Hot Standby Router Protocol (HSRP) provides redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first hop router failures. From the group of routers configured in a HSRP group, the one with the highest priority functions as the active router and the one with the second highest priority functions as the standby router. When a router becomes an HSRP active router, it adds the virtual IP address and the corresponding virtual MAC address to its Address Resolution Protocol (ARP) table. It also adds the virtual MAC address to the address filter table.
Under certain circumstances, the active HSRP router may stop responding to ARP requests sent for the virtual IP address. This can occur when more than one interface is configured for HSRP on the router, and HSRP snooping is enabled. The HSRP snooping process helps Internet Control Message Protocol (ICMP) redirect to co-exist with HSRP by allowing a router to learn the existence of other HSRP groups and their active routers in the same broadcast domain. The hosts could then be redirected to active routers of other groups to take an optimal path to the destination, if necessary.
When there are momentary Layer 2 loops in the network, a virtual IP address configured under an interface for an HSRP group might also be learned over a different interface through an HSRP snooping mechanism. Once this temporary loop is resolved, the router stops receiving the HSRP messages over the wrong interface for the particular virtual IP address. The router removes it from its ARP as well as address filter table. Due to a bug, this procedure also incorrectly removes the entry for the same virtual IP address configured under the actual interface from the tables, causing it to stop responding to ARP requests.
For a workaround, disable the HSRP snooping process by issuing the no standby redirects or standby redirects disable command from the global configuration mode. Remove and re-add the HSRP configuration under the corresponding interface that has stopped responding to ARP requests.
For more information, refer to Cisco bug ID CSCdx10170.
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