Spanning tree instances

[eagles-nest]

Hi

When I look at some lower end switches they state they support, for example, 128 spanning tree instances.  I was always under the impression that a spanning tree instance was created per vlan per port it runs on.  So if I have 10 vlans, 20 access ports and 2 trunks that would be (2 x 10) +20 = 40 instances.  Or when I issue the "show spanning summ totals" command I thought the instances were the number in the STP active column at the end.

However, when I do some testing on a small switch with several trunks I only get an issue with spanning tree instances when I create my 125th vlan.  This in addition to default created vlans takes my total to 129 and in the config I then see a "no spanning-tree vlan xxx" command inserted for the last created vlan.

This implies a spanning tree instance is per vlan no matter how many ports are active in that vlan.  Is this correct ?

Isn't it a bit dangerous that the switch inserts the no spanning-tree vlan xxx command ?  Surely this leaves the network open to loops if someone creates too many vlans in rapid pvst mode.

Any comments are appreciated.

Thanks.

[Joseph W. Doherty]

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Posting

When you're dealing with STPs, you need to distinguish between STP standards and proprietary extensions.  What you're thinking of, I believe, is one of Cisco's P(er)V(LAN)STPs.  That do have a STP "instance" per VLAN.  An "instance" supports a single L2 topology.  The concept might be a little clearer if you consider an "ordinary" STP that has only a single instance or MST where you can map VLANs per MST instance.

"Isn't it a bit dangerous that the switch inserts the no spanning-tree vlan xxx command ?  Surely this leaves the network open to loops if someone creates too many vlans in rapid pvst mode."

Yes, but if your running that many VLANs, MST should be considered.