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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. 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).
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Figure A-3. After Applying STP
VH-2402-L3 Management Guide 181
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