 allowed to forward traffic (though they still send and receive BPDUs). Always configure a switch to be the root bridge. Letting the switches configure themselves is dangerous, because they will choose the switch with the lowest MAC address, which will usually be a switch other than the one it should be. As a general rule, you should not let networking devices make critical decisions using default values. It will cause your network to behave in unexpected ways and will cause you to fail higher- level certification exams, which are designed to catch you in exactly this way. Usually, the device that should be the root bridge will be obvious. The root bridge should generally be one of the core switches in your design. Preventing Loops with Spanning Tree Protocol ID Version Message type Flags Root priority 12123456 Root ID 123456 Root MAC address Path cost Bridge priority 12123456 Bridge ID 123456 Bridge MAC address Port priority Port ID Message age Max age Hello timer Fwd delay Figure 8-4. BPDU format Every port on a switch goes through a series of spanning tree states when it is brought online, as illustrated in the flowchart in Figure 8-5. These states transition in a pattern depending on the information learned from BPDUs received on the port. Disabled Initializing Blocking Listening Learning Forwarding Figure 8-5. Spanning tree port states These are the spanning tree states: Initializing A port in the initializing state has either just been powered on or taken out of the administratively down state. Blocking A port in the blocking state is essentially unused. It does not forward or receive frames, with the following exceptions: I Chapter 8: Spanning Tree The port receives and processes BPDUs. The port receives and responds to messages relating to network management. Listening The listening state is similar to the blocking state, except that in this state, BPDUs are sent as well as received. Frame forwarding is still not allowed, and no addresses are learned. Learning A port in the learning state still does not f