D-Link DES-3226 User Manual - Troubleshooting Stp
DES-3226 NWay Standalone 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.
There are a number of circumstances in which the STA 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.
Full/Half Duplex Mismatch
A mismatch in the duplex state of two ports is a very common configuration error for a point-to-point
link. If one port is configured as a full duplex, and the other port is left in auto-negotiation mode, the
second port will end up in half-duplex because ports configured as half- or full-duplex do not negotiate.
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