what will happen if VSS preempt is configured

[carl_townshend]

Hi all

We have 2 core switches running VSS

Tmrw we will power off the Primary VSS switch which has priority of 150 and preempt configured.

my question is what will happen when its powered back up again, should I experience no loss and will it autopmatically recover and resume the primary vss switch?

cheers

Carl

[Leo Laohoo]

Tmrw we will power off the Primary VSS switch which has priority of 150 and preempt configured.

Cisco doesn't recommend this.  Let me explain.

Let say that you have the primary configured for preempt. 

The primary has a power failure and the secondary card takes over.  This is what happens next, if you have preempt, when the primary recovers.

1.  The primary powers up.  Because the of the preempt command, the primary will "demand a showdown" with the secondary.

2.  Showdown happens when both the primary and secondary reboots so the primary can recover.  This means that your chassis reboots TWICE.

Without the preempt, the primary card will bootup and seeing that there's already an existing primary card, the original primary card will join the VSS as "hot" standby.

[carl_townshend]

so will this cause downtime in both chassis ?

what exactly will happen when the primary comes back online ?

[Leo Laohoo]

so will this cause downtime in both chassis ?

Yes.  Significant outage for both chassis when both chassis will reboot so the secondary can hand-over to the primary.

what exactly will happen when the primary comes back online ?

If preempt was NOT configured, nothing.  The primary will quietly come back up as "hot standby" while the secondary will remain "Active" until the secondary supervisor card reboots.

If preempt is configured, the primary will come up, demand a "challenge" and causes both chassis to reboot. 

[carl_townshend]

Hi

Many thanks for your help

Ok, so my next question, Can I take the preempt command off ? and will this itself cause any outage ?

cheers

Carl

[Leo Laohoo]

In my opinion, taking the command off shouldn't cause an outage.

But I've never used this command, so I can't help you here, Carl.

[InayathUlla Sharieff]

Yes you can remove without any issues.

this is been recorded under below link:

When removing the preempt commands in Catalyst 6500 Series Switches in VSS Mode, will it reload the switches?

The switch preemption is not recommended. Therefore, removing the commands is a good practice and does not cause a reload. For more information on the Preemption feature on VSS, refer to Switch Preemption.

Ref: http://www.cisco.com/en/US/products/ps9336/products_tech_note09186a0080a7c837.shtml#faq2

HTH

Regards

Inayath

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