HSRP preempt

Unanswered Question
Aug 12th, 2009
User Badges:

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.


Thanks.


/rls

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (3 ratings)
Loading.
kwillacey Wed, 08/12/2009 - 07:00
User Badges:
  • Bronze, 100 points or more

the preempt is to allow the once active device to take over for the stanby once it comes back online.

Edison Ortiz Wed, 08/12/2009 - 07:01
User Badges:
  • Super Bronze, 10000 points or more
  • Hall of Fame,

    Founding Member

'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.


HTH,


__


Edison.

rsamuel37 Wed, 08/12/2009 - 07:13
User Badges:

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.


Thanks.


/rls

Jerry Ye Wed, 08/12/2009 - 07:31
User Badges:
  • Cisco Employee,

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.


HTH,

jerry

rsamuel37 Wed, 08/12/2009 - 07:51
User Badges:

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....

Jerry Ye Wed, 08/12/2009 - 07:54
User Badges:
  • Cisco Employee,

It is always a good practice to configure the standby priority on the interface, this will make routing more deterministic.


HTH,

jerry

rsamuel37 Wed, 08/12/2009 - 08:26
User Badges:

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?


/rls

Jon Marshall Wed, 08/12/2009 - 09:01
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

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.


Jon

rsamuel37 Wed, 08/12/2009 - 09:13
User Badges:

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...


Thanks.


/rls

Jon Marshall Wed, 08/12/2009 - 09:24
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Two different things.


If the interface that is the active HSRP interface dies then the standby will miss the hellos and then assume the active role.


With interface tracking you don't actually lose the active HSRP interface because the interface you are tracking is neither of the HSRP interfaces. What happens is that the router realises that the tracked interface has gone down and then decrements the HSRP active. The standby will then see that it has the higher priority and preempt, assuming that after decrementing the active priority the standby now has a higher priority.


Note that in the tracking situation it is nothing to do with not receiving hellos because the active HSRP interface has gone down because it hasn't, it has simply changed it's priority.


Jon

rsamuel37 Wed, 08/12/2009 - 09:33
User Badges:

So under normal circumstances, the decrement value plays no part unless you're tracking. If you just have two interfaces set to a priority 200 and 100, the 100 interface only takes over after 3 missed hellos from the 200 interface. That happens by design...then once the 200 interface comes back online, it resumes the active state assuming preempt is configured. If preempt is not configured the 100 interface keeps the active role.


Would that be a correct description?


/rls

Jon Marshall Wed, 08/12/2009 - 09:35
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Correct.


Jon

rsamuel37 Wed, 08/12/2009 - 09:42
User Badges:

Got it. Thanks for the patience and the answers.


Take care.

Actions

This Discussion