Troubleshooting the Spanning Tree Protocol

Symptoms of STP failure and consequent switching loops include:

  • Unusually high backplane utilization due to forwarding frames at line speed.
  • Rapid address re-learning, as the loop will allow a switch to see the same source address entering on multiple ports.
  • Rapidly incrementing frame counters on the affected ports as they receive frames at line speed.
  • Extremely poor link performance, high latency on ping replies, and TCP timeouts, as this traffic has to compete with broadcast frames caught in the loop.
  • Broadcast storms within a Layer 2 domain. Since broadcasts will be perpetuated by a loop, their effect will be felt throughout the switched network.

How is it possible for the switch to stop receiving BPDUs while the port is up? The most obvious answer is that the STP has been turned off. Another common cause is the unidirectional link. A link is considered unidirectional when:

  • Links are up on both sides of the connection.
  • Local side is not receiving the packets sent by remote side.
  • Remote side receives packets sent by local side.

Consider the scenario in Figure , the arrows indicate the flow of STP BPDUs.

During normal operation, bridge B is designated on the link B-C. Bridge B sends BPDUs down to C, which is blocking the port. The port is blocked while C is seeing BPDUs from B on that link.

Now, consider what happens if the link B-C fails in the direction of C. C will stop receiving traffic from B, but B will still receive traffic from C.

C will stop receiving BPDUs on the link B-C, and will age the information received with the last BPDU. This will take up to 20 seconds, depending on the maxAge STP timer. Once the STP information is aged out on the port, it will transition from blocking state to listening, learning, and eventually to forwarding STP state. This will create a forwarding loop, as there will be no blocking port in the triangle A-B-C. Packets will cycle along the path (B still receives packets from C) taking more and more bandwidth until the links will be filled up completely bringing the network down.

Lab Activity

Lab Exercise: Troubleshooting Problems at the Physical and Data Link Layers

After completing this lab, the student will be able to follow a logical troubleshooting process to define, isolate, and correct problems outlined in a trouble ticket.

Spanning Tree Protocol