Solved: Could not enable spaning tree on some VLAN

fromthesky · ‎09-15-2008

Hi,

an our custumer as an old C2900XL switch, with IOS 12.0(5.2)XU installed on it. The switch is in VTP client mode.

I noticed that on this switch the STP has been disabled for some VLANs (don't ask me why, it was configured before my company started to manage it), with the command "no spanning-tree vlan X".

I tried to re-enable the spanning tree for all the VLANs in various mode but the STP still remains disabled:

- with the command "spanning-tree vlan X"

- trying to copy a new startup configuration file without the lines "no spanning-tree" and rebooting the switch, but it doesn't worked. I still see those lines in the running config.

- trying to re-enable the STP from the vlan database configuration mode, but I didn't find a command to do that.

- putting the switch in VTP server mode an issuing the command "spanning-tree vlan X", still the same

Do you have any suggestion?

Thanks,

Matteo

Giuseppe Larosa · ‎09-15-2008

Hello Matteo,

a switch like that has a limitation in the maximum number of STP instances that is probably 64.

Another limit applies to vlans created and propagated by VTP and probably is 128.

So if the vlans in your network are in a number between 64 and 128 the switch is able to participate in VTP but vlans exceeding the limit on STP instances will have STP disabled.

If the vlan number becomes higher then the VTP limit the switch reverts to VTP transparent mode I've seen this during tests on 2950.

Hope to help

Giuseppe

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mahmoodmkl · ‎09-15-2008

HI

Is this switch connected to another switch.

Do u see all the vlans which are in the vlan database of all other switches.

Thanks

Mahmood

fromthesky · ‎09-15-2008

Yes to both the questions.

Hi,

Matteo

Giuseppe Larosa · ‎09-15-2008

Hello Matteo,

a switch like that has a limitation in the maximum number of STP instances that is probably 64.

Another limit applies to vlans created and propagated by VTP and probably is 128.

So if the vlans in your network are in a number between 64 and 128 the switch is able to participate in VTP but vlans exceeding the limit on STP instances will have STP disabled.

If the vlan number becomes higher then the VTP limit the switch reverts to VTP transparent mode I've seen this during tests on 2950.

Hope to help

Giuseppe

fromthesky · ‎10-13-2008

Hi Giuseppe,

effectively some months ago I noticed that switch was in transparent mode and I reduced the number of VLANs to bring it back in client mode. However I didn't thought to a similar limitation in STP, surely it's cause of the problem.

Our customer however decided to replace the switches because they're end-of-life.

Thanks,

Matteo

Giuseppe Larosa · ‎10-13-2008

Hello Matteo,

switch replacement is a solution.

But higher STP limits still exist for devices like C2960 up to C3550, so I would suggest to track the number of vlans used in the campus network.

Best Regards

Giuseppe

fromthesky · ‎10-13-2008

Yes, we will also recalculate which Vlans are realy needed before the replacement, and I think that most of the Vlans will be deleted.

Thanks,

Matteo

Jon Marshall · ‎10-13-2008

Matteo

Just one additional point. If you are replacing your switches make sure the new hardware supports MST so that in future if you do exceed the number of STP instance supported with PVST+ then you can utilize MST.

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml

Jon

fromthesky · ‎10-13-2008

Very usefull, thanks.

Matteo

glen.grant · ‎09-15-2008

You can usually get around this by manually pruning off vlans not needed to be allowed across the trunk link . Only allow the vlans needed on the switch itself, "switchport trunk allowed vlan X " . That being so old I don't know if that is the correct syntax on those or not , it is for a 2950 .