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Yesterday we added a few VLANs (218-220) to one of our core-switches (WS-C3560X-24, c3560e-universalk9-mz.122-55.SE3 running as vtp-server) and noticed that a configuration directive no spanning-tree vlan 218-220 was automatically inserted into the switches runnig-configuration.

We are working with these switches for years now and such a directive was never automatically inserted on vlan creation. The VLANs were created by issuing the vlan 218 and respective commands.

What caused this directive to be added to the switches config?

We didn't upgrade the switches firmware recently.

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Cisco switches do this automatically for new VLANs when you have exceeded the platform's maximum spanning-tree instance limit. With Cisco PVST+, the switch will run one spanning-tree instance on each VLAN on each active interface.

If you do a show spanning-tree summary at the CLI, you will likely find that the STP Active count will be at or near 128 or 256 for the platforms you mention (depending on platform, older platforms may be lower).

To reduce the number of active spanning-tree instances, you have a several options:

  1. If your switches are all Cisco and running in VTP server or client mode, then you can enable VLAN pruning on the VTP server(s) with the vtp pruningcommand. This will prune unnecessary VLANs from your trunk links automatically.
  2. If any of your switches are in VTP transparent mode or you use multiple vendors, you may need to manually prune unnecessary VLANs by using switchport trunk allowed vlan <VLAN list> command to the trunk interfaces.
  3. Reduce the number of active trunk ports or access ports. Trunk ports will make the largest difference. For instance, if you have multiple trunk ports between the same two switches, converting them to a single link aggregation group will reduce the number of logical connections.
  4. Remove unnecessary VLANs. Maybe you have some old unused VLANs that are no longer needed? Remove them.
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  • That's it. Thanks YLearn. Just for reference: bst.cloudapps.cisco.com/bugsearch/bug/CSCdy52706 – Guybrush Threepwood Sep 25 '17 at 11:03
  • @GuybrushThreepwood, Cisco used to have this more clearly documented in several of their documents, but the ones I find now don't reference this operation. Seems they buried this (intentionally or unintentionally) through the process of time. Yes the bug you reference does mention this type of operation, but the focus of the bug was removing the no spanning-tree vlan statement when removing VLANs which the 3550 didn't do (and shouldn't have as this is expected operation). – YLearn Sep 25 '17 at 19:55
  • Thanks YLearn, I am aware of this. I just didn't find a better reference; so thank you again for your information on the Cisco docs situation. – Guybrush Threepwood Sep 26 '17 at 13:57
  • I'm wondering how many people have run into this same problem before us.. a little hint on the IOS CLI would have saved us from a meltdown. What I learned (except from this 128 STP-instances-limitation): It's better to run MSTP in environments that deal with 100+ VLANs. – Guybrush Threepwood Sep 26 '17 at 14:24
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In my opinion, it could be a IOS bug - a "comestic" issue. There is a similar issue reported though not on C3560X but on Nexus model at here.

You can issue show spanning-tree vlan 218 and other vlans across your network to verify their spanning-tree state, then contact Cisco TAC for further advice. An IOS upgrade may solve this.

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  • This bug doesn't apply to the problem at hand. It is a cosmetic bug when you have a lot of VLANs with spanning tree disabled and some show up in the configuration as enabled. It is not about spanning-tree being disabled automatically on new VLANs, which is part of normal operation when you exceed a platforms spanning-tree instance limit. – YLearn Sep 22 '17 at 18:52

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