One way prevent a single point of failure at the default gateway, is to implement a virtual router. To implement this type of router redundancy, multiple routers are configured to work together to present the illusion of a single router to the hosts on the LAN, as shown in the figure. By sharing an IP address and a MAC address, two or more routers can act as a single virtual router.
The IP address of the virtual router is configured as the default gateway for the workstations on a specific IP segment. When frames are sent from host devices to the default gateway, the hosts use ARP to resolve the MAC address that is associated with the IP address of the default gateway. The ARP resolution returns the MAC address of the virtual router. Frames that are sent to the MAC address of the virtual router can then be physically processed by the currently active router within the virtual router group. A protocol is used to identify two or more routers as the devices that are responsible for processing frames that are sent to the MAC or IP address of a single virtual router. Host devices send traffic to the address of the virtual router. The physical router that forwards this traffic is transparent to the host devices.
A redundancy protocol provides the mechanism for determining which router should take the active role in forwarding traffic. It also determines when the forwarding role must be taken over by a standby router. The transition from one forwarding router to another is transparent to the end devices.
The ability of a network to dynamically recover from the failure of a device acting as a default gateway is known as first-hop redundancy.