Implementation
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 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.
Read more about this topic: Virtual Router Redundancy Protocol