Common Spanning Tree Problems

Spanning tree can be a bit of a challenge when it misbehaves. More to the point, spanning tree problems can be hard to diagnose if the network is not properly designed. Here are a couple of common problems and how to avoid them.

Duplex Mismatch

A bridge still receives and processes BPDUs on ports, even when they are in a blocked state. This allows the bridge to determine that a path to the root bridge is still available should the primary path fail.

If a port in the blocking state stops receiving BPDUs, the bridge no longer considers the port to be a path to the root bridge. In this case, the port should no longer be blocked, so the bridge puts the port into the forwarding state. Would this ever happen in the real world? It’s happened to me more than once.

A common spanning tree problem is shown in Figure 8-8. Here, two switches are connected 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.

Spanning tree half-duplex problem

Figure 8-8. Spanning tree half-duplex problem

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 ...

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