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HSRP Interface Tracking on IOS Routers

In this lab, I learned the following:

  • configuring a HSRP group on physical interfaces,
  • testing how a PC can reach a remote subnet through a redundant gateway, by configuring its default gateway as the HSRP virtual address,
  • testing how the second router becomes Active,
  • testing interface tracking.

For interface tracking, at first, I thought there was a problem with the lab.

configure interface tracking and a decrement value:

When I shut down ser0/0 on R2, I expected R2 to become Standby router. But reality bites:

Then I checked on R3. R3 is still Standby, not Active !

So what’s wrong? Here’s what Cisco says:

How Object Tracking Affects the Priority of an HSRP Router

The priority of a device can change dynamically if it has been configured for object tracking and the object that is being tracked goes down. The tracking process periodically polls the tracked objects and notes any change of value. The changes in the tracked object are communicated to HSRP, either immediately or after a specified delay. The object values are reported as either up or down. Examples of objects that can be tracked are the line protocol state of an interface or the reachability of an IP route. If the specified object goes down, the HSRP priority is reduced. The HSRP router with the higher priority can now become the active router if it has the standby preempt command configured.
So, even if R3 has a higher priority than R2, it can not become Active unless it has preemption feature configured; interesting.
So, to generalize:
-in interface tracking, if the interface goes down and the priority of its router (router X) went below router Y’s priority, router Y can become Active IF it has preemption feature configured.
This is also valid when a gateway is no longer reachable on its interface where we configured HSRP.
The missing command is: standby 1 preempt, on R3.
and now R3 is the Active router:
Published inNetworking Basics

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