[ednog] Anyone deploying MSTP and/or RSTP as alternatives to spanning tree?

Nicola Foggi nfoggi at depaul.edu
Thu Apr 28 16:44:47 EDT 2005

>>> "David Farmer" <farmer at umn.edu> 04/26/05 06:19PM >>>


> We had one small problem.  We configure new switchs via a script, the

> old script turned on VTP before MST.  This wasn't a problem until we

> got to our largest Core Node that has 178 VLANs.  So, 3750s support 
> 128 spanning tree instances and default to PVST (Per VLAN Spanning 
> Tree) as do most Cisco Switches.  So when VTP was enabled it tried 
> to create 178 spanning trees, it could so it disable Spanning Tree
> about 50 VLANs, and they remain disable after MST was configured. 


We ran across the limited # of spanning tree instances a couple of
times unfortunately...  we haven't looked at MST yet, but we did end up
manually pruning the vlan's off the trunked interfaces for vlans that
were not in use which solved the # of spanning tree.. it also solved a
problem we were having with the cam table's filling up and caused
multicast reliability problems...  We looked at enabling Cisco's
Auto-Pruning functionality, but in our test, because it relies on
spanning tree to determine if a vlan is needed on a trunk, if you are
passed the number of supported instances of spanning tree, the 3500 will
disable spanning tree for a potential vlan that is needed on the
switch... which then doesn't allow it on the trunk.

I haven't looked at it with the 3750's as most of our deployment is
3500XL/3550 but my guess is it's the same...

Nicola Foggi
Networks and Telecom
DePaul University

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