Virtual Router Redundancy Protocol (VRRP)

The Virtual Router Redundancy Protocol (VRRP) is an election protocol that dynamically assigns
responsibility for one or more virtual routers to the VRRP routers on a LAN, allowing several routers on
a multiaccess link to utilize the same virtual IP address. A VRRP router is configured to run the VRRP
protocol in conjunction with one or more other routers attached to a LAN. In a VRRP configuration, one
router is elected as the virtual router master, with the other routers acting as backups in case the virtual
router master fails.


There are several ways a LAN client can determine which router should be the first hop to a particular
remote destination. The client can use a dynamic process or static configuration. Examples of dynamic
router discovery are as follows:
• Proxy ARP—The client uses Address Resolution Protocol (ARP) to get to the destination it wants
to reach, and a router will respond to the ARP request with its own MAC address.
• Routing protocol—The client listens to dynamic routing protocol updates (for example, from
Routing Information Protocol [RIP]) and forms its own routing table.
• ICMP Router Discovery Protocol (IRDP) client—The client runs an Internet Control Message
Protocol (ICMP) router discovery client.


VRRP is supported on Fast Ethernet, BVI, and Gigabit Ethernet interfaces, on MPLS VPNs, VRF-aware
MPLS VPNs, and VLANs.


VRRP Benefits

Redundancy
VRRP enables you to configure multiple routers as the default gateway router, which reduces the
possibility of a single point of failure in a network.
Load Sharing
You can configure VRRP in such a way that traffic to and from LAN clients can be shared by multiple
routers, thereby sharing the traffic load more equitably among available routers.
Multiple Virtual Routers
VRRP supports up to 255 virtual routers (VRRP groups) on a router physical interface, subject to the
platform supporting multiple MAC addresses. Multiple virtual router support enables you to implement
redundancy and load sharing in your LAN topology.
Multiple IP Addresses
The virtual router can manage multiple IP addresses, including secondary IP addresses. Therefore, if you
have multiple subnets configured on a GigabitEthernet interface, you can configure VRRP on each
subnet.

Preemption
The redundancy scheme of VRRP enables you to preempt a virtual router backup that has taken over for
a failing virtual router master with a higher priority virtual router backup that has become available.
Advertisement Protocol
VRRP uses a dedicated Internet Assigned Numbers Authority (IANA) standard multicast address
(224.0.0.18) for VRRP advertisements. This addressing scheme minimizes the number of routers that
must service the multicasts and allows test equipment to accurately identify VRRP packets on a segment.
The IANA assigned VRRP the IP protocol number 112.


SUMMARY STEPS
1. enable
2. configure terminal
3. interface type number
4. ip address ip-address mask
5. vrrp group description text
6. vrrp group priority level
7. vrrp group preempt [delay minimum seconds]
8. vrrp group timers advertise [msec] interval
9. vrrp group timers learn
10. exit
11. no vrrp sso