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.