Troubleshooting STP

Some common things to look for when troubleshooting Spanning Tree Protocol include:

• Duplex mismatch—When one side of a link is half-duplex and the other is full-duplex. This causes late collisions and FCS errors.

• Unidirectional link failure—The link is up but data flows only in one direction. It can cause loops.

• Frame corruption—Physical errors on the line cause BPDUs to be lost, and the port incorrectly begins forwarding. This is caused by duplex mismatch, bad cable, or cable too long.

• Resource errors—STP is implemented in software, so a switch with an overloaded CPU or memory might neglect some STP duties.

• Port Fast configuration errors—Connecting a switch to two ports that have Port Fast enabled. This can cause a loop.

• STP tuning errors—Max age or forward delay set too short can cause a loop. A network diameter that is set too low causes BPDUs to be discarded and affects STP convergence.

Identifying a Bridging Loop

Suspect a loop if you see the following:

• You capture traffic on a link, and see the same frames multiple times.

• All users in a bridging domain have connectivity problems at the same time.

• There is abnormally high port utilization.

To remedy a loop quickly, shut redundant ports and then enable them one at a time. Some switches allow debugging of STP (not 3550/2950) to help in diagnosing problems.


What to Use Where

Confused by all the acronyms and STP features? Figure 3-3 shows the STP features you might use in your network and where you might use them.