Cisco Catalyst 3560-X Software Configuration Manual page 783

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Configuring UDLD

Event-driven detection and echoing

UDLD relies on echoing as its detection mechanism. Whenever a UDLD device learns about a new

neighbor or receives a resynchronization request from an out-of-sync neighbor, it restarts the

detection window on its side of the connection and sends echo messages in reply. Because this

behavior is the same on all UDLD neighbors, the sender of the echoes expects to receive an echo in

If the detection window ends and no valid reply message is received, the link might shut down,

depending on the UDLD mode. When UDLD is in normal mode, the link might be considered

undetermined and might not be shut down. When UDLD is in aggressive mode, the link is

considered unidirectional, and the port is disabled.

If UDLD in normal mode is in the advertisement or in the detection phase and all the neighbor cache

entries are aged out, UDLD restarts the link-up sequence to resynchronize with any potentially

out-of-sync neighbors.

If you enable aggressive mode when all the neighbors of a port have aged out either in the advertisement

or in the detection phase, UDLD restarts the link-up sequence to resynchronize with any potentially

out-of-sync neighbor. UDLD shuts down the port if, after the fast train of messages, the link state is still

undetermined.

Switch B successfully

receives traffic from

Switch A on this port.

shows an example of a unidirectional link condition.

UDLD Detection of a Unidirectional Link

However, Switch A does not receive traffic

from Switch B on the same port. If UDLD

is in aggressive mode, it detects the

problem and disables the port. If UDLD is

in normal mode, the logical link is

considered undetermined, and UDLD

does not disable the interface.

Catalyst 3750-X and 3560-X Switch Software Configuration Guide

Understanding UDLD

Troubleshooting

Catalyst 3750-x