D-Link DES-3326S User Manual page 333

DES-3326S Layer 3 Fast Ethernet Switch User's Guide
Any failure that allows a link to remain up while providing one-
way communication is very dangerous for STP.
In this example, port 2 on B can receive but not transmit
packets. Port 2 on C should be in the blocking state, but since
it can no longer receive BPDUs from port 2 on B, it will
transition to the forwarding state. If the failure exists at boot,
STP will not converge and rebooting the bridges will have no
effect. (Note: Rebooting would help temporarily in the
previous example).
This type of failure is difficult to detect because the Link-state
LEDs for Ethernet links rely on the transmit side of the cable
to detect a link. If a unidirectional failure on a link is
suspected, it is usually required to go to the console or other
management software and look at the packets received and
transmitted for the port. A unidirectional port will have many
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Understanding and Troubleshooting the Spanning Tree Protocol