HSRP(Hot Standby Router Protocol)

HSRP(Hot Standby Router Protocol) is a Cisco proprietary redundancy protocol. With this protocol one “virtual ip address” is assigned and always this address is used to go through the routers. At the backplane, the gateway interfaces’s ip address still exist. But it seems that only one interface exist and the ip address of this interface is virtual ip address.

While using HSRP, there is a HSRP group and in this group there are gateway routers for specific roles.

These routers are :

• Acitive Router
• Standby Router
• Listenning Routers

The Active Router is the router that is currently used as the gateway. The traffic goes throught this router.

The Standby Router is the backup of Active Router. When th active router fails then this router used as active.

Lastly the Listening Router is other routers particippating in this HSRP group.

Active router is controlled by standby router with periodic hello packets by default every 3 seconds. In case of a failure of active router, this mechanism provide switching to the backup router.

The router roles are decided with “priority” values. The router that has the highes priority, becomes active router. The default priority value is 100 In case of an equal priority values, ip addresses are used as the tie-breaker. The router with the highes ip address wins this election and bcome active router. And the other router that has the second hihg ip address become standby router.

HSRP progresses throught some states. These states are mentioned below respectively:

– Disabled: Not configured for HSRP or shut down
– Init: Starting state
– Learn: Wainting for hellos, to learn virtual address
– Listen: Aware of virtual ip, not elected stadby or active
– Speak: Participating in election,sending hellos
– Standby: Acting as backup,monitors and send hellos
– Active: Accepting and forwarding user traffic

Only in three states, hellos are sent. Speak,Standby and Active states. As you know standby and active routers already send hellos each other. In the last one, in speak state, hellos are sent to determine the active and standby routers.

Redundancy Protocols – Part 1
Redundancy Protocols – Part 3 (HSRP Configuration)
Redundancy Protocols – Part 4 (VRRP)
Redundancy Protocols – Part 5 (GLBP)