Is 'standby preempt' needed on both the active and standby router? My thinking is that if you don't have preempt configured on the active router that it won't allow the standby to take over even in the event of a failure but I want to be certain.
Any insight is appreciated.
'preempt' is only needed on the higher HSRP priority router as the lowest HSRP priority router will only take over when the higher HSRP priority router is not available hence no need to preempt an existing HSRP router.
The higher priority HSRP router will need to preempt the current HSRP router which has the lesser HSRP priority value.
Ok, so in that case you're saying it would need to be on the current 'standby' router so that when the active fails it can take over. If my intention is to have the active take back over once it comes back online would having 'preempt' on both sides do the job?
I'm trying to make sure that I have preempt configured to do what I intend ideally, but also that it's at least on the correct side (active or standby) to do what it should.
Like Edison said, preempt is needed on the higher priority router, you can verify it by show standby.
Assuming both routers are running, active is priority 105 and standby is 100. When the higher priority router goes down, the standby will take over (lost of heartbeat). Once the higher priority router comes back online, and it has preempt configured, it will take over again.
Ok...I got it now. Wasn't thinking it through before I responded. That makes sense since you specifically configure the priority on the interfaces that won't physically change....
What would be the outcome if you have preempt configured only on the lowest priority side, but not on the higher priority? Would the state change?
Yes it would but then when the router with the higher priority came back online it would not be able to preempt.
One last thing. There are times when you do need preempt on both routers and this is when you are tracking another interface. If that interface goes down then the active router will decrement it's priority so it is lower than the standby router. But if you haven't configured preempt on the standby it will not take over.
Ok great info, but one last question: Does the decrement value only come into play when you're tracking interfaces? i.e. if I have my priorities set to 200 and 190, when the 200 interface dies will it decrement by 10 to 190 and still have to wait for the 3 missed hello packets?
...or... if I have the priorities set to 200 and 191 for example, will the downed interface decrement to 190 and immediately move to the other link?
Hope I was clear enough to get my meaning...