TCN is a topology change notification in spanning tree.
What it means is that the switch you are seeing these BPDUs on is getting notification that somewhere in the spanning tree topology there was a change.
An example of when this could occur is if you dont use the portfast command on ports connected to end hosts. Then someone unplus the cable between the switch and the host to move it or something. This yeilds a TCN.
Other examples include bad cabling that can cause a link to flap, etc. Just watch where they are coming from by looking at the BPDU, note the source BID, and search the CAM/mac-address table for the switch. Also, you can do show spanning-tree and note the BID of your switch and upstream switch to see who its coming from.
Andy
