First Hop Redundancy Protocol (FHRP) are protocols which are designed to keep the default gateway available for the network participants. This is achieved by using two or more routers in a group to provide a backup for the router that carries the default gateway IP address. Such FHRP protocols are Hot Standby Router Protocl (HSRP) and Virtual Router Redundancy Protocl (VRRP).

HSRP allows a transparent failover of the first hop gateway router. It provides first hop routing redundancy for IP hosts on Ethernet networks configured with a gateway or default route. You can use HSRP in a group of routers for selecting an active router and a standby router. In a group of two routers, the active router is the router that routes packets; the standby router is the router that takes over when the active router fails or when preset conditions are met.

Many host implementations do not support any dynamic router discovery mechanisms but can be configured with a default router. Running a dynamic router discovery mechanism on every host is not feasible for a number of reasons, including administrative overhead, processing overhead, and security issues. HSRP provides failover services to these hosts.

When you use HSRP, you need an HSRP virtual IP address as the host’s default router (instead of the IP address of the actual router). The virtual IP address is an IP address that is shared among a group of routers that run HSRP. Configuring HSRP on a network segment will provide a virtual MAC address and a virtual IP address for the HSRP group. You need to configure the same virtual address on each HSRP-enabled interface in the group. You also configure a unique IP address and MAC address on each interface that acts as the real address. HSRP selects one of these interfaces to be the active router. The active router receives and routes packets destined for the virtual MAC address of the group.

HSRP detects when the active router fails. At that point, a selected standby router assumes control of the virtual MAC and IP addresses of the HSRP group. HSRP also selects a new standby router at that time.

HSRP uses a priority mechanism to determine which HSRP-configured interface becomes the default active router. To configure an interface as the active router, you assign it with a priority that is higher than the priority of all the other HSRP-configured interfaces in the group. The default priority is 100, so if you configure just one interface with a higher priority, that interface becomes the default active router.

Interfaces that run HSRP send and receive multicast User Datagram Protocol (UDP)-based hello messages to detect a failure and to designate active and standby routers. When the active router fails to send a hello message within a configurable period of time, the standby router with the highest priority becomes the active router. The transition of packet forwarding functions between the active and standby router is completely transparent to all hosts on the network.