FHRP
If a router or an interface/link (which is serving as the default gateway) fails, all the hosts using that default gateway will be isolated from outside networks.
As you’re already aware of, in a switched network, a client can only use one default gateway, even if there is more than one operational path.
A mechanism that can provide alternate default gateways in switched networks where two or more routers are connected to the same VLANs exists. Such a mechanism is offered by what we call “First Hop Redundancy Protocols” (FHRPs).
FHRP protocols allow us to implement what’s known as a “Virtual Router”, allowing us to have redundancy paths. Two or more routers are set to work together to virtually operate as one router and therefore provide us with an illusion of a single router. This virtual router will have its own IP address (assignable) and its own MAC address (generated).
All the physical routers present in this virtual group will share the assigned IP address and the generated MAC address and act as a single virtual router
Failover Router Steps
| Step 1 |
| The standby router stops seeing Hello messages from the forwarding router |
| Step 2 |
| The standby router assumes the role of the forwarding router |
| Step 3 |
| Since the new forwarding router is using both the IPv4 and MAC addresses of the virtual router, the hosts will notice no disruption in service |