First Hop Redundancy Protocol

FHRP allows networks to communicate, specifically routers and switches. This is with redundancy via using active/standby or master/slave hierarchy for routers.

End devices' default gateways will refer to virtual IP address of the hierarchal routers. Specifically, all routers together will choose a virtual IP address. Virtual MAC address will be provided by the set of routers for end devices' frames.

I recall one of 3 FHRP types allows for load balancing between subnets, and another type allows for load balancing within a single subnet. Subnets are L3 while VLANs is L2.

Switch between standby to active routers will happen due to not receiving 'Hello' message from active router. Once switch happens, it's non-preemptive, meaning it doesn't revert the switch.

Switches in network will update their own MAC address tables as the newly active router will send ARP replies, known as gratuitous ARP since it's sent without router receiving request.