Virtual Router Redundancy Protocol

Virtual Router Redundancy Protocol (VRRP) is a non-proprietary redundancy protocol described in RFC3768 designed to increase the availability of the default gateway servicing hosts on the same subnet. This increased reliability is achieved by advertising a virtual router as a default gateway to the host instead of one physical router. Two or more physical routers are then configured to stand for the virtual router, with only one doing the actual routing at any given time. If the current physical router that is routing the data on behalf of the virtual router fails, an arrangement is made for another physical router to automatically replace it. The physical router that is currently forwarding data on behalf of the virtual router is called the master router. Physical routers standing by to take over from the master router in case something goes wrong are called backup router.

A virtual router must use 00-00-5E-00-01-XX as its Media Access Control (MAC) address. The last byte of the address (XX) is the Virtual Router Identifier (VRID), 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. Physical routers within the virtual router must communicate within themselves using packet with multicast IP address 224.0.0.18 and IP protocol number 112.

Routers have a priority of between1-255 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 preempt the master router status rather than having to wait for the hold time to expire. This reduces the black hole period.

For more information, you can visit http://en.wikipedia.org/wiki/Virtual_Router_Redundancy_Protocol

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