Re:Networks are fragile. (Score 2, Informative)

Networks can be fragile and spanning tree can certainly cause some of the problems. That is why one must design the spanning tree topology. When you say "one switch declares itself a server of a given protocol", I assume you mean "declares itself the root of a VLAN." The root is determined by the lowest advertised bridge ID from each switch. The bridge ID is the bridge priority plus the bridge address. Cisco switches have a default bridge priority. So then it boils down to whichever switch has the lowest bridge address becomes the root, which could be any switch anywhere in your network. The network admin should decide which switch will be the root for a given VLAN and set the bridge priority lower. And then he/she selects another switch to be a backup root and sets its priority to be lower than the default but higher than the root's priority. So you if don't manually set the root then a new switch plugged into the network could very well become the root if all the switches have a default priority and the new switch has a lower bridge address than the current root.

If this happens, you can just turn off the offender to get your root back. In STP only the root talks. If the other switches don't hear from the root in something like 20 seconds, then they'll elect a new root.

-Kary