It looks to be a limitation of the platform. Taken from the release
notes for 12.2(55)SE for Catalyst 3750X switch:
The switch supports 128 spanning-tree instances. If a switch has more
active VLANs than supported spanning-tree instances, spanning tree can
be enabled on 128 VLANs and is disabled on the remaining VLANs. If you
have already used all available spanning-tree instances on a switch,
adding another VLAN anywhere in the VTP domain creates a VLAN on that
switch that is not running spanning-tree. If you have the default
allowed list on the trunk ports of that switch (which is to allow all
VLANs), the new VLAN is carried on all trunk ports. Depending on the
topology of the network, this could create a loop in the new VLAN that
would not be broken, particularly if there are several adjacent switches
that all have run out of spanning-tree instances. You can prevent this
possibility by setting allowed lists on the trunk ports of switches that
have used up their allocation of spanning-tree instances.
And I know the "feature" exists on the classic 10/100 Catalyst 3750s as
I have seen it happen on them as well.
Bill Foster
Network Support Specialist
UW-Madison
On 5/1/13 7:49 AM, cisco-nsp-requ...@puck.nether.net wrote:
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Today's Topics:
1. Spanning Tree Instances (Leigh Harrison)
2. Ang: ME3600X Output Drops (gustav.ulan...@steria.se)
----------------------------------------------------------------------
Message: 1
Date: Fri, 23 Mar 2012 12:48:06 +0000
From: "Leigh Harrison" <lharri...@convergencegroup.co.uk>
To: <cisco-nsp@puck.nether.net>
Cc: Clive Webster <cwebs...@convergencegroup.co.uk>
Subject: [c-nsp] Spanning Tree Instances
Message-ID:
<BE13605B4C12654284C937196842A00020254F@condc02.ConvergenceGroup.local>
Content-Type: text/plain; charset="us-ascii"
Hello all,
We have run into an issue on a 3750 switch where it has run out of spanning
tree instances.
Is this a limitation of PVST or is it a limitation of the switch? I can't seem
to find good clarity anywhere. I have some 6509's and nexus 7k's and I'm
wondering if they're going to suffer from the same fate...
Leigh
Leigh Harrison, CCIE#15331
Head of Design & Infrastructure
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