Question on HSRP failover detection

Answered Question
Feb 19th, 2008

Imagine I have

[RouterA, s0:10.0.0.1/8, fa0:192.168.1.1/24]


[RouterB, s0:172.16.1.1/24,fa:192.168.1.2/24]


Virtual IP for HSRP between the two routers, on the Ethernet interfaces is 192.168.1.3/24. Active router is RouterA. No HSRP is set on the serial interfaces, which are connected to different ISPs.


Question:

If line protocol or s0 physical interface goes down on s0 on RouterA, failover to RouterB will NOT occur, is that right? If I want a failover to be triggered under such scenario I would need to configure object tracking for this. Did I understand this right?

Correct Answer by lamav about 9 years 1 week ago

"If line protocol or s0 physical interface goes down on s0 on RouterA, failover to RouterB will NOT occur, is that right?"


Yes.


"If I want a failover to be triggered under such scenario I would need to configure object tracking for this. Did I understand this right?"


Interface tracking, yes.


HTH


Victor

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Richard Burts Tue, 02/19/2008 - 19:45

Marlon


You are close but not quite right. In a simple HSRP configuration you are right that a failure of the serial interface would not cause a failover of the HSRP function. But you do not need Object Tracking to get the failover. In HSRP there is an ability to track interfaces and interface track can cause a failover of HSRP.


Here are some suggestions if you want to configure interface track in HSRP:

- specify priority to select which router is to be active (you do not want to negotiate this). And set the priority to 105 on the active router.

- specify interface track on one or both routers. (if the interface goes down the router will lower its priority by 10 points - resulting in priority of 95).

- configure preempt on both routers. You need preempt on the standby router so that it can take over if the active router lowers its priority. And you want preempt on the original lead router so that when its interface comes back up and its priority returns to 105 that it can take over being active router again.


HTH


Rick

Correct Answer
lamav Tue, 02/19/2008 - 19:46

"If line protocol or s0 physical interface goes down on s0 on RouterA, failover to RouterB will NOT occur, is that right?"


Yes.


"If I want a failover to be triggered under such scenario I would need to configure object tracking for this. Did I understand this right?"


Interface tracking, yes.


HTH


Victor

news2010a Thu, 02/21/2008 - 07:12

One last question on this:


Now imagine that I DO NOT implement the tracking feature. Then the interface s0 on Router1 fails. As we agreed, HSRP will not failover to the other node.


As long as I have BGP between RouterA and Router2 correctly configured, even without the tracking feature packets which cannot go out of the interface s0 (in case it goes down) will still bounce to RouterB and flow through the ISP2, right?


I think I got it now.

Richard Burts Thu, 02/21/2008 - 07:39

Marlon


Yes. If you do not configure interface tracking and if the outbound serial on routerA fails then routerA remains the active HSRP router and when it receives packets to forward it will forward to router2 which will send to its ISP.


HTH


Rick

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