D-Link DES-3326 User Manual page 406

24-port NWay 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 packets
transmitted but none received, or vice versa, for example.
390 Understanding and Troubleshooting the Spanning Tree Protocol