Catalyst 6500 Release 12.2SY Software Configuration Guide
UniDirectional Link Detection (UDLD)
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UniDirectional Link Detection ( UDLD)

Table Of Contents

UniDirectional Link Detection ( UDLD)

Prerequisites for UDLD

Restrictions for UDLD

Information About UDLD

UDLD Overview

UDLD Aggressive Mode

Default Settings for UDLD

How to Configure UDLD

Enabling UDLD Globally

Enabling UDLD on LAN Interfaces

Disabling UDLD on Nonfiber-Optic LAN Interfaces

Disabling UDLD on Fiber-Optic LAN Interfaces

Configuring the UDLD Probe Message Interval

Resetting Disabled LAN Interfaces


UniDirectional Link Detection ( UDLD)


Prerequisites for UDLD

Restrictions for UDLD

Information About UDLD

Default Settings for UDLD

How to Configure UDLD


NoteFor complete syntax and usage information for the commands used in this chapter, see these publications:

http://www.cisco.com/en/US/products/ps9536/prod_command_reference_list.html

Cisco IOS Release 12.2SY supports only Ethernet interfaces. Cisco IOS Release 12.2SY does not support any WAN features or commands.



Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page:

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

Participate in the Technical Documentation Ideas forum


Prerequisites for UDLD

None.

Restrictions for UDLD

None.

Information About UDLD

UDLD Overview

UDLD Aggressive Mode

UDLD Overview

The Cisco-proprietary UDLD protocol allows devices connected through fiber-optic or copper (for example, Category 5 cabling) Ethernet cables connected to LAN ports to monitor the physical configuration of the cables and detect when a unidirectional link exists. When a unidirectional link is detected, UDLD shuts down the affected LAN port and alerts the user. Unidirectional links can cause a variety of problems, including spanning tree topology loops.

UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected LAN ports. When you enable both autonegotiation and UDLD, Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.

A unidirectional link occurs whenever traffic transmitted by the local device over a link is received by the neighbor but traffic transmitted from the neighbor is not received by the local device. If one of the fiber strands in a pair is disconnected, as long as autonegotiation is active, the link does not stay up. In this case, the logical link is undetermined, and UDLD does not take any action. If both fibers are working normally at Layer 1, then UDLD at Layer 2 determines whether those fibers are connected correctly and whether traffic is flowing bidirectionally between the correct neighbors. This check cannot be performed by autonegotiation, because autonegotiation operates at Layer 1.

LAN ports with UDLD enabled periodically transmit UDLD packets to neighbor devices. If the packets are echoed back within a specific time frame and they are lacking a specific acknowledgment (echo), the link is flagged as unidirectional and the LAN port is shut down. Devices on both ends of the link must support UDLD in order for the protocol to successfully identify and disable unidirectional links.


Note By default, UDLD is locally disabled on copper LAN ports to avoid sending unnecessary control traffic on this type of media since it is often used for access ports.


Figure 11-1 shows an example of a unidirectional link condition. Switch B successfully receives traffic from Switch A on the port. However, Switch A does not receive traffic from Switch B on the same port. UDLD detects the problem and disables the port.

Figure 11-1 Unidirectional Link

UDLD Aggressive Mode

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

To prevent spanning tree loops, nonaggressive 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 UDLD aggressive mode, you receive additional benefits in the following situations:

One side of a link has a port stuck (both Tx and Rx)

One side of a link remains up while the other side of the link has gone down

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


Note In UDLD normal mode, when a unidirectional error is detected, the port is not disabled. In UDLD aggressive mode, when a unidirectional error is detected, the port is disabled.


Default Settings for UDLD

Feature
Default Value

UDLD global enable state

Globally disabled.

UDLD aggressive mode

Disabled.

UDLD per-port enable state for fiber-optic media

Enabled on all Ethernet fiber-optic LAN ports.

UDLD per-port enable state for twisted-pair (copper) media

Disabled on all Ethernet 10/100 and 1000BASE-TX LAN ports.


How to Configure UDLD

Enabling UDLD Globally

Enabling UDLD on LAN Interfaces

Disabling UDLD on Nonfiber-Optic LAN Interfaces

Disabling UDLD on Fiber-Optic LAN Interfaces

Configuring the UDLD Probe Message Interval

Resetting Disabled LAN Interfaces

Enabling UDLD Globally

To enable UDLD globally on all fiber-optic LAN ports, perform this task:

Command
Purpose

Router(config)# udld {enable | aggressive}

Enables UDLD globally on fiber-optic LAN ports.

Note This command only configures fiber-optic LAN ports. Individual LAN port configuration overrides the setting of this command.


Enabling UDLD on LAN Interfaces

To enable UDLD on a LAN port, perform this task:

 
Command
Purpose

Step 1 

Router(config)# interface type slot/port

Selects the LAN port to configure.

Step 2 

Router(config-if)# udld port [aggressive]

Enables UDLD on a LAN port.

Enter the aggressive keyword to enable aggressive mode.

On a fiber-optic LAN port, this command overrides the udld enable global configuration command setting.

On fiber-optic LAN ports, the no udld port command reverts the LAN port configuration to the udld enable global configuration command setting.

Disabling UDLD on Nonfiber-Optic LAN Interfaces

To disable UDLD on a nonfiber-optic LAN port., perform this task:

 
Command
Purpose

Step 1 

Router(config)# interface type slot/port

Selects the LAN port to configure.

Step 2 

Router(config-if)# no udld port [aggressive]

Disables UDLD on a nonfiber-optic LAN port.

Disabling UDLD on Fiber-Optic LAN Interfaces

To disable UDLD on individual fiber-optic LAN ports, perform this task:

Command
Purpose

Router(config)# interface type slot/port

Selects the LAN port to configure.

Router(config-if)# udld port disable

Disables UDLD on a fiber-optic LAN port.

Note The no form of this command, which reverts to the udld enable global configuration command setting, is only supported on fiber-optic LAN ports.

Configuring the UDLD Probe Message Interval

To configure the time between UDLD probe messages on ports that are in advertisement mode and are currently determined to be bidirectional, perform this task:

Command
Purpose

Router(config)# udld message time interval

Configures the time between UDLD probe messages on ports that are in advertisement mode and are currently determined to be bidirectional; valid values are from 7 to 90 seconds.

Resetting Disabled LAN Interfaces

To reset all LAN ports that have been shut down by UDLD, perform this task:

Command
Purpose

Router# udld reset

Resets all LAN ports that have been shut down by UDLD.



Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page:

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

Participate in the Technical Documentation Ideas forum