Unidirectional Link - D-Link DES-3250TG User Manual

DES-3250TG Layer 2 Fast Ethernet Switch User's Guide
BPDUs) will be dropped. If the BPDUs sent from A to B are
dropped for longer than the MAX AGE, B will lose its
connection to the root (A) and will unblock its connection to C.
This will lead to a data loop.

Unidirectional Link

Unidirectional links can be caused by an undetected failure in
one side of a fiber cable, or a problem with a ports transceiver.
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
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