 multiple segments within a single collision domain. Hubs and switches are both considered bridges. While the spanning tree documentation always refers to bridges generically, my examples will show switches. Switches are the devices in which you will encounter spanning tree. When a switch receives a broadcast, it repeats the broadcast on every port (except the one on which it was received). In a looped environment, the broadcasts are repeated forever. The result is called a broadcast storm, and it will quickly bring a network to a halt. Figure 8-1 illustrates what can happen when there's a loop in a network. Switch A Broadcast Switch B Switch Figure 8-1. Broadcast storm The computer on Switch A sends out a broadcast frame. Switch A then sends a copy of the broadcast to Switch B and Switch C. Switch B repeats the broadcast to Switch C, and Switch C repeats the broadcast to Switch B; Switch B and Switch C also repeat the broadcast back to Switch A. Switch A then repeats the broadcast it heard from Switch B to Switch C-and the broadcast it heard from Switch C-to Switch B. This progression will continue indefinitely until the loop is somehow broken. Span- ning tree is an automated mechanism used to discover and break loops of this kind. Spanning tree was developed by Dr. Radia Perlman of Sun Microsys- items, Inc., who summed up the idea in a poem titled "Algorhyme" that's based on Joyce Kilmer's "Trees": I think that I shall never see A graph more lovely than a tree. A tree whose crucial property Is loop-free connectivity. A tree which must be sure to span. So packets can reach every LAN. First the Root must be selected By ID it is elected. Least cost paths from Root are traced In the tree these paths are placed. A mesh is made by folks like me Then bridges find a spanning tree. Broadcast Storms In the network shown in Figure 8-2, there's a simple loop between two switches. Switch A and Switch B are connected to each other with two links: F0/14 and F0/15 on Switch A are connected to the same ports on Switch B. I've d