Trunked links and STP Convergence

Unanswered Question
Jun 8th, 2009

Hi all,

We in the final steps of a fork-lift upgrade going from sup2 catos to sup720 IOS. Here's the issue we have been working under every tight outage windows, so we swung half the links over onto to the new pair of 6k, but left one of the links shutdown just in case we needed to fall back to the old system. In our new design the ports are trunked links to the core. I assumed that when we plugged in the second link and turned up the interface that it would act similarly to etherchannel and that the port would come up without causing STP convergence, thus not "needing" an outage window. Well we tried it last night and that wasn't the case, plugged the second link in and sure enough, convergence. Could some one explain why with trunked links it did this?

I have this problem too.
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Jon Marshall Mon, 06/08/2009 - 06:33

Lindsay

"I assumed that when we plugged in the second link and turned up the interface that it would act similarly to etherchannel and that the port would come up without causing STP convergence"

Unless the port is part of an existing etherchannel then there will have to be STP convergence because there is now another path within the L2 network and STP needs to converge so as to avoid a loop.

And because it's a trunk link then every vlan that goes across that trunk will be subject to STP convergence.

Jon

Lsimancek_2 Mon, 06/08/2009 - 08:04

Thanks Jon, I hadn't thought about it really in depth or from the L2 side. Makes sense. I guess its another set of outages while we swing links over :(

Thanks tho!

johnstone_cisco Mon, 06/08/2009 - 07:54

Hi,

In the past when I have been working on our switching infrastructure, I confirm the following 2 settings:

- spanning tree priorities (in relation to the root)

- VTP settings

If they are correct,I tend to find little problems with STP reconvergence.

Is this not the case?

Thanks

brian

Lsimancek_2 Mon, 06/08/2009 - 08:03

Brian,

The priorities and vtp settings were as to be expected in our new enviroment. It wasn't until Jon explained that L2 links are still going to see a loop/convengence while it figures out its links that it clicked in my head that the switch was still seeing the loop.

Its not that it caused any problems just we were hoping that any sort of outage would be minumal at best to avoid having to planning for "outages". Turns out not to be the case, the 5-30 sec convergence is long enough that we have to list is as an outage and go threw application testing and verification afterwards.

thanks

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