Specifications

CHAPTER 3
Spanning Tree
To enable Loop Guard on a specific interface, use the following:
(config-if)# spanning-tree guard loop
Loop Guard automatically reenables the port if it starts receiving BPDUs again.
Troubleshooting STP
Some common things to look for when troubleshooting Spanning Tree Protocol include:
n Duplex mismatch: When one side of a link is half-duplex and the other is full-duplex. This causes late collisions
and FCS errors.
n Unidirectional link failure: The link is up but data flows only in one direction. It can cause loops.
n 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.
n Resource errors: STP is implemented in software, so a switch with an overloaded CPU or memory might neglect
some STP duties.
n Port Fast configuration errors: Connecting a switch to two ports that have Port Fast enabled. This can cause a
loop.
n 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.
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CCNP SWITCH 642-813 Quick Reference by Denise Donohue
9781587140112.qxd 11/23/09 11:35 AM Page 46
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