Special frames sent to well-known MAC address 01:00:0C:CC:CC:CC
If no echo frame with our ID has been seen from the peer for a certain amount of time, the port is suspected to be unidirectional.
In Normal mode, if the physical state of port (as reported by Layer 1) is still up, UDLD marks this port as Undetermined, but does NOT shut down or disable the port, which continues to operate under it’s current STP status.
If UDLD is set to Agressive mode, once the switch loses it’s neighbor it actively tries to re-establish the relationship by sending a UDLD frame 8 times every 1 second. If the neighbor does not respond after that, port is considered to be unidirectional and brought to Errdisable state.
UDLD Aggressive will only brings link to errdisable state when it detects Bidirectional to Unidirectional state transition. This prevents link from becoming errdisabled when you configure Aggressive mode just on one side. The UDLD state of such link will be Unknown