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| 6.2 |  | Implementing Router Redundancy in a Switched Network |
| 6.2.4 |  | Virtual Router Redundancy Protocol (VRRP) |

VRRP is an IETF proposed standard (RFC 2338) nearly identical to the Cisco proprietary HSRP. As a standards based solution introduced in 1998, VRRP is now being made available to provide interoperability using equipment from multiple vendors.

IP routing redundancy is designed to allow for transparent fail-over at the first-hop router.

Both HSRP and VRRP enable two or more devices to work together in a group, sharing a single virtual IP address. The virtual IP address is configured in each end user workstation as a default gateway address and is cached in the host ARP cache.

In an HSRP or VRRP group, one router is elected to handle all requests sent to the virtual IP address. An HSRP group has one active router, at least one standby router, and perhaps many listening routers. A VRRP group has one master router and one or more backup routers.

When a group initializes, messages are exchanged to elect the active or master router. Within these messages, information such as group identifier, priority, virtual IP address, and hello advertisement interval is exchanged. The routers in a group continue to exchange these messages at a configured interval. In HSRP, both the active and standby routers send periodic hello messages. In VRRP, only the master sends periodic messages, known as advertisements.

Using the information in these messages, a standby or backup router can determine whether it needs to take over the active or master role in a group. If a hello message is not received from the active or master router in the defined interval, the standby or backup router automatically assumes the role of active or master. When the original active or master router becomes available again, it will send out a hello or advertisement message and regain the lead IP packet-forwarding role within the group. All of this is completely transparent to the end user traversing this first-hop group. Connectivity is maintained with little or no loss of traffic and functionality.

Cisco recommends using HSRP because HSRP is a proven technology that has superior convergence characteristics. Use VRRP only when local subnet interoperability is required with equipment from another vendor.

Troubleshooting VRRP
Customizing the behavior of VRRP is optional. As soon as a VRRP group is enabled, that group is operating. Always customize VRRP before enabling VRRP. Otherwise, a router could become the master virtual router before the customization is complete.