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Community Member

Spanning Tree Root Bridge Dispute C3750-V2

I am seeing unusal spanning tree behaviour on VLAN 1 when adding a new switch stack to our network. The network is part of a user estate consisting of a pair of C6509s each connect to a number of C3750 switch stacks. Each stack has a single trunk to each of the C6509s with the native VLAN set to a shutdown VLAN (2). Spanning tree mode is configured to rapid-pvst. C6509-1 is configure as to be the root bridge using: spanning-tree vlan 1,2,x,x,x,x priority 8192 and C6509-2 is configured to: spanning-tree vlan 1,2,x,x,x,x priority 16384 Each C3750 stack is configured with: spanning-tree uplinkfast (I know this isn't active under rpvst but ensure priority for all VLANs is 49152) All this works as expected, then along comes the C3750-V2 (15.0.2.SE2), we can add the stack successfully to the network but both the C6509s give the following for "show spanning-tree vlan 1": Gi1/17 Desg BLK 4 128.17 P2p Dispute The same coomand on the C3750-V2 show a bridge priority of 49153 and "This bridge is root" under the root ID. To add to the confusion all the other VLANs on the switch are showing C6509-1 as the root bridge. Have I misconfigured something or has anyone else seen this behaviour? Regards Dan

Hi, Can you check if the vlan



Can you check if the vlan 1 is allowed on both ends of the trunks between the 6509 and the 3750 stack? 


Hope this helps.

Community Member

That's interesting (and

That's interesting (and annoyingly overlooked): All ends of the trunk show the full set of vlans under "allowed and active in the management domain" but vlan 1 is missing from the 6509 end under "forwarding state and not pruned". Putting the trunk native vlan back to 1 obviously brings vlan 1 back to a forwarding state and returns spanning tree back to a consistent state. This isn't consistent with the operation of the 3750 or what I'd expect. Again am I overlooking the obvious or is this expected on the V2?

Hi, If you dont have any



If you dont have any acitve ports associated with vlan 1 then it will not be active and in forwarding state even if it is allowed on the trunk, after you put back the native vlan to 1 then vlan 1 will become active and therefore the 6500 will claim the root state and the dispute will go away.



Community Member

That is correct. However on

That is correct. However on the old 3750 stacks there are no active vlan 1 ports and the native vlan has been changed and vlan remains in a forwarding state. I thought the default vlan always stayed in a forwarding state, part of the reason vlan 1 cannot be deleted, to handle management traffic such as cdp (cdp is working across the trunk). I guess that makes it impractical to ever change the native vlan and a problem if you have security people who still believe vlan hopping is more than a theoretical threat. (As an after thought turning off VTP pruning also fixes the problem) Thanks for responding
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