Virtual Router Redundancy Protocol

High Availability of routers

RFC 5798 (2010)

The Virtual Router Redundancy Protocol ( VRRP ) is a method for increasing the availability of key gateways in local networks with redundant routers.

The VRRP was developed by Ascend Communications, DEC, IBM, Microsoft and Nokia in 1998. Today, however, it is also available in the routers from many other manufacturers such as Alcatel, Cisco or DrayTek. The VRRP was developed to establish an open standard, which relates to the great problem of the reliability of data paths, namely the availability of the so-called default gateways in local networks.

When routing of data packets through networks the failure of legs is counteracted on the way to the target by the dynamic routing. However, if from the first router on a path, this error can not be solved by dynamic routing, as a host usually has only a static default gateway and usually only one supported. Due to a failure of the first router, the default gateway, the affected hosts of the communication are cut off with the outside world, even if still active router on the same subnet are available.

At this point is where the VRRP. Multiple physical routers are combined into a logical group. This group of routers in the network presents itself now as one logical virtual router.

For this purpose, the logical router, a virtual IP address and one virtual MAC address is assigned. One of the routers within the group is defined as the master virtual router, it then binds the virtual MAC address and the virtual IP address of its network interface and informs other routers of the group, acting as a virtual backup router.

If the master router, the virtual IP address and virtual MAC address be transferred to the backup router within three seconds, which thus becomes the new master router. Both the MAC and the IP address are transferred, so that the affected hosts do not need to update their ARP cache. The consequences of the failure of the first router on the route can be reduced. This principle is called Hot Standby redundancy.

Since VRRP uses patents held by Cisco, the developers of the free OpenBSD operating system have developed an alternative protocol CARP.