We had some serious network connection problems caused by spanning tree topology changes. Our root switch is set with the lowest STP priority and this root switch also never changes, so this setting is right.
The exact problem was that when a client connects to a client access switch, the client access switch generates a TCN which is flooded around the network.
This was only happening to one switch. The problem on this switch seems to be portfast: when I enable portfast on the client ports, I can plug-in clients and printers safely, and when I disable portfast again. Tcn's are flooded on the network and all the clients in the whole network will lose their connection for 5 to 10 seconds.
I think this is strange behavior. Although it's a best practice to enable portfast for edge ports, it should never cause any network problem, the only disadvantage should be a longer STP negotiation shouldn’t it?