Simple HSRP Question

[sullivan.p]

Hi All,

Hoping to get some clarification around something I'm having trouble finding a definitive answer on in the Cisco Docs.

My Question - If an HSRP interface whose priority is decremented based on a IP SLA tracked object being offline is re-incremented before the preemption delay of the other HSRP router has completed, does that 're-incrementation' of the preferred HSRP interface reset or cancel the preemption?

Or is it once a preemption action has been initiated, does the re-incrementation of the preferred interface have no effect on the preemption and it continues to failover the active HSRP interface to the lower priority HSRP interface?

I have an HA Internet scenario, using two routers in an HSRP v2 setup, with upstream objects being tracked using IP SLA targets to decrement the configured priority of Router A (Priority 100 HSRP Preferred Router) by 60. the configured priority of Router B is 50 (HSRP - Backup RTR). Therefore when the upsteam IP SLA targets are identified as not responding, the preferred Router A's HSRP interface priority is decremented by 60, and a preemption takes place. My intended configuration is that this preemption is delayed by 30 secs to avoid false positives, but I just want to be sure that this is in fact what I can expect.

That if the IP SLA targets configured on Router A come back online before the preemption delay period has completed, that the priority of Router A's HSRP interface is re-incremented to 100 and the preemption is cancelled before Router B has had a chance to takeover.

Hope that makes sense. Any help much appreciated, and if you can point me in the direction of any KB articles that would be very useful.

Thanks,

Patrick

[handoko wiyanto]

hi Patrick,

would you post that HSRP and IP SLA config to the forum here?

regards,

[smehrnia · Accepted solution]

Hi there,

I have never experienced this condition in a Lab before, but from my readings, due to the fact that "preempt delay" is a timer that is used to give some time to Standby router to populate its routing table before becomming active. I guess as soon as the coupe message is exchanged, the standby router sets to become active, and whatever rolls back will take effect after the timer expires, and a new coupe message exchanged.

preempt timer is not there to prevent flapping, I guess. btw I'd be happy if you let me know if its otherwise.

Link:

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094a91.shtml#preemption

plz Rate if it helped,

Soroush.

[siddhartham · Accepted solution]

As Soroush explained, Prrempt delay specifies the time the new HSRP standby Router waits before take over the active role from the peer HSRP router.

In your case:

1) Initial condition

--- RouterA-priority100-Active router    RouterB-Pr50-Standby Router

2) IP SLA Target on RouterA stopped responding

--- RouterA-priority40    RouterB-Pr50 ---Router B immediately becomes the Active Router

3)Preempt Dealy min 180 and preempt command configured on RouterA

--- When the priority on RouterA is back to 100 (means the IP SLA target start responding) RouterA will start the count down timer and wait 3 min (180sec) then take over the active role from Router B.

[sullivan.p]

Thanks for the responses, tested this in the lab today and it indeed looks like each coup message requires its own preemption delay period.

Thanks again,

Patrick.