Cisco WS-C2960-24LC-S Software Configuration Manual page 429

Chapter 22 Configuring UDLD
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.
Figure 22-1
Figure 22-1
Switch B successfully
receives traffic from
Switch A on this port.
OL-8603-04
shows an example of a unidirectional link condition.
UDLD Detection of a Unidirectional Link
TX
TX
Switch A
RX
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.
RX
Switch B
Catalyst 2960 Switch Software Configuration Guide
Understanding UDLD
22-3