First Hop Reduncy Protocol

First Hop Redundancy protocols (FHRP) provide gateway redundancy to end hosts. When a host in a network boots up, it get a gateway IP using DHCP. If the gateway goes down, the host cannot reach devices outside of that network. Using FHRP we can have active and standby routers to avoid failure of gateway.

How gateway redundancy works:

  • Multiple routers are bundled in a group and the group is associated to a virtual IP
  • When a end host is assigned a gateway IP manually or via DHCP, the virtual IP address is used as the gateway IP address
  • All routers in the group know the virtual IP
  • The active physical router responds to the ARP request with a virtual mac address
  • Routers in the group exchange keepalive messages to maintain their status
  • If an active router goes down, another router in the group will take its place

Few FHRP features to be aware of are:

  • An interface can have multiple groups
  • Preemption can be used
  • Authentication can be used
  • IP SLA, interface tracking and object tracking can be used

The following are examples of few FHRP protocols

Hot Standby Router protocol (HSRP)
Virtual Router Redundancy Protocol (VRRP)
Gateway load balancing protocol (GLBP)