24-port NWay Ethernet Switch User’s Guide
Understanding and Troubleshooting the Spanning Tree Protocol
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In the above example, port 1 on B is configured as a full-duplex
port and port 1 on A is either configured as a half-duplex port, or
left in auto-negotiation mode. Because port 1 on B is configured
as a full-duplex port, it does not do the carrier sense when
accessing the link. B will then start sending packets even if A is
using the link. A will then detect collisions and begin to run the
flow control algorithm. If there is enough traffic between B and
A, all packets (including 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.