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| 4.3 |  | Troubleshooting Switched Ethernet Networks |
| 4.3.2 |  | Troubleshooting STP loops |

Once an STP loop has been identified it is important to isolate the reason behind the loop. One of the first things to check is if the Spanning Tree Protocol is running on each of the switches. A switch should only have STP disabled if it is not part of a physically looped topology. Unless there is a very good reason for turning STP off, every switch should have STP enabled.

The IOS command to verify STP operation is:

> Switch#show spanning-tree

It will be clear from the output of the show spanning-tree command if STP is not running.

It is important to remember that spanning tree operates on a per-vlan basis by default on Cisco devices (PVST/PVST+) and executing a show spanning-tree command will report on VLAN 1 only. Use the show spanning-tree [vlan ID] command to verify STP operations for other VLANs.

Should STP not be operating, it can be enabled using the spanning-tree vlan ID command.