HSRP nuance

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So studying hard to renew CCNA Wednesday. Came across something i hadn't seen before regarding HSRP. It seems if you simply enable HSRP and don't set a priority, the HSRP pair will compare IP addresses and the higher interface IP wins. That router will become the active device for the virtual IP/MAC.

Can anyone tell me what would happen if preempt was enabled on a router in this scenario where it had the lower IP address? Does that in any way change the behavior?

Asking because I ran into a study question that was worded like this, "Which configuration do you apply to an HSRP router so that it is most likely to come up if the active router goes down?"

General consensus seems to point to the answer as enabling preempt, although I don't really understand why.

Regards,
Adam Tyler

I found this on Cisco's website discussing HSRP. Maybe it will be helpful.

Q. Which HSRP router requires that I configure preempt?
A. An HSRP-enabled router with preempt configured attempts to assume control as the active router when its Hot Standby priority is higher than the current active router. The standby preempt command is needed in situations when you want an occurring state change of a tracked interface to cause a standby router to take over from the active router. For example, an active router tracks another interface and decrements its priority when that interface goes down. The standby router priority is now higher and it sees the state change in the hello packet priority field. If preempt is not configured, it cannot take over and failover does not occur.

I think this means if the active router does NOT go down but a tracked interface does, it would decrement the priority of the router by the configured amount and the preempt on the standby router would allow it to take over after seeing that the priority on the active router is now lower than its own. If the active router wasn't tracking an interface, the priority would stay the same and the preempt command wouldn't cause the standby to take over because the active router's priority hasn't changed. If the active router failed, the priority wouldn't matter because it stopped responding and the standby would take over after the HSRP hold time expired without receiving a hello.

Good question. I'm contributing to the thread and don't consider my reply an answer.