HSRP Short for Hot Standby Routing Protocol, a proprietary protocol from Cisco. HSRP is a routing protocol that provides backup to a router in the event of failure. Using HSRP, several routers are connected to the same segment of an Ethernet, FDDI or token-ring network and work together to present the appearance of a single virtual router on the LAN. The routers share the same IP and MAC addresses, therefore in the event of failure of one router, the hosts on the LAN are able to continue forwarding packets to a consistent IP and MAC address. The process of transferring the routing responsibilities from one device to another is transparent to the user.
HSRP configuration requires only one command on the two or more routers that want to share default router responsibilities with HSRP interface sub command:
standby <group-no> ip <virtualipadd>
The first value defines HSRP group number which must match on all the routers within the group. The group number lets one router support multiple HSRP groups at a time on the same interface and it allows router to identify each other based on the group. The command also configures the virtual ip address shared by the routers in the same group. The virtual ip address is the address the hosts in the VLAN use as their default-gateway.
Note: If the group-number is not entered, then it will default to a group number of 0.
Example:
R1>enable
R1#configure teminal
R1(config)#interface fastethernet
0/0
R1(config-if)#standby 11 ip
192.168.10.100
The above command configures router R1 as a member of HSRP standby group with group number 11 and virtual ip address as 192.168.10.100 on interface fa0/0.
The command :
standby <group-number> preempt
is used to force an interface to resume active router state. Note that the priority of the router should be higher than the current Active router.
Members of HSRP group
1. Virtual router: virtual router is what is seen by the end user device. The virtual router has its own IP and MAC addresses.
2. Active router: Forwards packets sent to the virtual router. An active router assumes the IP and MAC addresses of the virtual router.
3. Standby router: Standby router monitors the state of HSRP by using Hello massages. It assumes the role of Active router, should the current Active router fail.
When an Active router fails in HSRP environment, Standby router assumes the Active router role. This new Active router will remain as Active router even if the failed Active router comeback to service, irrespective of the priority levels.
