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Spanning Tree Protocol
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.
Figure D-8: Example unidirectional link
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.
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. For example, a unidirectional port will have
many packets transmitted but none received, or vice versa.
HP ProLiant BL e-Class C-GbE Interconnect Switch User Guide D-11
HP CONFIDENTIAL Codename: DeLorean Part Number: 263682-002 Last Saved On: 2/5/03 10:46 AM