Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6
Cisco Systems
301 Midenhall Way
Cary
NC
27513
USA
acee@cisco.com
Cisco Systems
Sarjapur Outer Ring Road
Bangalore
Karnataka
560103
India
addogra@cisco.com
Computer Science, Boston University
111 Cummington Mall
Boston
MA
02215
USA
nadas@bu.edu
General
RFC
VRRP
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. This document obsoletes VRRP Version 3
.
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 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. This document obsoletes VRRP Version 3
.
VRRP provides a function similar to the proprietary protocols "Hot Standby Router Protocol (HSRP)"
and "IP Standby Protocol" .
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.
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) or OSPF version 2
, running an ICMP router discovery client
, 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 a lost (i.e., dead) neighbor; such a delay may
introduce unacceptably long "black hole" periods.
The use of a statically configured default route is quite popular; it
minimizes configuration and processing overhead on the end-host and
is supported by virtually every IPv4 implementation. This mode of
operation is likely to persist as dynamic host configuration
protocols are deployed, which typically provide
configuration for an end-host IPv4 address and default gateway.
However, this creates a single point of failure. Loss of the default
router results in a catastrophic event, isolating all end-hosts that
are unable to detect an available alternate path.
The Virtual Router Redundancy Protocol (VRRP) is designed to
eliminate the single point of failure inherent in 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.
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 . 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.
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.
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 when, and only when, they appear in all capitals,
as shown here.
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.
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
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.
Required Features
This section describes the set of features that were considered
mandatory and that guided the design of VRRP.
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 black holes.
- Minimize the steady-state bandwidth overhead and processing
complexity.
- Function over a wide variety of multiaccess LAN technologies
capable of supporting IPvX traffic.
- Allow multiple virtual routers on a network for load-balancing.
- Support multiple logical IPvX subnets on a single LAN segment.
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.
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.
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:
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 the MAC
learning; and 3) trigger periodic messages from the Active Router to
maintain the MAC address cache.
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; this specification leverages that earlier
approach for 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.
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.
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.
Sample Configurations
Sample Configuration 1
The following figure shows a simple network with two VRRP routers
implementing one virtual router.
* *<---------IPvX B
| |
| |
-------------+------------+--+-----------+-----------+-----------+
^ ^ ^ ^
| | | |
Default Router | | | |
IPvX addreses ---> (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 (that is, 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 (that is, IPvX is IPv6 everywhere in the figure),
each router has a link-local IPv6 address on the LAN interface (Router-1
is assigned IPv6 Link-Local A and Router-2 is assigned IPv6 Link-Local B),
and each host learns a default route from Router
Advertisements through one of the routers (in this example, they all
use Router-1's IPv6 Link-Local A).
In an IPv4 VRRP environment, each router has the exact same
permanently assigned 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 virtual router ID) with the address owned by a
router.
In an IPv6 VRRP environment, each router has the exact same
Link-Local IPv6 address. 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; 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.
Sample Configuration 2
The following figure shows a configuration with two virtual routers
with the hosts splitting their traffic between them.
* *<---------- IPvX B
| |
| |
----------+-------------+-+-----------+-----------+-----------+
^ ^ ^ ^
| | | |
Default Router | | | |
IPvX addreses ---> (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 (that is, 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 ), 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 (that is, IPvX is IPv6 everywhere in the
figure), half of the hosts have learned a default route through
Router-1's IPv6 A, and half are using Router-2's IPv6 B. The configuration
of virtual router VRID=1 is exactly the same as in the first example
(see ), 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.
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.
VRRP Packet Format
This section defines the format of the VRRP packet and the relevant
fields in the IP header.
IPv4 Field Descriptions
Source Address
This is the primary IPv4 address of the interface from which the packet is being
sent.
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.
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.
Protocol
The IPv4 protocol number assigned by the IANA for VRRP is 112
(decimal).
IPv6 Field Descriptions
Source Address
This is the IPv6 link-local address of the interface from which the packet is
being sent.
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.
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.
Next Header
The IPv6 Next Header protocol assigned by the IANA for VRRP is 112
(decimal).
VRRP Field Descriptions
Version
The version field specifies the VRRP protocol version of this packet.
This document defines version 3.
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.
Virtual Rtr ID (VRID)
The Virtual Rtr ID field identifies the virtual router for which this
packet is reporting status.
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.
IPvX Addr Count
This is the number of either IPv4 addresses or IPv6 addresses
contained in this VRRP advertisement. The minimum value is 1.
0
This reserved field MUST be set to zero on transmission and ignored on
reception.
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 becausel low-
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.
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 . 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 .
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.
Protocol State Machine
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
(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.
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.
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.
State Transition Diagram
| |<-------------+
| | Initialize | |
| +------| |----------+ |
| | +---------------+ | |
| | | |
| V V |
+---------------+ +---------------+
| |---------------------->| |
| Active | | Backup |
| |<----------------------| |
+---------------+ +---------------+
]]>
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.
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.
Backup
The purpose of the {Backup} state is to monitor the availability and
state of the Active Router. The Solicited-Node multicast address
is referenced in the psuedo-code below.
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.
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.
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.
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"
.
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.
Operational Issues
IPv4
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.
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.
When a VRRP router restarts or boots, it SHOULD NOT send any ARP
messages using its physical MAC address for the IPv4 address it owns;
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.
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.
IPv6
ICMPv6 Redirects
ICMPv6 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, 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.
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;
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.
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 . 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.
IPvX
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.
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.
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.
VRRPv3 and VRRPv2 Interoperation
Assumptions
-
VRRPv2 and VRRPv3 interoperation is optional.
-
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.
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.
VRRPv3 Support of VRRPv2 Considerations
Slow, High-Priority Active Routers
See also , "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.
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.
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.
Security Considerations
VRRP for IPvX does not currently include any type of authentication.
Earlier versions of the VRRP (for IPv4) specification included
several types of authentication ranging from none to strong.
Operational experience and further analysis determined that these did
not provide sufficient security to overcome the vulnerability of
misconfigured secrets, causing multiple 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 nodefrom 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 do
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;
send redirects to tell the hosts to send their traffic somewhere
else; send unsolicited ND replies; answer ND requests; etc. All of
this 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 . 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.
Contributors and Acknowledgments
The IPv6 text in this specification is based on . 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 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 . The
authors of that specification would like to thank Glen Zorn, Michael
Lane, Clark Bremer, Hal Peterson, Tony Li, Barbara Denny, Joel
Halpern, Steve Bellovin, Thomas Narten, Rob Montgomery, Rob Coltun,
Radia Perlman, Russ Housley, Harald Alvestrand, Steve Bellovin, Ned
Freed, Ted Hardie, Russ Housley, Bert Wijnen, Bill Fenner, and Alex
Zinin for their comments and suggestions.
Thanks to Stewart Bryant and Sasha Vainshtein for comments on the
current document (RFC 5798 BIS).
IANA Considerations
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).
Information technology - Telecommunications and information exchange between systems - Local area networks - Media ac control (MAC) bridges
International Organization for Standardization
Key words for use in RFCs to Indicate Requirement Levels
Harvard University
Internet Protocol, Version 6 (IPv6) Specification
Cisco
Nokia
Virtual Router Redundancy Protocol
Nokia
IP Version 6 Addressing Architecture
Cisco
Nokia
Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification
Cisco
Transwitch
Tropos Networks
Neighbor Discovery for IP version 6
IBM
Sun Microsystems
Daydreamer
Elevate Technologies
Virtual Router Redundancy Protocol (VRRP) Version 3 for IPv4 and IPv6
Ericsson
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
Huawei Technologies
Informative References
Virtual Router Redundancy Protocol for IPv6
Development of Router Clusters to Provide Fast Failover in IP Networks",
Digital Technical Journal, Volume 9 Number 3
Higginson, P. and M. Shand
IPX Router Specification Version 1.10
Novell Incorporated
Computing the Internet Checksum
ISI
Cray Research
BBN Laboratories
OSPF Version 2
Ascend Communications, Inc.
ICMP Router Discovery Messages
Xerox PARC
IP Multicast over Token-Ring Local Area Networks
Consultant
Dynamic Host Configuration Protocol
Bucknell University
Cisco Hot Standby Router Protocol (HSRP)
Juniper Networks
Juniper Networks
Cisco Systems
Cisco Systems
Virtual Router Redundancy Protocol
BAscend Communications
Ascend Communications
Microsoft
Nokia
Nokia
Nokia
Digital Equipment Corp
Digital Equipment Corp
IBM Corporation
Bay Networks
RIP Version 2
Bay Networks
Transmission of IPv6 Packets over Ethernet Networks
Fermilab
SEcure Neighbor Discovery (SEND)
Ericsson
DoCoMo Communications Labs USA
Microsoft
Ericsson
IBM Token-Ring Network, Architecture Specification, Publication SC30-3374-02, Third Edition
IBM Incorporated
Operation over FDDI, Token Ring, and ATM LANE
Operation over FDDI
FDDI interfaces remove from the FDDI ring frames that have a source
MAC address matching the device's hardware address. Under some
conditions, such as router isolations, ring failures, protocol
transitions, etc., VRRP may cause there to be more than one Active
Router. If an Active Router installs the virtual router MAC address
as the hardware address on an FDDI device, then other Active Routers'
ADVERTISEMENTS will be removed from the ring during the Active Router
convergence, and convergence will fail.
To avoid this, an implementation SHOULD configure the virtual router
MAC address by adding a unicast MAC filter in the FDDI device, rather
than changing its hardware MAC address. This will prevent an Active
Router from removing any ADVERTISEMENTS it did not originate.
Operation over Token Ring
Token Ring has several characteristics that make running VRRP
difficult. These include the following:
-
In order to switch to a new Active Router located on a different bridge
Token-Ring segment from the previous Active Router when using
source-route bridges, a mechanism is required to update cached source-route
information.
-
No general multicast mechanism is supported across old and new
Token-Ring adapter implementations. While many newer Token-Ring
adapters support group addresses, Token-Ring functional-address
support is the only generally available multicast mechanism. Due
to the limited number of Token-Ring functional addresses, these
may collide with other usage of the same Token-Ring functional
addresses.
Due to these difficulties, the preferred mode of operation over Token
Ring will be to use a Token-Ring functional address for the VRID
virtual MAC address. Token-Ring functional addresses have the two
high-order bits in the first MAC address octet set to B'1'. They
range from 03-00-00-00-00-80 to 03-00-02-00-00-00 (canonical format).
However, unlike multicast addresses, there is only one unique
functional address per bit position. The functional addresses
03-00-00-10-00-00 through 03-00-02-00-00-00 are reserved by the
Token-Ring Architecture for user-defined applications.
However, since there are only 12 user-defined Token-Ring functional
addresses, there may be other non-IPvX protocols using the same
functional address. Since the Novell IPX protocol uses the
03-00-00-10-00-00 functional address, operation of VRRP over Token
Ring will avoid using this functional address. In general, Token-Ring
VRRP users will be responsible for resolution of other user-defined
Token-Ring functional address conflicts.
VRIDs are mapped directly to Token-Ring functional addresses. In
order to decrease the likelihood of functional-address conflicts,
allocation will begin with the largest functional address. Most
non-IPvX protocols use the first or first couple user-defined functional
addresses, and it is expected that VRRP users will choose VRIDs
sequentially, starting with 1.
Or, more succinctly, octets 3 and 4 of the functional address are
equal to (0x4000 >> (VRID - 1)) in non-canonical format.
Since a functional address cannot be used as a MAC-level source
address, the real MAC address is used as the MAC source address in
VRRP advertisements. This is not a problem for bridges, since
packets addressed to functional addresses will be sent on the
spanning-tree explorer path .
The functional-address mode of operation MUST be implemented by
routers supporting VRRP on Token Ring.
Additionally, VRRP routers MAY support the unicast mode of operation to
take advantage of newer Token-Ring adapter implementations that
support non-promiscuous reception for multiple unicast MAC addresses
and to avoid both the multicast traffic and usage conflicts
associated with the use of Token-Ring functional addresses. Unicast
mode uses the same mapping of VRIDs to virtual MAC addresses as
Ethernet. However, one important difference exists. ND
request/reply packets contain the virtual MAC address as the source
MAC address. The reason for this is that some Token-Ring driver
implementations keep a cache of MAC address/source-routing
information independent of the ND cache.
Hence, these implementations have to receive a packet with the
virtual MAC address as the source address in order to transmit to
that MAC address on a source-route-bridged network.
Unicast mode on Token Ring has one limitation that should be
considered. If there are VRID routers on different
source-route-bridged segments, and there are host implementations
that keep their source-route information in the ND cache and do
not listen to gratuitous NDs, these hosts will not update their ND source-route
information correctly when a switchover occurs. The only possible
solution is to put all routers with the same VRID on the same source-route-bridged
segment and use techniques to prevent that bridge segment from being a single point
of failure. These techniques are beyond the scope of this document.
For both the multicast and unicast mode of operation, VRRP
advertisements sent to 224.0.0.18 should be encapsulated as described
in .
Operation over ATM LANE
Operation of VRRP over ATM LANE on routers with ATM LANE interfaces
and/or routers behind proxy LAN Emulation Clients (LECs) are beyond
the scope of this document.