on spanning tree problem is shown in Figure 8-8. Here, two switches are con- nected with two links: F0/0 on Switch A is connected to F0/0 on Switch B, and F0/1 on Switch A is connected to F0/1 on Switch B. Switch A is the root bridge. All ports are in the forwarding state, except for F0/1 on Switch B, which is blocking. The network is stable because spanning tree has broken the potential loop. The arrows show BPDUs being sent. Port F0/0 on Switch A is the only port that is set to autonegotiation. Autonegotiation has determined that the port should be set to 100 Mbps and half-duplex mode. The Chapter 8: Spanning Tree Auto (100/Half) 100/Full Forwarding Forwarding Forwarding Blocking Switch A Switch B 100/Full 100/Full Figure 8-8. Spanning tree half-duplex problem other ports are all hardcoded to 100/full. Spanning tree is sending BPDUs out of all ports and is receiving them on all ports-even the one that is blocking. Always make sure both sides of an Ethernet link are configured the same way regarding speed and duplex. See Chapter 3 for details. When a port is in half-duplex mode, it listens for collisions before transmitting. A port in full-duplex mode does not. When a half-duplex port is connected with a full-duplex port, the full-duplex port will send continuously, causing the half-duplex port to en- counter many collisions. After a collision, the port will perform the back-off algorithm, and wait to resend the packet that collided. In our example, the half-duplex port is the active link with data being sent across it. When the data rate gets high, the collision problem gets worse, resulting in frames-including BPDUs-being dropped. Switch B will listen for BPDUs over the two links shown in Figure 8-8. If it doesn't see any BPDUs over the F0/0 link for a set amount of time, Switch B will no longer consider the F0/0 link to be a valid path to the root bridge. Because this was the primary path to the root bridge, and the root bridge can no longer be seen, Switch B will change F0/1 from blocking to forwarding to re