The Host Standby Router Protocol (HSRP)Cisco proprietary protocolDetailed in RFC 2281Provides gateway redundancy by sharing a Virtual IP and MAC addresses between two or more redundant gateways belong to the same HSRP group

HSRP Terms.Active router: The router that is currently forwarding packets for the virtual routerStandby router: The primary backup routerStandby group: The set of routers participating in HSRP that jointly emulate a virtual routerThe primary function of the HSRP standby router (virtual) is to monitor the functioning status of the HSRP group and to quickly assume packet-forwarding responsibility if the active router fails.

These are the steps that take place when a router or Layer-3 device (switch) fails:

1.The standby router stops receiving hello messages from the forwarding router.2.The standby router assumes the role of the forwarding router.3.Because the new forwarding router (standby router) assumes both the IP and MAC addresses of the virtual router, the connected network devices see no disruption in service

Debug

Router_1# debug standby4d01h: SB1: Vlan1 Hello out 10.1.1.1 Active pri 100 ip 10.1.1.254 4d01h: SB1: Vlan1 Hello in 10.1.1.2 Standby pri 100 ip 10.1.1.254 4d01h: SB2: Vlan2 Hello in 10.2.1.2 Standby pri 100 ip 10.2.1.254 4d01h: SB2: Vlan2 Hello out 10.2.1.1 Active pri 100 ip 10.2.1.254

Hello out is the hello packet sent on the specified ip addressfrom line 1(4d01h: SB1: Vlan1 Hello out 10.1.1.1 Active pri 100 ip 10.1.1.254 ) the router is sending a hello packet on 10.1.1.1 in Vlan1.from line 4 (4d01h: SB2: Vlan2 Hello out 10.2.1.1 Active pri 100 ip 10.2.1.254) the router is sending a hello packet on 10.2.1.1 in Vlan2.

Hello in defines hello packets received from the specified ip addressfrom line 2 (4d01h: SB1: Vlan1 Hello in 10.1.1.2 Standby pri 100 ip 10.1.1.254 ) the hello packet was recieved by the router from 10.1.1.2 in Vlan1from line 3 (4d01h: SB2: Vlan2 Hello in 10.2.1.2 Standby pri 100 ip 10.2.1.254) the hello packet was received by the router from 10.2.1.2 in Vlan2

configure HSRP

configuring the IP address on the interface

standby group-number ip ip-address command to configure HSRP

The group number can be any value between 0 and 255 in HSRPv1 and must be the same configuration on neighbouring routers.

In HSRPv2, the group number can be any value between 0 and 4095

IP address configured is that of the virtual router IP address for the HSRP group

Assigning a priority value to each router in a standby group help in controlling the order in which active routers for that group are selected.

highest priority in an HSRP group becomes the active router. same priority, the router with the highest configured IP address becomes the default active router. Routers do not have the preempt configured, any router that boots up faster than the others in the standby group becomes the active router, regardless of the configured priority

you can still activate the former active router to resume the forwarding role by configuring the preempt command with a lower priority.

Verify HSRP

Show standby

Show standby brief