UDLD means Unidirectional Link Detection. it is mostly used on fiber links, but it is can also used on copper, to monitor the physical configuration of the cables and detect when a unidirectional condition exists,otherwise known as one way traffic.
Both sides of a connection must support udld for the protocol to effectively detect when a link goes unidirectional. when found to be unidirectional by aggressive mode the port is disabled and an alert is sent. in normal mode it is considered undetermined:
unless, for both types of media, one of the ports cannot send or receive
or one port is down and the other is up,
and in the case of fiber alone, one of the strands is disconnected
udld will disable the port
firstly, udld is globally disabled.
if enabled globally with udld enable (normal mode) or udld aggressive it is enabled on all fiber ports. for copper ports, use the interface configuration command “udld port for normal mode and udld port aggressive, you guessed it, for aggressive mode.