Doubt about SPT involvement

Unanswered Question
Jan 18th, 2010

I have configured a spanning tree root for vlan 999 named NULL and it is only having one port assigned to it. The port is shutdown. However, when i configured the spanning tree root with thepriority for this vlan, it is not showing under the "sh spanning-tree sumary" output. That means SPT is not taking place for this vlan. The surpising part is, i did also configured vlan 800 and 1000 for as the root in the same switch but it showed up.

Can anyone tell me what is the reason vlan 999 is not showing up while other showed up?

I have this problem too.
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bmcginn Mon, 01/18/2010 - 14:24

Hi There,

This may be way off track, but you can try this to see if it's right.

If you enable the VLAN on a port (ie switchport access vlan 99) that is not shutdown, and that has something connected to it (has link light), does the VLAN then show up?

I think a VLAN needs to be enabled and active on a swith before it starts worrying about STP for it.  At least that's what my recollection is, as i mentioned before i could be way off track..


karmesh3243 Mon, 01/18/2010 - 15:02


Thansk for ur reply. Yes, the vlan is up but the port is shut down. Doest it mean it doesnt particiapte in STP?

bmcginn Mon, 01/18/2010 - 15:12

Karmesh, no problems.

As Guiseppe mentioned, there needs to be an active port in that VLAN.  A switchport access vlan 999 is fine but you also need it to be active, ie interface up and line protocol is up.  Also mentioned by Guiseppe, is if a trunk port is up, and you're allowing the vlan in that trunk, then it participates in STP.


PS.  To answer you're question, if a port is in the vlan but the line protocol is not up, and there are no trunks or other active ports in that vlan, then no STP isn't active for that vlan on that switch.

Giuseppe Larosa Mon, 01/18/2010 - 14:24

Hello Karmesh,

an STP instance for vlan999 is started when at least one port is active for vlan999 either access port or trunk port that permits vlan 999.

check your configuration vlans 800 and 1000 may be permitted on a trunk port and this is enough to start STP instances for them

Hope to help


bmcginn Mon, 01/18/2010 - 14:43

Excellent, its great to get vindication.  Thanks Giuseppe for confirming my recollection


karmesh3243 Mon, 01/18/2010 - 15:01


Thanks for your remark. Yes, i was thinking about what you wuld have said but i tried enabling the port and yet i did not see any changes to te STP summary output. Does it take sometime to be shown as STP process takes a while right? Since the test was done in a production network, we cant wait quite long to see the output. Just to clarifiy, have you tested this in a lab?


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