Configuring UDLD Verify before forwarding

When an UDLD enabled port transitions to link-up, the port will begin with a UDLD blocking state. UDLD will probe via protocol packet exchange to determine the bidirectional state of the link. Until UDLD has completed the probe, all data traffic will be blocked. If the link is found to be bidirectional, UDLD will unblock the port for data traffic to pass. Once UDLD unblocks the port, other protocols will see the port as up and data traffic can be safely forwarded.

The default mode of a switch is “forward first then verify’’. Enabling UDLD link-up will default to “forward first then verify”. To change the mode to “verify then forward”, you need to configure using the commands found in section 6.72.


Link-UP data traffic will resumed after probing the link partner completes. All other protocols running will see the port as down.