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We are operating several Cisco 2960S stacks here and, for some time, are experiencing regular Spanning Tree TCNs (Topology Change Notifications).

When using show spanning-tree detail to get to the source of the TCN, they originate from different 2960S stack members.

As the config has been transferred from older switches, the ports are configured as trunks, with the data VLAN being the native VLAN and the VoIP VLANs being included as tagged in the trunk. I know, today the correct configuration would be switchport mode access combined with switchport voice vlan - but at the moment it is at it is.

Part of the problem was based on the fact that some ports were only configured as portfast, not portfast trunk, so this has to be fixed, as the portfast only command is not applied to ports in trunk mode. However, some ports are configured as portfast trunk, which as of my understanding of Cisco documentation, should avoid generating TCNs on link changes.

Sadly, they do.

Executing show spanning-tree interface Gi1/0/x portfast shows portfast enabled for all VLANs on that interface, but show spanning-tree detail together with show logging shows that these interfaces are indeed the source for the Spanning Tree changes.

It is made sure that there are no "special" devices connected to those ports, just VoIP phones and desktop workstations.

The version run is 12.2(55)SE3, the devices are 2960S-48LPS-L and 2960S-48FPS-L.

Looking for known bugs, there was a bug in a 12.1 release for 3550 switches that caused such behaviour, but this is the wrong device and a much older version, and I guess somebody should have experienced that bug before.

Any idea where to look, what to try?

  • You need to tell the switch that the port is an edge port "spanning-tree portfast edge trunk" in the interface configuration will do that with more modern IOS but I'm not sure which version that first appeared. So not only does that say that it is a trunk, it says that it is an edge port. Just be absolutely sure not to plug a port configured as an edge trunk into a switch in your network. – GeorgeB Jun 12 '15 at 4:51
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This post is older in age so I'm not sure if you are still experiencing this problem. I would like to see the documentation you are referring too regarding the portfast trunk command. The portfast trunk configuration should be applied to hosts that you are trunking with, for example an ESX host or a Load Balancer. For switch to switch, this configuration should not be there.

There is a lot of information I would need to see to better understand the topology and what spanning-tree mode you are running in (PVST+, RPVST+ or MST). I suspect from what I've read you are running PVST+. I would like to first start with the output of show spanning-tree vlan x detail and a snippet of this TCN you are seeing. I would also like to see show run | i spanning.

I suspect you may have a port(s) that is flapping and the TCN message should lead us directly to the culprit.

So you have a VoIP phone plugged into a switchport that is configured switchport access vlan and computers/hosts plugged into other switchports with switchport access vlan ? Yeah you know you can set it up for switchport voice vlan. However being setup without the switchport voice vlan, I do not see this being an issue with TCN's.

TCN's are generated when a port transitions from one state to another assuming you are not changing the priority values.

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Although this question is rather old, someone may find it while googling the very same problem, so here comes our "solution":

We couldn't figure out the source of the problem, as the problem was non-reproducable and only happened about once a day on varying access-ports, with about 500 access-ports with the exactly same config in use.

After updating all 2960 to a release from the 15.x line, the problems disappared. The config didn't change, so it seems to have been silently fixed in one of the releases in between.

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