Spanning Tree Protocol
Troubleshooting STP
This section describes several troubleshooting tips.
Spanning Tree Protocol Failure
A failure in the STP generally leads to a bridging loop. A bridging loop in an STP environment comes from a port that should be in the blocking state, but is forwarding packets.
Figure
In this example, B has been elected as the designated bridge, and Port 2 on C is in the blocking state. The election of B as the designated bridge is determined by the exchange of BPDUs between B and C. B had a better BPDU than C. B continues sending BPDUs advertising its superiority over the other bridges on this LAN. Should C fail to receive these BPDUs for longer than the max age (default of 20 seconds), it could start to transition its Port 2 from the blocking state to the forwarding state.
NOTE: A port must continue to receive BPDUs advertising superior paths to remain in the blocking state.
There are a number of circumstances in which STP can fail, mostly related to the loss of a large number of BPDUs. These situations will cause a port in the blocking state to transition to the forwarding state.
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