It 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:

* 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's primary IPv4
address

* else // IPv6

- Set the source IPv6 address to the interface's 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 [RFC9542] in the following format:

IPv4 case: 00-00-5E-00-01-{VRID} (in hex, in network byte order)

The first three octets are derived from the IANA's Organizationally
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 network byte 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
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 a 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].

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 can be used normally when VRRP is running among 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 the 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 among 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 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 entails 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, Secure Shell (SSH) access to a particular VRRP
Router is required, an IPv4 address known to belong to that router
SHOULD 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 can be used normally when VRRP is running among 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
that the end-host used when making its next-hop routing decision. If
a VRRP Router is acting as the Active Router for Virtual Router(s)
containing address(es) it does not own, then it has to 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 entails 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 Advertisements, ND Neighbor Advertisements, and ND
Neighbor 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 may fail, as the Backup Router MAY answer that it
owns the address. One solution is to not run Duplicate Address
Detection in this case.

8.2.3. Router Advertisements

When a Backup VRRP Router has become the 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 the 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 avoid these forwarding loops is to
add/delete a host Drop Route for each non-owned IPvX address when
transitioning to/from the Active state.

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 the priorities so that
lower priority Backup Routers do not transition to the Active state
before receiving an advertisement from the highest priority Backup
Router when it transitions to the Active Router. If multiple VRRP
Routers advertising the same priority are detected, this condition
MAY be logged as a warning (subject to rate-limiting).

Since the Skew_Time is reduced as the priority is increased, 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 it is
sub-second.

When a Virtual Router is configured this way and is the Backup
Router, it MUST 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 or VRRPv3 implementation with which it is interoperating if
the VRRPv2 or 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
prevented a hostile node from causing all properly functioning
routers from going into the 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/ND and create the same effect as
having all routers go into the 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, and most of these
vulnerabilities are addressed independently, e.g., SEcure Neighbor
Discovery (SEND) [RFC3971].

VRRP includes a mechanism (setting IPv4 TTL or IPv6 Hop Limit to 255
and checking the value on receipt) that protects against VRRP packets
being injected from another remote network [RFC5082]. 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 SEND is deployed, VRRP is
compatible with the "trust anchor" and "trust anchor or CGA" modes of
SEND [RFC3971]. The SEND configuration needs 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.

Also in the context of IPv6 operation, it is RECOMMENDED that the
link-level security guidelines in Section 2.3 of [RFC9099] be
followed.

10. IANA Considerations

IANA has updated all IANA registry references to [RFC5798] to
references to RFC 9568, i.e., this document. The individual IANA
references are listed below.

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

In the "Local Network Control Block (224.0.0.0 - 224.0.0.255
(224.0.0/24))" registry 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" registry 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 [RFC9542], IANA has assigned
blocks of Ethernet unicast addresses as follows (in hexadecimal):

+======================+===========================+===========+
| Addresses | Usage | Reference |
+======================+===========================+===========+
| 00-01-00 to 00-01-FF | VRRP (Virtual Router | RFC 9568 |
| | Redundancy Protocol) | |
+----------------------+---------------------------+-----------+
| 00-02-00 to 00-02-FF | VRRP IPv6 (Virtual Router | RFC 9568 |
| | Redundancy Protocol IPv6) | |
+----------------------+---------------------------+-----------+

Table 1

11. References

11.1. 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>.

[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
<https://www.rfc-editor.org/info/rfc5082>.

[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>.

[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>.

[RFC9542] Eastlake 3rd, D., Abley, J., and Y. Li, "IANA
Considerations and IETF Protocol and Documentation Usage
for IEEE 802 Parameters", BCP 141, RFC 9542,
DOI 10.17487/RFC9542, April 2024,
<https://www.rfc-editor.org/info/rfc9542>.

11.2. 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]
National Institute of Standards and Technology (NIST),
"Guidance for NIST Staff on Using Inclusive Language in
Documentary Standards,", NISTIR 8366,
DOI 10.6028/NIST.IR.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>.

[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>.

[RFC4311] Hinden, R. and D. Thaler, "IPv6 Host-to-Router Load
Sharing", RFC 4311, DOI 10.17487/RFC4311, November 2005,
<https://www.rfc-editor.org/info/rfc4311>.

[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>.

[RFC9099] Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey,
"Operational Security Considerations for IPv6 Networks",
RFC 9099, DOI 10.17487/RFC9099, August 2021,
<https://www.rfc-editor.org/info/rfc9099>.

[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, Internet-Draft,
draft-ietf-vrrp-ipv6-spec-08, 5 March 2007,
<https://datatracker.ietf.org/doc/html/draft-ietf-vrrp-
ipv6-spec-08>.

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 authors 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 M. Bellovin, Thomas Narten, Rob Montgomery, Rob
Coltun, Radia Perlman, Russ Housley, Harald Alvestrand, Ned Freed,
Ted Hardie, 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 document that eventually became [RFC5798].

Thanks to Stewart Bryant, Sasha Vainshtein, Pascal Thubert, Alexander
Okonnikov, Ben Niven-Jenkins, Tim Chown, Mališa Vučinić, Russ White,
Donald Eastlake, Dave Thaler, Eric Kline, and Vijay Gurbani for
comments on the current document (RFC 9568). 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 9568).

Authors' Addresses

Acee Lindem
LabN Consulting, L.L.C.
301 Midenhall Way
Cary, NC 27513
United States of America
Email: [email protected]

Aditya Dogra
Cisco Systems
Sarjapur Outer Ring Road
Bangalore 560103
Karnataka
India
Email: [email protected]