D-Link DES-3250TG User Manual page 230

DES-3250TG Layer 2 Fast Ethernet Switch User's Guide
Troubleshooting STP
Spanning Tree Protocol Failure
A failure in the STA 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.
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.
It should be noted: A port must continue to receive BPDUs
advertising superior paths to remain in the blocking state.
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