udld (configuration mode)

To configure the Unidirectional Link Detection (UDLD) protocol on the switch, use the udld command. To disable UDLD, use the no form of this command.

udld {aggressive | message-time timer-time | reset}

no udld {aggressive | message-time | reset}

Syntax Description


Enables UDLD in aggressive mode on the switch.

message-time timer-time

Sets the period of time between UDLD probe messages on ports that are in advertisement mode and are currently determined to be bidirectional. The range is from 7 to 90 seconds. The default is 15 seconds.


Resets all the ports that are shut down by UDLD and permit traffic to begin passing through them again. Other features, such as spanning tree, will behave normally if enabled.

Command Default


Command Modes

Global configuration mode

Command History



This command was introduced.

Usage Guidelines

UDLD aggressive mode is disabled by default. You can configure UDLD aggressive mode only on point-to-point links between network devices that support UDLD aggressive mode. If UDLD aggressive mode is enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving UDLD frames, UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is disabled.

To prevent spanning tree loops, normal UDLD with the default interval of 15 seconds is fast enough to shut down a unidirectional link before a blocking port transitions to the forwarding state (with default spanning tree parameters).

When you enable the UDLD aggressive mode, the following occurs:

•One side of a link has a port stuck (both transmission and receive)

•One side of a link remains up while the other side of the link is down

In these cases, the UDLD aggressive mode disables one of the ports on the link, which prevents traffic from being discarded.


This example shows how to enable the aggressive UDLD mode for the switch:

switch# configure terminal switch(config)# udld aggressive 

This example shows how to reset all ports that were shut down by UDLD:

switch# configure terminal switch(config)# udld reset 

Related Commands


show udld

Displays the administrative and operational UDLD status.