HSRP Interface Tracking is an important feature to provide redundancy on the router group and switch between them if anything unexpected happens to the active router.
The active router is the router which has the highest priority. The active router can switch to another router if the main router decreases its priority or another router is set to a higher value. Remember for direct switching, the preempt command condition needs to be enabled.
HSRP tracking is able to decrease a router priority based on its track record. It can track interfaces and decreases a router priority should the interface that is monitored go down. The command to track an specific interface is:
(config)#interface
(config-if)#standby track // interface is the interface of the monitored interface, e.g. interface serial 0 going to another router.
The decrement value needs to be set from 1-255, and this feature will decrease the priority number if the monitored interface goes down. The default value (if not set) is 10.
For example, we have 2 router (router A and B), where router A is the active router set with value = 150 and router B is standby with value = 140. Serial 0 to another router is monitored with HSRP track on router A, where the decrement value = 20. If the serial 0 interface is down on router A, then the priority will be decreased to 150-20 = 130. This value is lower than router B with the priority 140. Therefore, router B will immediately take over as active router. If at some point serial 0 of router A is up again, the priority value will be set back to 150 and router A will be the active router again and router B will act as standby. Watch out on setting track priority decrement values, because if the decrement value is to low e.g. on router A is set to only 5, nothing will happen if serial 0 goes down. This happens because the decrease value of router A is 150-5=145 which is still higher than router B priority of 140. Changing decrement values can be done directly on live networks and will effect immediately (for example change from 5 to 15).
There is also a possibility to monitor multiple interfaces. In this case, it will check for both interfaces and will decrement only if one of the interfaces is down, or will accumulate the total amount of values set for both interfaces if both of them go down.
To see and debug HSRP enter "#debug standby" command.
-- 23 November 2010 --
The active router is the router which has the highest priority. The active router can switch to another router if the main router decreases its priority or another router is set to a higher value. Remember for direct switching, the preempt command condition needs to be enabled.
HSRP tracking is able to decrease a router priority based on its track record. It can track interfaces and decreases a router priority should the interface that is monitored go down. The command to track an specific interface is:
(config)#interface
(config-if)#standby
The decrement value needs to be set from 1-255, and this feature will decrease the priority number if the monitored interface goes down. The default value (if not set) is 10.
For example, we have 2 router (router A and B), where router A is the active router set with value = 150 and router B is standby with value = 140. Serial 0 to another router is monitored with HSRP track on router A, where the decrement value = 20. If the serial 0 interface is down on router A, then the priority will be decreased to 150-20 = 130. This value is lower than router B with the priority 140. Therefore, router B will immediately take over as active router. If at some point serial 0 of router A is up again, the priority value will be set back to 150 and router A will be the active router again and router B will act as standby. Watch out on setting track priority decrement values, because if the decrement value is to low e.g. on router A is set to only 5, nothing will happen if serial 0 goes down. This happens because the decrease value of router A is 150-5=145 which is still higher than router B priority of 140. Changing decrement values can be done directly on live networks and will effect immediately (for example change from 5 to 15).
There is also a possibility to monitor multiple interfaces. In this case, it will check for both interfaces and will decrement only if one of the interfaces is down, or will accumulate the total amount of values set for both interfaces if both of them go down.
To see and debug HSRP enter "#debug standby" command.
-- 23 November 2010 --
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