Switch A Broadcast Switch B Switch C 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. Spanning 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 disabled spanning tree, which is enabled by default, to demonstrate the power of a broadcast storm. Both ports are trunks. There are various devices on other ports on the switches, which create normal broadcasts (such as ARP and DHCP broadcasts). There is nothing unusual about this network, aside from spanning tree being disabled. F0/14 F0/14 F0/15 F0/15 Switch A Switch B Figure 8-2. Simple Layer-2 loop Interface F0/15 has already been configured and is operating properly. The output from the show interface fo/15 command shows the input and output rates to be very low (both are around 1,000 bits per second and