ot bridge. Switch B is only receiving BPDUs from the root bridge on one port: G0/1. Because Switch B is not receiving BPDUs from the root bridge on port G0/2, spanning tree does not block the port. Broadcasts received on Switch B on G0/1 will be retransmitted out G0/2. A loop is born. This problem can be difficult to uncover. A bridging loop causes mayhem in the network because the CPU utilization on network devices can quickly reach 100 per- cent, causing outages. The first thing inexperienced engineers try is rebooting one or more devices in the network. In the case of a unidirectional link, rebooting will not resolve the issue. When that fails, the loop is usually detected, and the engineer shuts down one of the links. But when he shuts down the link, the proof of the unidirec- tional link is often lost. Physical layer first! Always suspect that something physical is wrong when diagnosing connectivity problems. It can save you hours of headaches, especially if all the other clues don't seem to add up to any- thing substantial. Also, don't assume that it works today just because it worked yesterday. It doesn't take much for someone to crush a fiber strand when closing a cabinet door. With the latest versions of IOS and CatOS, unidirectional link problems are handled by a protocol called Unidirectional Link Detection (UDLD). UDLD is on by default and should be left on. If you see UDLD errors, look for issues similar to what I've just described. Designing to Prevent Spanning Tree Problems Proper design can help minimize spanning tree problems. One of the simplest ways to help keep trouble to a minimum is to document and know your network. If you have to figure out how your network operates when there's a problem, the problem may last longer than your job. Use Routing Instead of Switching for Redundancy The saying used to be, "Switch when you can and route when you have to." But in today's world of fast layer-3 switching, this mantra no longer holds. With layer-3 switches, you can route at switching speeds. For ma