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

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 advertised based on the IP protocol version. Within a VRRP router, the virtual routers in each of the IPv4 and IPv6 address families are independent of one another. 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" for its inclusive language guidelines. This document obsoletes VRRP Version 3 . VRRP provides a function similar to the proprietary protocols "Hot Standby Router Protocol (HSRP)" and "IP Standby Protocol" .

RFC 5798 Differences The following changes have been made from RFC 5798:

The term for the VRRP router assuming forwarding responsibility has been changed to "Active Router" to be consistent with IETF inclusive terminology. Additionally, inconsistencies in RFC 5798 terminology for both "Active Router" and "Backup Router" were corrected. Additionally, the undesirable term for attracting and dropping unreachable packets has been changed.
Errata pertaining to the state machines in were corrected.
Appendices describing operation over legacy technologies (FDDI, Token Ring, and ATM LAN Emulation) were removed.
A recommendation was added indicating that IPv6 Unsolicited Neighbor Advertisements SHOULD be accepted by the Active and Backup Routers .
Checking that the Maximum Adverisement Intervals match is recommended although this will not result in the VRRP packet being dropped .
Miscellaneous editorial changes were made for readability.
The IANA considerations section was augmented include all the IPv4/IPv6 multicast address allocations and ethernet MAC address allocations.
Additional "Security Considerations" were added.

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 not be feasible for a number of reasons, including administrative overhead, processing overhead, security issues, or the lack of an implementation for a particular platform. Neighbor or router discovery protocols may require active participation by all hosts on a network, requiring large timer values to reduce protocol overhead associated with the associated protocol packet processing for each host. This can result in a significant delay in the detection of an unreachable router and, such a delay may introduce unacceptably long periods of unreachability for the default route. The use of a statically configured default route is quite popular since it minimizes configuration and processing overhead on the end-host and is supported by virtually every IPv4 implementation. This mode of operation is likely to persist as dynamic host configuration protocols are deployed, which typically provide configuration for an end-host IPv4 address and default gateway. However, this creates a single point of failure. Loss of the default router results in a catastrophic event, isolating all end-hosts that are unable to detect an available alternate path. The Virtual Router Redundancy Protocol (VRRP) is designed to eliminate the single point of failure inherent in a network utilizing static default routing. VRRP specifies an election protocol that dynamically assigns responsibility for a virtual router to one of the VRRP routers on a LAN. The VRRP router controlling the IPv4 address(es) associated with a virtual router is called the Active Router and forwards packets sent to these IPv4 addresses. The election process provides dynamic failover of the forwarding responsibility should the Active Router become unavailable. Any of the virtual router's IPv4 addresses on a LAN can then be used as the default first hop router by end-hosts. The advantage gained from using VRRP is a higher availability default path without requiring configuration of dynamic routing or a router discovery protocol on every end-host.

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 the default routers on a LAN 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 configuring the timer intervals to be more aggressive (note that the current lower limit for this is 5 seconds), this would have the downside of significantly increasing the overhead of ND traffic, especially when there are many hosts all trying to determine the reachability of one or more routers. The Virtual Router Redundancy Protocol for IPv6 provides a much faster switchover to an alternate default router than can be obtained using standard ND procedures. Using VRRP, a Backup Router can take over for a failed default router in around three seconds (using VRRP default parameters). This is done without any interaction with the hosts and a minimum amount of VRRP traffic.

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 guarantees 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, 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 unreachability.
Minimize the steady-state bandwidth overhead and processing complexity.
Function over a wide variety of multiaccess LAN technologies capable of supporting IPvX traffic.
Allow multiple virtual routers on a network for load-balancing.
Support multiple logical IPvX subnets on a single LAN segment.

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 transition between Active and Backup Routers can result in a disruption in service. The protocol should ensure that, after Active Router election, no state transition is triggered by any Backup Router of equal or lower preference as long as the Active Router continues to function properly. Some environments may find it beneficial to avoid the state transition triggered when a router that is preferred over the current Active Router becomes available. It may be useful to support an override of the immediate restoration to the preferred path.

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:

Use the virtual router MAC address as the source in a packet sent by the Active Router to trigger MAC learning.
Trigger a message immediately after transitioning to the Active Router to update MAC learning.
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 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 a significant disruption of traffic. To mitigate this problem, priority forwarding of VRRP packets should be considered. The Active Router SHOULD observe that this situation is occurring and log the problem.

VRRP Overview VRRP specifies an election protocol to provide the virtual router function described earlier. All protocol messaging is performed using either IPv4 or IPv6 multicast datagrams. Thus, the protocol can operate over a variety of multiaccess LAN technologies supporting IPvX multicast. Each link of a VRRP virtual router has a single well-known MAC address allocated to it. This document currently only details the mapping to networks using an IEEE 802 48-bit MAC address. The virtual router MAC address is used as the source in all periodic VRRP messages sent by the Active Router to enable MAC learning by layer-2 bridges on an extended LAN. A virtual router is defined by its virtual router identifier (VRID) and a set of either IPv4 or IPv6 address(es). A VRRP router may associate a virtual router with its real address on an interface. The scope of each virtual router is restricted to a single LAN. A VRRP router may be configured with additional virtual router mappings and priority for virtual routers it is willing to back up. The mapping between the VRID and its IPvX address(es) must be coordinated among all VRRP routers on a LAN. There is no restriction against reusing a VRID with a different address mapping on different LANs, nor is there a restriction against using the same VRID number for a set of IPv4 addresses and a set of IPv6 addresses. However, these are two different virtual routers. To minimize network traffic, only the Active Router for each virtual router sends periodic VRRP Advertisement messages. A Backup Router will not attempt to preempt the Active Router unless 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 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 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.

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 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 ), and a second virtual router has been added to cover the IPv4 address owned by Router-2 (VRID=2, IPv4_Address=B). In this case, Router-2 will assert itself as Active Router for VRID=2 while Router-1 will act as a Backup Router. This scenario demonstrates a deployment providing load-splitting when both routers are available, while providing full redundancy for robustness. In the IPv6 example above, i.e., IPvX is IPv6 everywhere in the figure, half of the hosts have learned a default route through Router-1's IPv6 A, and half are using Router-2's IPv6 B. The configuration of virtual router VRID=1 is exactly the same as in the first example (see ), 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 indicate higher priority. This field is an 8-bit unsigned integer field. The priority value for the VRRP router that owns the IPvX address associated with the virtual router MUST be 255 (decimal). VRRP routers backing up a virtual router MUST use priority values between 1-254 (decimal). The default priority value for VRRP routers backing up a virtual router is 100 (decimal). 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.

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

Maximum Advertisement Interval (Max Advertise Interval) The Maximum Advertisement Interval is a 12-bit field that indicates the time interval (in centiseconds) between ADVERTISEMENTS. The default is 100 centiseconds (1 second). Note that higher-priority Active Routers with slower transmission rates than their Backup Routers are unstable. This is because lower-priority VRRP routers configured to faster rates could join the LAN and decide they should be Active Routers before they have heard anything from the higher-priority Active Router with a slower rate. When this happens, it is temporary; once the lower-priority node does hear from the higher-priority Active Router, it will relinquish Active Router status.

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 "IPvX Addr Count" field. These fields are used for troubleshooting misconfigured routers. If more than one address is sent, it is recommended that all routers be configured to send these addresses in the same order to simplify comparisons. For IPv4 addresses, this refers to one or more IPv4 addresses that are backed up by the virtual router. For IPv6, the first address must be the IPv6 link-local address associated with the virtual router. This field contains either one or more IPv4 addresses, or one or more IPv6 addresses. The addresses, IPv4 or IPv6 but not both, MUST be the same as the VRRP 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 relinquishing responsibility for the virtual router. The range 1-254 (decimal) is available for VRRP routers backing up the virtual router. Higher values indicate higher priorities. The default value is 100 (decimal).
IPv4_Addresses: One or more IPv4 addresses associated with this virtual router. Configured list of addresses with no default.
IPv6_Addresses: One or more IPv6 addresses associated with this virtual router. Configured list of addresses with no default. The first address must be the Link-Local address associated with the virtual router.
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 the Active Router down (centiseconds). Calculated as: (3 * Active_Adver_Interval) + Skew_time
Preempt_Mode: Controls whether a (starting or restarting) higher-priority Backup Router preempts a lower-priority Active Router. Values are True to allow preemption and False to prohibit preemption. Default is True. Note: The exception is that the router that owns the IPvX address associated with the virtual router always preempts, independent of the setting of this flag.
Accept_Mode: Controls whether a virtual router in Active state will accept packets addressed to the address owner's IPvX address as its own even if it is not the IPvX address owner. The default is False. Deployments that rely on, for example, pinging the address owner's IPvX address may wish to configure Accept_Mode to True. Note: IPv6 Neighbor Solicitations and Neighbor Advertisements MUST NOT be dropped when Accept_Mode is False.
Virtual_Router_MAC_Address: The MAC address used for the source MAC address in VRRP advertisements and advertised in ARP 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 for 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 pseudo-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 to which the virtual router is attached.

Sending and Receiving VRRP Packets

Receiving VRRP Packets The following functions must be performed when a VRRP packet is received: A receiver SHOULD also verify that the "Maximum Advertisement Interval" in the received VRRP packet matches the "Maximum Advertisement Interval" configured for the VRID. Instability can occur with differing intervals (refer to ). If this check fails, the receiver SHOULD log the event (subject to rate-limiting) and MAY indicate via network management that a misconfiguration was detected. A receiver MAY also verify that "IPvX Addr Count" and the list of IPvX address(es) match the IPvX Address(es) configured for the VRID. If this check fails, the receiver SHOULD log (subject to rate-limiting) the event and MAY indicate via network management that a misconfiguration was detected.

Transmitting VRRP Packets The following operations MUST be performed when transmitting a VRRP packet: Note: VRRP packets are transmitted with the virtual router MAC address as the source MAC address to ensure that learning bridges correctly determine the LAN segment to which the virtual router is attached.

Virtual Router MAC Address The virtual router MAC address associated with a virtual router is an IEEE 802 MAC Address in the following format: IPv4 case: 00-00-5E-00-01-{VRID} (in hex, in Internet-standard bit- order) The first three octets are derived from the IANA's Organizational Unique Identifier (OUI). The next two octets (00-01) indicate the address block assigned to the VRRP protocol for the IPv4 protocol. {VRID} is the 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, i.e., "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 host to always use the same MAC address regardless of the current Active Router. When a VRRP router restarts or boots, it SHOULD NOT send any ARP messages using its physical MAC address for an IPv4 address 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, gratuitous ARP requests and ARP responses must be delayed until both the IPv4 address and the virtual router MAC address are configured.
When, for example, SSH access to a particular VRRP router is required, an 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 host to always use the same MAC address regardless of the current Active Router. When an Active Router sends an ND Neighbor Solicitation message for a host's IPv6 address, the Active Router MUST include the virtual 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.

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.

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 . Since these are sent to the IPv6 all-nodes multicast address (FF02:1) or the IPv6 all-routers multicast address (FF02:2), they will be received. Unsolicited Neighbor Advertisements are sent both in the case where the link-level addresses change and for gratuitous neighbor discovery by first hop routers . Additional configuration MAY be required in order for Unsolicited Neighbor Advertisements to update the corresponding neighbor cache.

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 mechanism for VRRP routers to these forwarding loops is to add/delete a host reject route for each non-owned IPvX address when transitioning to/from Active state.

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 Interoperation As mentioned above, this support is intended for upgrade scenarios and is NOT RECOMMENDED for permanent deployments. An implementation MAY implement a configuration flag that tells it to listen for and send both VRRPv2 and VRRPv3 advertisements. When a virtual router is configured this way and is the Active Router, it MUST send both types at the configured rate, even if sub-second. 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.

Interoperation Considerations

Slow, High-Priority Active Routers See also , "Maximum Advertisement Interval (Max Advertise Interval)". The VRRPv2 Active Router interacting with a sub-second VRRPv3 Backup router is the most important example of this. A VRRPv2 implementation should not be given a higher priority than a VRRPv2/VRRPv3 implementation with which it is interoperating if the VRRPv2/VRRPv3 router's advertisement rate is sub-second.

Overwhelming VRRPv2 Backups It seems possible that a VRRPv3 Active Router sending at centisecond rates could potentially overwhelm a VRRPv2 Backup Router with potentially non-deterministic results. In this upgrade case, a deployment should initially run the VRRPv3 Active Routers with lower frequencies, e.g., 100 centiseconds, until the VRRPv2 routers are upgraded. Then, once the deployment has verified that VRRPv3 is working properly, the VRRPv2 support may be disabled and the desired sub-second rates may configured.

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 Router, creating multiple Active Routers. 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.
Sending packets with the router's MAC address as the source MAC address in the L2 header to tell the L2 switches to send packets addressed to the router to the malicious node instead of the router.
Sending redirects to tell hosts to send their traffic somewhere else.
Sending unsolicited ND replies.
Answering ND requests, etc.

All of these can be done independently of implementing VRRP. VRRP does not add to these vulnerabilities. VRRP includes a mechanism (setting TTL = 255, checking on receipt) that protects against VRRP packets being injected from another remote network. This limits most vulnerabilities to attacks on the local network. VRRP does not provide any confidentiality. Confidentiality is not necessary for the correct operation of VRRP, and there is no information in the VRRP messages that must be kept secret from other nodes on the LAN. In the context of IPv6 operation, if SEcure Neighbor Discovery (SEND) is deployed, VRRP is compatible with the "trust anchor" and "trust anchor or CGA" modes of SEND . 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 Steve Nadas for his work merging/editting and into the draft that eventually became RFC 5798 . 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. Thanks to Dhruv Dhody and Donald Eastlake for comments and suggestions for improving the IANA section. Thanks to Sasha Vainshtein for recommending "Maximum Advertisement Interval" validation.

IANA Considerations In the "Local Network Control Block (224.0.0.0 - 224.0.0.255 (224.0.0/24))" of the "IPv4 Multicast Address Space Registry" , IANA has assigned the IPv4 multicast address 224.0.0.18 for VRRP. In the "Link-Local Scope Multicast Addresses" block of the "IPv6 Multicast Address Space Registry" , IANA has assigned the IPv6 link-local scope multicast address FF02:0:0:0:0:0:0:12 for VRRP for IPv6. In the "IANA Unicast 48-bit MAC Addresses" registry , IANA has assigned blocks of Ethernet unicast addresses as follows (in hexadecimal):