Unidirectional Link Detection (UDLD)
A switch notices when a physical connection is broken by the absence of
Layer 1 electrical keepalives (Ethernet calls this a link beat). However, sometimes
a cable is intact enough to maintain keepalives, but not to pass data in
both directions. This is a Unidirectional Link. Unidirectional Link Detection
(UDLD) detects a unidirectional link by sending periodic hellos out to the
interface. It also uses probes, which must be acknowledged by the device on
the other end of the link. UDLD operates at Layer 2. The port is shut down if
a unidirectional link is found.
To enable UDLD on all fiber-optic interfaces, use the following command:
(config)# udld enable
Although this command is given at global config mode, it applies only to
fiber ports.
To enable UDLD on non-fiber ports, give the same command at interface
config mode.
To disable UDLD on a specific fiber port, use the following command:
(config-if)# udld disable
To disable UDLD on a specific non-fiber port, use the following command:
(config-if)#no udld enable
To re-enable all interfaces shut by UDLD, use the following:
#udld reset
To verify UDLD status, use the following:
#show udld interface