will vtp pruning cause spantree priority to reset to 32778

[d-tay]

I have 2 catalyst 6509 connecting to two downstream catalyst 3550 switches. more catalyst 2950 switches are connected to the catalyst 3550 in a loop form for redundancy.

As i have more than 400 vlans and stp instances, i do a manual vtp pruning on the core 6509 switches ports which is connected to downstream catalyst 3550.

i only allow vlan1 and the user vlans (vlan 330) to go across the trunk port.i also set the root spanning tree on the catalyst 6509 for all the vlans.

when i do show spanning-tree vlan 10 on the catalyst 3550, the spanning tree priority for vlan 10 is set to 32778 and therefore the root bridge address is not the core catalyst 6509. However spanning-tree vlan 330 priorty was 8192 and the root bridge address is correct.

Is that normal ? i am concerned because one of my catalyst 2950 switches CPU utilisation was constantly 90% until i shutdown its redundant port to prevent a physical loop.

I appreciate any feedback.

Thanks

[vincent-n]

STP is normally "over-looked" when it comes to configuring VLANs on Cisco switches (even for experienced administrator). You'll have to configure the priority manually for each VLAN that you have in your network. Don't forget that STP priority is VLAN independent hence you can have a switch acting as root bridge for one VLAN and another for other VLAN. What happened in your case is that someone created VLAN 10 in the core switch, he/she forgot to specify the STP priority for the core switch and some poor 2950 switch (with lower MAC address than the 6509) is now the root bridge for VLAN10. Simply schedule for an outage and set the appropriate priority on the 6509. Have fun.

[d-tay]

I have done it. I have configure 6509 as the root bridge for vlan 10. However, when i do a show spanning tree vlan 10 on the catalyst 2950 or 3550, its stp priority is stil 32778 whereas when i do a show spanning tree on the catalyst 6509, the root bridge is itself.

Appreciate yr feedback very much

[milan.kulik · Accepted solution]

Hi,

if you "only allow vlan1 and the user vlans (vlan 330) to go across the trunk port" the VLAN10 BPDUs are not sent from 6509 to 3550 (they would be sent via VLAN10 but it's prohibited on the trunk).

So 3550 doesn't know there is 6509 with better STP priority in VLAN10 (VLAN10 is discontigous, in fact). So it seems there is another root with a default priority in the VLAN10 second part including 3550.

You can test it: allow VLAN10 on the trunk and you'll see there will be 6509 with priority 8192 detected as VLAN10 root on your 3550.

I don't think it should have anything common with CPU load unless your 2950 is old - I've noticed some STP problems with 2950 running an old IOS, so I would upgrade it to the newest one anyway.

Regards,

Milan

[d-tay]

Hi Milan,

Thanks for yr feedback.