Configuring UDLD

Understanding How UDLD Works

UDLD Software and Hardware Requirements

Default UDLD Configuration

Configuring UDLD

Enabling UDLD Globally

Enabling UDLD on Individual Ports

Disabling UDLD on Individual Ports

Disabling UDLD Globally

Displaying the UDLD Configuration

Configuring UDLD

This chapter describes how to configure the UniDirectional Link Detection (UDLD) protocol on the Catalyst enterprise LAN switches.

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

This chapter consists of these sections:

The UDLD protocol allows devices connected through fiber-optic or copper (for example, Category 5 cabling) 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 the local device over a link is received by the neighbor, but traffic transmitted from the neighbor is not received by the local device. Unidirectional links can cause a variety of problems, including spanning-tree topology loops.

The switch periodically transmits UDLD packets to neighbor devices on ports with UDLD enabled. If the packets are not echoed back to the sender, the link is flagged as unidirectional and the 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.

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

UDLD Software and Hardware Requirements

UDLD requires the following hardware and software:

For fiber-optic links:
- Supervisor engine software release 5.1 or later
- Ethernet, Fast Ethernet, or Gigabit Ethernet fiber-optic switching modules
For copper links:
- Supervisor engine software release 5.2 or later
- Ethernet or Fast Ethernet copper switching modules

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

Enabled on all Ethernet, Fast Ethernet, and Gigabit Ethernet ports using fiber-optic media

Disabled on all Ethernet and Fast Ethernet ports using copper media

Feature	Default Value
UDLD global enable state	Globally disabled
UDLD per-port enable state	Enabled on all Ethernet, Fast Ethernet, and Gigabit Ethernet ports using fiber-optic media Disabled on all Ethernet and Fast Ethernet ports using copper media

Configuring UDLD

These sections describe how to configure UDLD:

Enabling UDLD Globally

You must enable UDLD globally before any port can use UDLD.

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 the UDLD enable state.

show udld

Task	Command
Display the 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)

The link state is "undetermined" in these situations:

The local port is running UDLD and the neighbor is not
The port is not connected
UDLD is disabled on the port
The port is disabled

The link state is "bidirectional" when the local and neighbor ports are successfully exchanging UDLD packets. The link state is "shutdown" when a unidirectional link is detected and the port has been shut down.

Table of Contents

Configuring UDLD