Interface tracking in HSRP

The coolest thing about HSRP is it's ability to track interfaces and decrement the switch's HSRP priority in response to interface state changes. With HSRP, the highest priority wins the active switch role. All other switches with interfaces in the same HSRP group are designated as standby switches. The default HSRP priority is 100 so if you have to manually change the priority to something higher if you want a particular switch to be the active switch in the group. It's important to remember to use the preempt feature also to ensure that the manually designated active switch returns to it's active role after an outage.

HSRP could be used to determine the best route out of the LAN by using interface tracking. If you have a switch with a bouncing WAN link then you probably don't want it to function as the active switch and therefore the default gateway for your LAN hosts.

Let's say that you have a couple layer 3 switches in your distribution layer running HSRP. Each switch may have redundant WAN links to your ISP. You can set up interface tracking on the WAN links so that each time one of the links drops, the HSRP priority of the switch decrements. Eventually, the priority is lower than one of the standby switches and that switch will take over the active role. This ensures that network traffic is using the most reliable switch to reach the outside world. This is just one example and there are a lot of uses for interface tracking.

The default decrement value is 10 meaning that each time the tracked interface(s) drops, the HSRP priority is decremented by 10. You can change this to any value you want. You can track multiple interfaces, like this:

interface ethernet0

ip address 10.1.1.1 255.255.255.0

standby ip 10.1.1.3

standby priority 110

standby track serial0

standby track serial1

This is a really nifty way to ensure you are using the best path for your traffic flow -- kinda like route dampening in BGP. The other alternative is to change the decrement value so that it immediately becomes a standby router and abdicates the active role. It's very flexible.