New VLAN impact, PVST+

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Aug 27th, 2008
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Point of debate. On a LAN running PVST+, if a new VLAN is added, does the convergence impact the existing VLANs? Assume all cross-link interfaces are default trunked, carrying all VLANs.

I've heard both that the trunked port will go through the convergence process, impacting traffic on all VLANs, and that since PVST+ uses separate STP instances per VLAN, the trunk is not impacted, just the new VLAN.

The question seems academic until considered on a large production network, where any time lost for convergence on an existing VLAN could cause issues.

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Jon Marshall Wed, 08/27/2008 - 11:58
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Per vlan STP means just that ie. adding a new vlan should only create STP convergence for that vlan.

Obviously adding a new switch connected via a trunk is a totally different thing.


paul.matthews Fri, 09/05/2008 - 07:30
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Jon's correct. The whole point of PVST it so that each VLAN can run its own instance of spanning tree, and changes in one VLAN should not affect another.

Having said that, no network change is completely risk free, so consideration of timings is worthwhile.

letsgomets Fri, 09/05/2008 - 11:50
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PVST+ is Per VLAN Spanning Tree with the support for uplink fast and backbone fast.

With that being said each VLAN runs its own spanning tree instance. By you adding a new vlan you will not have a topology change on any other vlan.

All trunk ports will go through the STP process for that VLAN only.


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