Internet DRAFT - draft-addogra-rtgwg-vrrp-rfc5798bis
draft-addogra-rtgwg-vrrp-rfc5798bis
Network Working Group A. Lindem
Internet-Draft A. Dogra
Obsoletes: 5798 (if approved) Cisco Systems
Intended status: Standards Track 9 July 2022
Expires: 10 January 2023
Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6
draft-addogra-rtgwg-vrrp-rfc5798bis-12
Abstract
This document defines the Virtual Router Redundancy Protocol (VRRP)
for IPv4 and IPv6. It is version three (3) of the protocol, and it
is based on VRRP (version 2) for IPv4 that is defined in RFC 3768 and
in "Virtual Router Redundancy Protocol for IPv6". 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 VRRP 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 VRRP Backup Routers infer
the address family of the virtual addresses being carried based on
the transport protocol. Within a VRRP router, the virtual routers in
each of the IPv4 and IPv6 address families are a domain unto
themselves and do not overlap. The election process provides dynamic
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 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. This document obsoletes VRRP Version
3 [RFC5798].
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. RFC 5798 Differences . . . . . . . . . . . . . . . . . . 4
2. A Note on Terminology . . . . . . . . . . . . . . . . . . . . 5
3. IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Requirements Language . . . . . . . . . . . . . . . . . . . . 7
6. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Required Features . . . . . . . . . . . . . . . . . . . . . . 8
8.1. IPvX Address Backup . . . . . . . . . . . . . . . . . . . 8
8.2. Preferred Path Indication . . . . . . . . . . . . . . . . 9
8.3. Minimization of Unnecessary Service Disruptions . . . . . 9
8.4. Efficient Operation over Extended LANs . . . . . . . . . 9
8.5. Sub-Second Operation for IPv4 and IPv6 . . . . . . . . . 10
9. VRRP Overview . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Sample Configurations . . . . . . . . . . . . . . . . . . . . 11
10.1. Sample Configuration 1 . . . . . . . . . . . . . . . . . 12
10.2. Sample Configuration 2 . . . . . . . . . . . . . . . . . 14
11. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. VRRP Packet Format . . . . . . . . . . . . . . . . . . . 15
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11.1.1. IPv4 Field Descriptions . . . . . . . . . . . . . . 16
11.1.1.1. Source Address . . . . . . . . . . . . . . . . . 16
11.1.1.2. Destination Address . . . . . . . . . . . . . . 16
11.1.1.3. TTL . . . . . . . . . . . . . . . . . . . . . . 16
11.1.1.4. Protocol . . . . . . . . . . . . . . . . . . . . 17
11.1.2. IPv6 Field Descriptions . . . . . . . . . . . . . . 17
11.1.2.1. Source Address . . . . . . . . . . . . . . . . . 17
11.1.2.2. Destination Address . . . . . . . . . . . . . . 17
11.1.2.3. Hop Limit . . . . . . . . . . . . . . . . . . . 17
11.1.2.4. Next Header . . . . . . . . . . . . . . . . . . 17
11.2. VRRP Field Descriptions . . . . . . . . . . . . . . . . 17
11.2.1. Version . . . . . . . . . . . . . . . . . . . . . . 17
11.2.2. Type . . . . . . . . . . . . . . . . . . . . . . . . 17
11.2.3. Virtual Rtr ID (VRID) . . . . . . . . . . . . . . . 18
11.2.4. Priority . . . . . . . . . . . . . . . . . . . . . . 18
11.2.5. IPvX Addr Count . . . . . . . . . . . . . . . . . . 18
11.2.6. 0 - Reserved . . . . . . . . . . . . . . . . . . . . 18
11.2.7. Maximum Advertisement Interval (Max Adver Int) . . . 18
11.2.8. Checksum . . . . . . . . . . . . . . . . . . . . . . 19
11.2.9. IPvX Address(es) . . . . . . . . . . . . . . . . . . 19
12. Protocol State Machine . . . . . . . . . . . . . . . . . . . 19
12.1. Parameters Per Virtual Router . . . . . . . . . . . . . 19
12.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.3. State Transition Diagram . . . . . . . . . . . . . . . . 21
12.4. State Descriptions . . . . . . . . . . . . . . . . . . . 22
12.4.1. Initialize . . . . . . . . . . . . . . . . . . . . . 22
12.5. Backup . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.6. Active . . . . . . . . . . . . . . . . . . . . . . . . . 26
12.7. Virtual Router MAC Address . . . . . . . . . . . . . . . 28
12.8. IPv6 Interface Identifiers . . . . . . . . . . . . . . . 29
13. Operational Issues . . . . . . . . . . . . . . . . . . . . . 29
13.1. IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . 29
13.1.1. ICMP Redirects . . . . . . . . . . . . . . . . . . . 29
13.1.2. Host ARP Requests . . . . . . . . . . . . . . . . . 29
13.1.3. Proxy ARP . . . . . . . . . . . . . . . . . . . . . 30
13.2. IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . 30
13.2.1. ICMPv6 Redirects . . . . . . . . . . . . . . . . . . 30
13.2.2. ND Neighbor Solicitation . . . . . . . . . . . . . . 31
13.2.3. Router Advertisements . . . . . . . . . . . . . . . 31
13.2.4. Unsolicited Neighbor Advertisements . . . . . . . . 32
13.3. IPvX . . . . . . . . . . . . . . . . . . . . . . . . . . 32
13.3.1. Potential Forwarding Loop . . . . . . . . . . . . . 32
13.3.2. Recommendations Regarding Setting Priority Values . 32
13.4. VRRPv3 and VRRPv2 Interoperation . . . . . . . . . . . . 33
13.4.1. Assumptions . . . . . . . . . . . . . . . . . . . . 33
13.4.2. VRRPv3 Support of VRRPv2 . . . . . . . . . . . . . . 33
13.4.3. VRRPv3 Support of VRRPv2 Considerations . . . . . . 33
13.4.3.1. Slow, High-Priority Active Routers . . . . . . . 33
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13.4.3.2. Overwhelming VRRPv2 Backups . . . . . . . . . . 34
14. Security Considerations . . . . . . . . . . . . . . . . . . . 34
15. Contributors and Acknowledgments . . . . . . . . . . . . . . 35
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
17. Normative References . . . . . . . . . . . . . . . . . . . . 36
18. Informative References . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
This document defines the Virtual Router Redundancy Protocol (VRRP)
for IPv4 and IPv6. It is version three (3) of the protocol. It is
based on VRRP (version 2) for IPv4 that is defined in [RFC3768] and
in [VRRP-IPv6]. 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 VRRP
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 VRRP Backup Routers infer the address family of
the virtual addresses being carried based on the transport protocol.
Within a VRRP router, the virtual routers in each of the IPv4 and
IPv6 address families are a domain unto themselves and do not
overlap. The election process provides dynamic failover in the
forwarding responsibility should the Active Router become
unavailable.
The VRRP terminology has been updated 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:
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.
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2. Errata pertaining to the state machines in Section 12 were
corrected.
3. Appendicies describing operation over legacy technologies (FDDI,
Token Ring, and ATM LAN Emulation) were removed.
4. Miscellaneous editorial changes were made for readability.
5. The allocation of 224.0.0.18 was added to the IANA considerations
section.
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.
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 be
infeasible 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 associated protocol packets 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
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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 an 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 in 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 router discovery protocols on
every end-host.
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 multicast periodically at a rate at which the
hosts will learn about the default routers in a few minutes. They
are not sent frequently enough to rely on the absence of the Router
Advertisement to detect router failures.
Neighbor Discovery (ND) includes a mechanism called Neighbor
Unreachability Detection to detect the failure of a neighbor node
(router or host) or the forwarding path to a neighbor. This is done
by sending unicast ND Neighbor Solicitation messages to the neighbor
node. To reduce the overhead of sending Neighbor Solicitations, they
are only sent to neighbors to which the node is actively sending
traffic and only after there has been no positive indication that the
router is up for a period of time. Using the default parameters in
ND, it will take a host about 38 seconds to learn that a router is
unreachable before it will switch to another default router. This
delay would be very noticeable to users and cause some transport
protocol implementations to time out.
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While the ND unreachability detection could be made quicker by
changing the parameters to be more aggressive (note that the current
lower limit for this is 5 seconds), this would have the downside of
significantly increasing the overhead of ND traffic, especially when
there are many hosts all trying to determine the reachability of one
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.
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.
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 requests, generation
of ICMP redirect messages, and security issues are addressed.
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.
IP Address Owner The VRRP router that has the virtual router's
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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 mode, VRRP
advertisements are always sent using the
primary IPv4 address as the source of the
IPv4 packet. In IPv6, the link-local address
of the interface over which the packet is
transmitted is used.
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 become 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.
8. Required Features
This section describes the set of features that were considered
mandatory and that guided the design of VRRP.
8.1. IPvX Address Backup
Backup of an IPvX address or addresses is the primary function of
VRRP. When providing election of a 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.
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* Allow multiple virtual routers on a network for load-balancing.
* Support multiple logical IPvX subnets on a single LAN segment.
8.2. 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 preferential router currently
available.
8.3. Minimization of Unnecessary Service Disruptions
Once Active Router election has been performed, any unnecessary
transitions 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.
8.4. 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.
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2. Trigger a message immediately after transitioning to the Active
Router to update the MAC learning.
3. Trigger periodic messages from the Active Router to maintain the
MAC address cache.
8.5. 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 when using small
VRRP_Advertisement_Intervals may occur when a router is generating
more packets on a LAN than it can transmit, and a queue builds up on
the router. When this occurs, it is possible that packets being
transmitted onto the VRRP-protected LAN could see larger queueing
delay than the smallest VRRP Advertisement_Interval. In this case,
the Active_Down_Interval 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 Active Router cause a
switch back to Backup Router. Furthermore, this process can repeat
many times per second, causing significant disruption to 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.
9. 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 in an extended LAN.
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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 it 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 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 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 (< 1 second ). Thus, the VRRP optimizations represent
significant simplifications in the protocol design while incurring an
insignificant probability of brief network disruption.
10. Sample Configurations
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10.1. Sample Configuration 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, Token Ring, or FDDI
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 for
the VRRP protocol 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
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owner of IPv4 B. A virtual router is then defined by associating a
unique identifier (the virtual router ID) with the address owned by a
router.
In an IPv6 VRRP environment, each router supports reception and
transmission with the exact same Link-Local IPv6 address. In an IPv6
VRRP environment, each router will support transmission and reception
for the Link-Local IPv6 addresses associated with both VRIDs.
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 virtual router ID)
with the address owned by a router.
Finally, in both the IPv4 and IPv6 cases, the VRRP protocol manages
virtual router failover to a Backup Router.
The IPv4 example above shows a virtual router configured to cover the
IPv4 address owned by Router-1 (VRID=1, IPv4_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 IP address owner for the
virtual router IP 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 IPv4 address owner. If
Router-1 should fail, then the VRRP protocol will transition Router-2
to Active Router, temporarily taking over forwarding responsibility
for IPv4 A to provide uninterrupted service to the hosts. When
Router-1 returns to service, it will re-assert itself as Active
Router.
The IPv6 example above shows a virtual router configured to cover the
IPv6 address owned by Router-1 (VRID=1, IPv6_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 IPv6 address owner for
the virtual router IPv6 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 IPv6 address
owner. If Router-1 should fail, then the VRRP protocol will
transition Router-2 to Active Router, temporarily taking over
forwarding responsibility for IPv6 A to provide uninterrupted service
to the IPv6 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.
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10.2. Sample Configuration 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 A) (IPvX A)
| | | |
IPvX H1->* IPvX H2->* IPvX H3->* IPvX H4->*
+--+--+ +--+--+ +--+--+ +--+--+
| H1 | | H2 | | H3 | | H4 |
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = Ethernet, Token Ring, or FDDI
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 10.1), 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
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configuration of virtual router VRID=1 is exactly the same as in the
first example (see Section 10.1), 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 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.
11. Protocol
The purpose of the VRRP packet is to communicate to all VRRP routers
the priority and the state 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.
11.1. VRRP Packet Format
This section defines the format of the VRRP packet and the relevant
fields in the IP header.
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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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Max Adver Interval | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| IPvX Address(es) |
+ +
+ +
+ +
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
11.1.1. IPv4 Field Descriptions
11.1.1.1. Source Address
This is the primary IPv4 address of the interface from which the
packet is being sent.
11.1.1.2. 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.
11.1.1.3. 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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11.1.1.4. Protocol
The IPv4 protocol number assigned by the IANA for VRRP is 112
(decimal).
11.1.2. IPv6 Field Descriptions
11.1.2.1. Source Address
This is the IPv6 link-local address of the interface from which the
packet is being sent.
11.1.2.2. 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.
11.1.2.3. 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.
11.1.2.4. Next Header
The IPv6 Next Header protocol assigned by the IANA for VRRP is 112
(decimal).
11.2. VRRP Field Descriptions
11.2.1. Version
The version field specifies the VRRP protocol version of this packet.
This document defines version 3.
11.2.2. 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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11.2.3. Virtual Rtr ID (VRID)
The Virtual Rtr ID field identifies the virtual router for which this
packet is reporting status.
11.2.4. Priority
The priority field specifies the sending VRRP router's priority for
the virtual router. Higher values equal higher priority. This field
is an 8-bit unsigned integer field.
The priority value for the VRRP router that owns the IPvX address
associated with the virtual router MUST be 255 (decimal).
VRRP routers backing up a virtual router MUST use priority values
between 1-254 (decimal). The default priority value for VRRP routers
backing up a virtual router is 100 (decimal).
The priority value zero (0) has special meaning, indicating that the
current 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 Router to time out.
11.2.5. IPvX Addr Count
This is the number of either IPv4 addresses or IPv6 addresses
contained in this VRRP advertisement. The minimum value is 1.
11.2.6. 0 - Reserved
This reserved field MUST be set to zero on transmission and ignored
on reception.
11.2.7. Maximum Advertisement Interval (Max Adver Int)
The Maximum Advertisement Interval is a 12-bit field that indicates
the time interval (in centiseconds) between ADVERTISEMENTS. The
default is 100 centiseconds (1 second).
Note that higher-priority Active Routers with slower transmission
rates than their Backup Routers are unstable. This is because lower-
priority nodes configured to faster rates could come online 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.
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11.2.8. Checksum
The checksum field is used to detect data corruption in the VRRP
message.
The checksum is the 16-bit one's complement of the one's complement
sum of the entire VRRP message starting with the version field and a
"pseudo-header" as defined in Section 8.1 of [RFC2460]. The next
header field in the "pseudo-header" should be set to 112 (decimal)
for VRRP. For computing the checksum, the checksum field is set to
zero. See RFC1071 for more detail [RFC1071].
11.2.9. 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 "IP
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 protocol packet address family.
12. Protocol State Machine
12.1. 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 releasing responsibility
for the virtual router. The range 1-254
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(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.
Advertisement_Interval Time interval between ADVERTISEMENTS
(centiseconds). Default is 100
centiseconds (1 second).
Active_Adver_Interval Advertisement interval contained in
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 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.
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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 responses as the MAC
address to use for IPvX Addresses.
12.2. Timers
Active_Down_Timer Timer that fires when a VRRP Advertisement
has not been received for
Active_Down_Interval.
Adver_Timer Timer that fires to trigger transmission of
a VRRP Advertisement based on the
Advertisement_Interval.
12.3. State Transition Diagram
+---------------+
+--------->| |<-------------+
| | Initialize | |
| +------| |----------+ |
| | +---------------+ | |
| | | |
| V V |
+---------------+ +---------------+
| |---------------------->| |
| Active | | Backup |
| |<----------------------| |
+---------------+ +---------------+
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12.4. 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 election in which it is participating.
12.4.1. 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 of
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 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 request
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 does not own virtual address
(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 // priority was not 255
(175) endif // startup event was received
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12.5. 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 this 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 request
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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 zero, 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 Adver Interval
contained in the ADVERTISEMENT
(455) @ Recompute the Active_Down_Interval
(460) @ Reset the Active_Down_Timer to
Active_Down_Interval
(465) * else // preempt was true and priority was less
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(470) @ Discard the ADVERTISEMENT
(475) *endif // preempt test
(480) +endif // was priority zero?
(485) -endif // was advertisement received?
(490) endwhile // Backup state
12.6. 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 this 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.
(650) - MUST accept packets addressed to the IPvX address(es)
associated with the virtual router if it is the IPvX
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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 zero, 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 Adver Interval
contained in the ADVERTISEMENT
(750) @ Recompute the Skew_Time
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(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
(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 the virtual router is attached
to.
12.7. 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 for IPv4 protocol. {VRID} is the
VRRP Virtual Router Identifier. This mapping provides for up to 255
IPv4 VRRP routers on a network.
IPv6 case: 00-00-5E-00-02-{VRID} (in hex, in Internet-standard bit-
order)
The first three octets are derived from the IANA's OUI. The next two
octets (00-02) indicate the address block assigned to the VRRP
protocol for the IPv6 protocol. {VRID} is the VRRP Virtual Router
Identifier. This mapping provides for up to 255 IPv6 VRRP routers on
a network.
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12.8. IPv6 Interface Identifiers
IPv6 routers running VRRP MUST create their Interface Identifiers in
the normal manner. e.g., "Transmission of IPv6 Packets over Ethernet
Networks" [RFC2464]. They MUST NOT use the virtual router MAC
address to create the Modified Extended Unique Identifier (EUI)-64
identifiers.
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.
13. Operational Issues
13.1. IPv4
13.1.1. 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 addresses 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.
13.1.2. 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 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 client to always use the same MAC
address regardless of the current Active Router.
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When a VRRP router restarts or boots, it SHOULD NOT send any ARP
messages using its physical MAC address for an IPv4 address it owns
and, it should only send ARP messages that include virtual MAC
addresses.
This may entail the following:
* When configuring an interface, Active Routers should broadcast a
gratuitous ARP request containing the virtual router MAC address
for each IPv4 address on that interface.
* At system boot, when initializing interfaces for VRRP operation,
delay gratuitous ARP requests and ARP responses until both the
IPv4 address and the virtual router MAC address are configured.
* When, for example, SSH access to a particular VRRP router is
required, an IP address known to belong to that router must be
used.
13.1.3. 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.
13.2. IPv6
13.2.1. ICMPv6 Redirects
ICMPv6 redirects may be used normally when VRRP is running between a
group of routers [RFC4443]. This allows VRRP to be used in
environments where the topology is not symmetric, e.g., the VRRP
routers do not connect to the same destinations.
The IPv6 source address of an ICMPv6 redirect should be the address
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 addresses 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.
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13.2.2. ND Neighbor Solicitation
When a host sends an ND Neighbor Solicitation message for the virtual
router IPv6 address, the Active Router MUST respond to the ND
Neighbor Solicitation message with the virtual MAC address for the
virtual router. The Active Router MUST NOT respond with its physical
MAC address. This allows the client 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 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 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.
13.2.3. 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 12.6. The Backup Routers must
be configured to send the same Router Advertisement options as the
address owner.
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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.
13.2.4. 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 changes [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.
13.3. IPvX
13.3.1. 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, e.g., Token Ring, this can result in a forwarding loop that
is only terminated when the IPvX TTL expires.
One such mechanism for VRRP routers is to add/delete a reject host
route for each adopted IPvX address when transitioning to/from Active
state.
13.3.2. Recommendations Regarding Setting Priority Values
A priority value of 255 designates a particular router as the "IPvX
address owner". Care must be taken not to configure more than one
router on the link in this way for a single VRID.
Routers with priority 255 will, as soon as they start up, preempt all
lower-priority routers. No more than one router on the link is to be
configured with priority 255, especially if preemption is set. If no
router has this priority, and preemption is disabled, then no
preemption will occur.
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When there are multiple Backup Routers, their priority values should
be uniformly distributed. For example, if one Backup Router has the
default priority of 100 and another Backup Router is added, a
priority of 50 would be a better choice for it than 99 or 100, in
order to facilitate faster convergence.
13.4. VRRPv3 and VRRPv2 Interoperation
13.4.1. 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.
13.4.2. VRRPv3 Support of VRRPv2
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.
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.
13.4.3. VRRPv3 Support of VRRPv2 Considerations
13.4.3.1. Slow, High-Priority Active Routers
See also Section 11.2.7, "Maximum Advertisement Interval (Max Adver
Int)".
The VRRPv2 Active Router interacting with a sub-second VRRPv3 Backup
router is the most important example of this.
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A VRRPv2 implementation should not be given a higher priority than a
VRRPv2/VRRPv3 implementation it is interoperating with a VRRPv2/
VRRPv3 router's advertisement rate is sub-second.
13.4.3.2. 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 then the desired sub-second rates may configured.
14. 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 none to strong. Operational experience
and further analysis determined that these did not provide sufficient
security to overcome the vulnerability of misconfigured secrets,
causing multiple 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 a VRRP Active Routers, creating multiple Active Router.
Authentication of VRRP messages could have 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.
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* 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 the 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
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.
15. 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
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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/editting [RFC3768] and
[VRRP-IPv6] into the draft that eventually became RFC 5798 [RFC5798].
Thanks to Stewart Bryant, Sasha Vainshtein, and Pascal Thubert 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 appendicies.
16. IANA Considerations
IANA has assigned the IPv4 multicast address 224.0.0.18 for VRRP.
IANA has assigned an IPv6 link-local scope multicast address for VRRP
for IPv6. The IPv6 multicast address is FF02:0:0:0:0:0:0:12.
IANA has reserved a block of IANA Ethernet unicast addresses for VRRP
for IPv6 in the range 00-00-5E-00-02-00 to 00-00-5E-00-02-FF (in
hex).
17. 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>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC3768] Hinden, R., Ed., "Virtual Router Redundancy Protocol
(VRRP)", RFC 3768, DOI 10.17487/RFC3768, April 2004,
<https://www.rfc-editor.org/info/rfc3768>.
[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>.
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[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>.
[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>.
[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>.
18. 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>.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
<https://www.rfc-editor.org/info/rfc2464>.
[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>.
[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
Cisco Systems
301 Midenhall Way
Cary, NC 27513
United States of America
Email: acee@cisco.com
Aditya Dogra
Cisco Systems
Sarjapur Outer Ring Road
Bangalore 560103
Karnataka
India
Email: addogra@cisco.com
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