U Commands

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

aggressive

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.

reset

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

Disabled

Command Modes

Global configuration mode

Command History

Release

Modification

4.0(1a)N1(1)

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.

Examples

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

Command

Description

show udld

Displays the administrative and operational UDLD status.

udld (Ethernet)

To enable and configure the Unidirectional Link Detection (UDLD) protocol on an Ethernet interface, use the udld command. To disable UDLD, use the no form of this command.

udld { aggressive | disable | enable }

no udld { aggressive | disable | enable }

Syntax Description

aggressive

Enables UDLD in aggressive mode on the interface.

disable

Disables UDLD on the interface.

enable

Enables UDLD in normal mode on the interface.

Command Default

None

Command Modes

Interface configuration mode

Command History

Release

Modification

4.0(1a)N1(1)

This command was introduced.

Usage Guidelines

You can configure normal or aggressive UDLD modes for an Ethernet interface. Before you can enable a UDLD mode for an interface, you must make sure that UDLD is enabled on the switch. UDLD must also be enabled on the other linked interface and its device.

To use the normal UDLD mode on a link, you must configure one of the ports for normal mode and configure the port on the other end for the normal or aggressive mode. To use the aggressive UDLD mode, you must configure both ends of the link for aggressive mode.

Examples

This example shows how to enable the normal UDLD mode for an Ethernet port: 


switch# configure terminal
switch(config)# interface ethernet 1/1
switch(config-if)# udld enable

This example shows how to enable the aggressive UDLD mode for an Ethernet port: 


switch(config-if)# udld aggressive

This example shows how to disable UDLD for an Ethernet port: 


switch(config-if)# udld disable

Related Commands

Command

Description

show udld

Displays the administrative and operational UDLD status.