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VTP and STP

I have a number of switches with VTP pruning enabled. Each of the switches has a limit on the supported number of STP instances. I believe that when VTP pruning is enabled it does not limit the number of STP instances created (assumedly VTP pruning just limits the VLAN traffic that passes over a link, not the VLANs available and STP creates an instance per available VLAN). My question is this:

If I limit the allowed VLANs on all the trunks on a given switch (using the switchport trunk allowed vlan command)does that reduce the spanning-tree instances used on that switch? Assuming the answer to that is yes my next question is if I only limit the VLANs on one end of a link does that impact the number of STP instances for both switches or just the one that is limiting the VLANs?

For the purposes of my questions assume that I am using pvst or rapid-pvst and also have VTP pruning enabled.

Thanks in advance for any clarification

Peter

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Hall of Fame Super Blue

Re: VTP and STP

Peter

No it doesn't. Spanning tree will still run for all the vlans. Using the allowed vlan command stops STP running across the link so if you have

sw1 -> sw2

switches are connected via a trunk link. If you have a vlan 10 on both switches but you don't allow it on the trunk link then both switches assume they are root for vlan 10. It will not stop each switch running an STP instance for vlan 10.

Jon

2 REPLIES
Hall of Fame Super Blue

Re: VTP and STP

Peter

No it doesn't. Spanning tree will still run for all the vlans. Using the allowed vlan command stops STP running across the link so if you have

sw1 -> sw2

switches are connected via a trunk link. If you have a vlan 10 on both switches but you don't allow it on the trunk link then both switches assume they are root for vlan 10. It will not stop each switch running an STP instance for vlan 10.

Jon

New Member

Re: VTP and STP

Jon,

Thanks - that makes a little more sense to me.

Peter

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