Cisco Systems 3130 User Manual

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. 

 shows an example of a unidirectional link condition.

These sections contain this configuration information:

TX

TX

RX

RX

Switch A

Switch B

Switch B successfully

receives traffic from

Switch A on this port.

98648

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.