Core issue
Hot Standby Router Protocol (HSRP) provides redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first hop router failures. HSRP allows multiple routers on a single LAN to share a virtual IP and MAC address, which is configured as the default gateway on the hosts. From the group of routers configured in a HSRP group, one router is elected as the active router and another as a standby router. The active router assumes the role of forwarding packets sent to the virtual IP address. If the active router fails, the standby router takes over as the new active router. HSRP is useful for hosts that do not support a router discovery protocol, such as Internet Control Message Protocol (ICMP) Router Discovery Protocol (IRDP) and it cannot switch to a new router when the router configured as the default gateway fails or reloads.
Resolution
All HSRP commands are configured under the interface configuration mode of a router. To configure HSRP, issue the standby [group-number] ip [ip-address] command and assign the virtual IP address for a group. Use the same group number and virtual IP address on other routers, which provide redundancy for that group. This is the only command that is necessary to enable HSRP.
Other HSRP commands are optional and can be configured if required. To influence a particular router to be elected as the active router, issue the standby [group-number] priority [priority] command and configure the priority. The one with the highest priority functions as the active router and the one with the second highest priority functions as the standby router. The interface IP address is used as the tie breaker in the event of the same priority on multiple routers. The active router is the one with the highest IP address. If the local router has priority over the current active router, issue the standby [group-number] preempt command. The local router should attempt to take its place as the active router.
You can configure HSRP to use authentication and accept HSRP packets only from trusted devices. HSRP can also be configured to track the status of an interface and adjust its priority to function as an active or standby router for a group. This is useful if more than one router can reach a particular network and one router is preferred over the other for forwarding packets, based on the status of a link. A router can be configured to use the Burned-in MAC Address (BIA) of an interface as the virtual MAC address instead of the standard HSRP MAC address. This is useful in situations when using the HSRP MAC address can create problems, as in Token Ring source route bridge environments and FDDI networks.
For more information on HSRP configuration with examples, refer to the Configuring HSRP section of Configuring IP Services.
