The Virtual Router Redundancy Protocol is a computer networking protocol that provides
for automatic assignment of available Internet Protocol routers to participating hosts.
This increases the availability and reliability of routing paths via automatic default gateway
selections on an IP subnetwork.
The protocol achieves this by creation of virtual routers, which are an abstract representation
of multiple routers, i.e. master and backup routers, acting as a group.
The default gateway of a participating host is assigned to the virtual router instead
of a physical router.
If the physical router that is routing packets on behalf of the virtual router fails, another
physical router is selected to automatically replace it.
The physical router that is forwarding packets at any given time is called the master router.
VRRP provides information on the state of a router, not the routes processed and exchanged
by that router.
Each VRRP instance is limited, in scope, to a single subnet.
It does not advertise IP routes beyond that subnet or affect the routing table in any
way.
VRRP can be used in Ethernet, MPLS and token ring networks with Internet Protocol Version
4, as well as IPv6.
The protocol is described in IETF publication RFC 5798, which is an open standard, but Cisco
claims that a similar protocol with essentially the same facility is patented and licensed;
however in reply to a direct request Robert Barr of Cisco replied in 2001 that they will
not assert any patent claims unless someone tried to assert a claim against Cisco.
IBM also claims covering patents and their statement is readable on the IETF webpage.
Implementation A virtual router must use 00-00-5E-00-01-XX
as its Media Access Control address.
The last byte of the address is the Virtual Router IDentifier, which is different for
each virtual router in the network.
This address is used by only one physical router at a time, and it will reply with this
MAC address when an ARP request is sent for the virtual router's IP address.
The destination MAC address for VRRP 'hello' messages is set to the Ethernet multicast
MAC address 01-00-5E-00-00-12 Physical routers within the virtual router
must communicate within themselves using packets with multicast IP address 224.0.0.18 and IP
protocol number 112.
Routers have a priority of between 1-255 and the router with the highest priority will
become the master.
When a planned withdrawal of a master router is to take place, its priority can be lowered
which means a backup router will pre-empt the master router status rather than having
to wait for the hold time to expire.
This reduces the black hole period.
The default priority is 100 for backups and 255 for a master.
Elections of master routers A failure to receive a multicast packet from
the master router for a period longer than three times the advertisement timer causes
the backup routers to assume that the master router is dead.
The virtual router then transitions into an unsteady state and an election process is
initiated to select the next master router from the backup routers.
This is fulfilled through the use of multicast packets.
Backup router(s) are only supposed to send multicast packets during an election process.
One exception to this rule is when a physical router is configured with a higher priority
than the current master, which means that on connection to the network it will preempt
the master status.
This allows a system administrator to force a physical router to the master state immediately
after booting, for example when that particular router is more powerful than others within
the virtual router.
The backup router with the highest priority becomes the master router by raising its priority
above that of the current master.
It will then take responsibility for routing packets sent to the virtual gateway's MAC
address.
In cases where backup routers all have the same priority, the backup router with the
highest IP address becomes the master router.
All physical routers acting as a virtual router must be in the same LAN segment.
Communication within the virtual router takes place periodically.
This period can be adjusted by changing advertisement interval timers.
The shorter the advertisement interval, the shorter the black hole period, though at the
expense of more traffic in the network.
Security is achieved by responding only to first hop packets, though other mechanisms
are provided to reinforce this, particularly against local attacks.
Election process is made orderly through the use of skew time, derived from a router's
priority and used to reduce the chance of the thundering herd problem occurring during
election.
The skew time is given by the formula RFC 3768 .
Backup router utilization can be improved by load sharing.
For more on this, see RFC 5798.
History Example: Configuring VRRP from JUNOS http:www.juniper.neten_UStopicsvrrp-configuring-example.html
VRRP is based on Cisco's proprietary Hot Standby Router Protocol concepts.
The protocols, while similar in concept, are not compatible.
(Cisco Example) VLAN Tagging
track 1 interface Serial00.1 ip routing ! Points at the interface that needs to be
! Prioritized interface fastethernet0/0.1 ! VLANs have to
be on a Sub-Interface, It is ! best practice to match the Sub-Interface
# ! and the VLAN #
encapsulation dot1q 1 ! Enables IEEE 802.1Q VLAN frame tagging,
! followed by the VLAN # that this ! sub-interface will route
ip address x.x.x.x 255.255.255.0 ! Make sure the IP is on the same subnet as
! the virtual Gateway1 vrrp 1 priority 110 ! The Priority of the
Gateway1 vrrp 1 ip ! The Virtual Gateway
for the VLAN 1 vrrp 1 preempt delay minimum 20 ! If the other
router fails it will wait 20 ! sec before becoming the master
vrrp 1 track 1 decrement 15 ! If the S00.1 Link fails, This command
! drops the priority by 15 !
interface fastethernet0/0.5 ! VLANs have to be on a Sub-Interface, It is
! best practice to match the Sub-Interface #
! and the VLAN # encapsulation dot1q 5 ! Enables IEEE 802.1Q
VLAN frame tagging, ! followed by the VLAN # that this
! sub-interface will route ip address x.x.x.x 255.255.255.0 ! Make sure
the IP is on the same subnet as ! the virtual Gateway2
vrrp 5 priority 110 ! The Priority of the Gateway2
vrrp 5 ip ! The Virtual Gateway for the VLAN 5
vrrp 5 preempt delay minimum 20 ! If the other router fails it will wait 20
! sec before becoming the master vrrp 5 track 1 decrement 15 ! If the Fa0/0.5
Link fails, This command ! drops the priority by 15
! router bgp
network mask 255.255.255.0 ! Broadcasts Gateway1 out the WAN through BGP
network mask 255.255.255.0 ! Broadcasts Gateway2 out the WAN through BGP
See also Juniper Networks Understanding VRRP http:www.juniper.neten_UStopicsvrrp-overview-ha.html
Common Address Redundancy Protocol - A non-proprietary, patent-free, and unrestricted alternative
to HSRP and VRRP.
Gateway Load Balancing Protocol - A Cisco Systems proprietary router redundancy protocol
providing load balancing Hot Standby Routing Protocol - A Cisco Systems
proprietary router redundancy protocol R-SMLT - An Avaya proprietary router redundancy
and router load balancing protocol - replacement for VRRP in Avaya core networks
SMLT An Avaya redundancy protocol First Hop Redundancy Protocols - Lists of
default gateway redundancy protocols References
External links Keepalived 1.2.x adds ipv6 support
The current VRRP RFC which obsoletes RFC3768 The IETF VRRP mailing list archive
A detailed VRRP article A detailed vrid article
Controversy involving VRRP and Cisco patents Analysis of VRRPv 2 Issues and Solutions
Implementations A GPL licensed implementation of VRRP designed
for Linux operating systems A BSD licensed implementation of VRRP for
Unix-like operating systems A GPL licensed implementation of VRRPv2 for
Linux operating systems Configuring VRRP on Cisco IOS
Configuring VRRP on 3com NETBuilder Vyatta, a commercial and open-source router
/ firewall with VRRP functionality JBM C120 - A cellular enabled enterprise class
router