VRRP Priority and Preemption Question

Unanswered Question
Jul 19th, 2009

Hi, folks:

Here's the question...

There are 2 routers in a VRRP group...

The active router is tracking an object...

The object fails...

The active router decrements its priority by 20 (configurable, I know)

Backup routr becomes primary...

The tracked object comes back up...

Will the active router once again assume its respinsibilities as active by returning its priority to what it was originally configured for?

Or does the backup stay active?

I know the answer has to do with configuring preemption.

It seems that VRRP works the opposite from HSRP...

http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_vrrp.html#wp1054595

"By default, a preemptive scheme is enabled whereby a higher priority virtual router backup that becomes available takes over for the virtual router backup that was elected to become virtual router master. You can disable this preemptive scheme using the no vrrp preempt command. If preemption is disabled, the virtual router backup that is elected to become virtual router master remains the master until the original virtual router master recovers and becomes master again."

Thanks

I have this problem too.
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Giuseppe Larosa Sun, 07/19/2009 - 08:44

Hello Joe/Victor,

I read the link you have provided.

My understanding is the following:

VRRP has the concept of owner that is the router that has on the interface the same ip address as the VRRP VIP.

Without an owner on LAN the VRRP process is stucked in an idle state.

VRRP preemption or VRRP no preemption applies to the other nodes: in any case when the owner comes back (or restores its priority) takes over.

The effect VRRP preemption disable should have is that of avoiding that a third device can take over on the second node (the one that took the role of active) during the time owner is not the active node.

Hope to help

Giuseppe

ex-engineer Sun, 07/19/2009 - 09:14

Hi, Giueseppe:

VRRP does not have to work in the manner you describe.

You dont have to have the physical IP address and the VIP as being the same address on the master router. That is an option, not a requirement.

You can configure VRRP just like HSRP with regard to IP address asignments.

The question I ask is specific:

By default, will the router that is configured as master regain its role as master once the object it is tracking is back up?

Or do I need to specfically configure preemption for this to happen?

In HSRP, you need to configure preemption. This allows an HSRP active router to regain its role as the active router once the object its tracking comes back up.

According to that link and the paragraph I just referenced, it seems like you need to DISABLE preemption for VRRP to behave that way.

My requirement is that I want the router originally acting as master/active to regain its role as master/active once the object it is tracking comes back up.

Thanks

EDIT - By the way, preemption is enabled by default on the code version I am using. 12.2.33 SRD

Giuseppe Larosa Sun, 07/19/2009 - 10:11

Hello Victor,

yes, I may be wrong about the owner.

In the same document you have linked the preemption is described as:

Preemption

The redundancy scheme of VRRP enables you to preempt a virtual router backup that has taken over for a failing virtual router master with a higher priority virtual router backup that has become available.

That is in line with my understanding.

So if you mean the original master of the group it should retake its role regardless of preemption configuration.

Again I didn't test this recently but it looks like VRRP treats differently the original master of the group.

If you have made tests that demonstrate a different behaviour it can be correct/acceptable.

And VRRP defaults to preemption enabled in all IOS versions.

Hope to help

Giuseppe

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