Cisco Small Business 200 Series Administration Manual page 137

Port Management: Unidirectional Link Detection
UDLD Operation
How UDLD Works
When UDLD is enabled on a port, the following actions are performed:
• UDLD initiates the detection state on the port.
In this state, UDLD periodically sends messages on every active interface to all neighbors. These
messages contain the device ID of all known neighbors. It sends these messages according to a
user-defined message time.
• 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
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.
Cisco Small Business 200 Series Smart Switch Administration Guide
Reactivating a Shutdown Port.
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