When the Hot Standby Router Protocol (HSRP) runs, an instability in HSRP during a network disruption or an active router transition may occur. The transition can be caused by a new router added with a higher HSRP priority and configured for preemption. This results in flapping of the HSRP state between active and standby on the existing active router.
The existing active router notices a change in its HSRP state from current active to the new standby state. This triggers a change in the physical link state and causes a Spanning Tree Protocol (STP) transition on the switch.
Since the STP takes more time to move a port to a forwarding state that is greater than the HSRP holdtime, the router does not receive HSRP hellos from the higher priority router within the holdtime. The router moves to an active state.
After the STP timers expire, the switch port enters a forwarding state. The router receives HSRP hellos from the higher priority router and moves to a standby state. This behavior repeats and causes the router to switch between active and standby states.
For a workaround to this problem, upgrade to Cisco IOS Software Release 12.1(3) or later or perform one of these options:
- Configure the STP PortFast on the switch port to reduce the time to move the port to a forwarding state.
- Configure the HSRP holdtime to a value higher than the time it takes the STP to move a port to the forwarding state.
- When the HSRP state changes, configure the router to use the Burned-In Address (BIA) for the HSRP MAC address (instead of the standard address used) to prevent the router from triggering a change in the physical link status.
For more information and workarounds, refer to Avoiding HSRP Instability in a Switching Environment with Various Router Platforms.