VTP and STP

Answered Question
Jul 7th, 2008
User Badges:

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

Correct Answer by Jon Marshall about 8 years 9 months ago

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

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
Correct Answer
Jon Marshall Tue, 07/08/2008 - 00:25
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

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

pthaynes Tue, 07/08/2008 - 15:16
User Badges:

Jon,


Thanks - that makes a little more sense to me.


Peter

Actions

This Discussion