Introduction to Router Redundancy
A virtual router is a method to prevent a single point of failure at the default gateway. To implement virtual router redundancy, several routers are configured to work jointly as a single router to the hosts on the LAN. The routers share an IP address and a MAC address and act as a single virtual router.
The IP address of the virtual router is used as the default gateway for the local network on a particular IP segment. When hosts on the local network sending data to the internet using the default gateway, the sending host resolve the MAC address of the default gateway using ARP.
The ARP returns the MAC address of the virtual router and the data that are sent to the MAC address of the virtual router can then be physically processed by the currently forwarding (active) router within the virtual router group.
The redundancy protocol is used to recognize two or more routers as the devices that are responsible for processing data frames for the MAC or IP address of a single virtual router.
The Host devices on the local network send traffic to the address of the virtual router but the physical router process and forwards this traffic is transparent to the host devices.
The redundancy protocol decides which router should take the active role in forwarding traffic. The protocol also decides when the forwarding role must be taken over by a standby router. The switch from one forwarding router to another is transparent to the end devices.
The end devices don’t know about the change of the physical router. Thus the network dynamically recovers from the failure using router redundancy. The device acting as the default gateway in the router redundancy protocol known as the first hop redundancy.