Troubleshooting Spanning Tree Issues in Campus LANs

July 2000 · Reading time: 4 min

Spanning Tree Protocol (STP) plays a crucial role in preventing loops in Layer 2 networks. However, when things go wrong, it can cause widespread disruption across campus LANs. Troubleshooting these problems requires not only a sound understanding of how STP operates, but also a practical approach to isolating issues as they arise.

Understanding the Symptoms

One of the first signs of STP trouble is intermittent connectivity or broadcast storms. These often stem from a failure in topology change propagation or a switch mistakenly taking on the root role due to misconfiguration or link issues.

Basic Steps for Troubleshooting

• Use show spanning-tree to verify the root bridge and port roles.
• Check for topology changes using debug spanning-tree events.
• Ensure portfast is not enabled on uplinks.
• Validate consistency of bridge priorities across the network.
• Use show cdp neighbors to verify physical topology against logical STP view.

Common Misconfigurations

In the mid/late 90s, it was common to find legacy switches in the access layer with default STP priorities, allowing them to compete for root bridge status. Adjusting bridge priorities and enforcing Root Guard on access-facing ports helps protect the STP topology.

Lessons from the Field

In one case, a newly added switch in a student lab area began acting as the STP root after a power cycle. The cause was a default priority and faster boot time. Once the lab switch was pre-configured with a higher bridge priority and Root Guard was enabled on the uplink, stability returned.

 

Eduardo Wnorowski is a network engineer focused on Cisco-based campus designs and troubleshooting. He writes about practical networking challenges from the trenches.