Reference Guide

UDLD 411
• UDLD receives UDLD messages from neighboring devices. It caches these
messages until the expiration time (3 times message time) has passed. If a
new message is received before the expiration time, the information in
that message replaces the previous one.
• When the expiration time expires, the device does the following with the
information received:
–
If the neighbor message contains the local device ID —The link
status of the port is set to bidirectional.
–
If the neighbor message does not contain the local device ID —The
link status of the port is set to unidirectional, and the port is shut
down.
• If UDLD messages are not received from a neighboring device during the
expiration time frame, the link status of the port is sent to undetermined
and the following occurs:
–
Device is in normal UDLD mode: A notification is issued.
–
Device is in aggressive UDLD mode. The port is shut down.
While the interface is in the bidirectional or the undetermined state, the
device periodically sends a message each message time seconds. The above
steps are performed over and over.
A port that was shut down can be reactivated manually in the Port
Management > Error Recovery Settings page. For more information, see
Reactivating a Shutdown Port .
If an interface is down and UDLD is enabled, the device removes all neighbor
information and sends at least one ULDL message to the neighbors
informing them that the port is down. When the port is brought up, the
UDLD state is changed to detection.
UDLD Not Supported or is Disabled on a Neighbor
If UDLD is not supported or disabled on a neighbor, then no UDLD
messages are received from that neighbor. In this case, the device cannot
determine whether the link is unidirectional or bidirectional. The status of
the interface is then set to undetermined. The actions taken by the device
depend on whether the UDLD mode is normal or aggressive.