Configuring UDLD

This chapter describes how to configure the Unidirectional Link Detection (UDLD) protocol on the Catalyst 6000 family switches.

Note For complete syntax and usage information for the commands used in this chapter, refer to the Catalyst 6000 Family Command Reference publication.

This chapter consists of these sections:

The UDLD protocol allows devices connected through fiber-optic or twisted-pair Ethernet cables 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 port and alerts the user.

A unidirectional link occurs whenever traffic transmitted by one device over a link is received by the neighbor, but traffic from the neighbor is not received. Unidirectional links can cause a variety of problems, including spanning-tree topology loops.

UDLD packets are sent periodically to neighbor devices connected through the links. If the packets are not echoed back to the sender, the link is flagged as unidirectional and the port is shut down. All connected devices must support UDLD in order for the protocol to successfully identify and disable unidirectional links.

Figure 24-1 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 24-1: Unidirectional Link

Default UDLD Configuration

Table 24-1 shows the default UDLD configuration.

Table 24-1: UDLD Default Configuration

Feature Default Value

UDLD global enable state

Globally disabled

UDLD per-port enable state for fiber-optic media

Enabled on all Ethernet fiber-optic ports

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

Disabled on all Ethernet 10/100 and 1000BaseTX ports

Feature	Default Value
UDLD global enable state	Globally disabled
UDLD per-port enable state for fiber-optic media	Enabled on all Ethernet fiber-optic ports
UDLD per-port enable state for twisted-pair (copper) media	Disabled on all Ethernet 10/100 and 1000BaseTX ports

Configuring UDLD

These sections describe how to configure UDLD:

Enabling UDLD Globally

To enable UDLD globally on the switch, perform this task in privileged mode:

Task Command

Step 1 Enable UDLD globally on the switch.

set udld enable

Step 2 Verify the configuration.

show udld

Task	Command
Step 1 Enable UDLD globally on the switch.	set udld enable
Step 2 Verify the configuration.	show udld

This example shows how to enable UDLD globally and verify the configuration:

Console> (enable) set udld enable
UDLD enabled globally
Console> (enable) show udld
UDLD      : enabled
Console> (enable)

Enabling UDLD on Individual Ports

To enable UDLD on individual ports, perform this task in privileged mode:

Task Command

Step 1 Enable UDLD on a specific port.

set udld enable mod_num/port_num

Step 2 Verify the configuration.

show udld port [mod_num[/port_num]]

Task	Command
Step 1 Enable UDLD on a specific port.	set udld enable mod_num/port_num
Step 2 Verify the configuration.	show udld port [mod_num[/port_num]]

This example shows how to enable UDLD on port 4/1 and verify the configuration:

Console> (enable) set udld enable 4/1
UDLD enabled on port 4/1.
Console> (enable) show udld port 4/1
UDLD      : enabled
Port      Admin Status  Link State
--------  ------------  ----------------
 4/1      enabled       bidirectional
Console> (enable)

Disabling UDLD on Individual Ports

To disable UDLD on individual ports, perform this task in privileged mode:

Task Command

Step 1 Disable UDLD on a specific port.

set udld disable mod_num/port_num

Step 2 Verify the configuration.

show udld port [mod_num[/port_num]]

Task	Command
Step 1 Disable UDLD on a specific port.	set udld disable mod_num/port_num
Step 2 Verify the configuration.	show udld port [mod_num[/port_num]]

This example shows how to disable UDLD on port 4/1:

Console> (enable) set udld disable 4/1
UDLD disabled on port 4/1.
Console> (enable)

Disabling UDLD Globally

To disable UDLD globally on the switch, perform this task in privileged mode:

Task Command

Step 1 Disable UDLD globally on the switch.

set udld disable

Step 2 Verify the configuration.

show udld

Task	Command
Step 1 Disable UDLD globally on the switch.	set udld disable
Step 2 Verify the configuration.	show udld

This example shows how to disable UDLD globally on the switch:

Console> (enable) set udld disable
UDLD disabled globally
Console> (enable)

Displaying the UDLD Configuration

To display the UDLD enable state, perform this task in privileged mode:

Task Command

Display UDLD enable state.

show udld

Task	Command
Display UDLD enable state.	show udld

This example shows how to display the UDLD enable state:

Console> (enable) show udld
UDLD      : enabled
Console> (enable)

To display UDLD configuration for a module or port, perform this task in privileged mode:

Task Command

Display the UDLD configuration for a module or port.

show udld port [mod_num[/port_num]]

Task	Command
Display the UDLD configuration for a module or port.	show udld port [mod_num[/port_num]]

This example shows how to display the UDLD configuration for ports on module 4:

Console> (enable) show udld port 4
UDLD      : enabled
Port      Admin Status  Link State
--------  ------------  ----------------
 4/1      disabled      not applicable
 4/2      disabled      not applicable
 4/3      disabled      not applicable
 4/4      disabled      not applicable
 4/5      enabled       undetermined
 4/6      enabled       undetermined
 4/7      enabled       bidirectional
 4/8      enabled       bidirectional
 4/9      enabled       bidirectional
 4/10     enabled       bidirectional
 4/11     enabled       shutdown
 4/12     enabled       shutdown
Console> (enable)

Table 24-2 describes the fields in the show udld command output.

Table 24-2: show udld Command Output Fields

Field Description

UDLD

Status of whether UDLD is enabled or disabled.

Port

Module and port number(s).

Admin Status

Status of whether administration status is enabled or disabled.

Link State

Status of the link: undetermined (detection in progress, neighbor UDLD has been disabled), not applicable (UDLD has been disabled), shutdown (unidirectional link has been detected and the port disabled), or bidirectional (bidirectional link has been detected and the port disabled).

Field	Description
UDLD	Status of whether UDLD is enabled or disabled.
Port	Module and port number(s).
Admin Status	Status of whether administration status is enabled or disabled.
Link State	Status of the link: undetermined (detection in progress, neighbor UDLD has been disabled), not applicable (UDLD has been disabled), shutdown (unidirectional link has been detected and the port disabled), or bidirectional (bidirectional link has been detected and the port disabled).

Table of Contents

Configuring UDLD