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This chapter describes how to configure the Unidirectional Link Detection (UDLD) protocol on the Catalyst 5000, 4000, 2948G, and 2926G series switches.
This chapter consists of these sections:
The UDLD protocol allows devices connected through fiber-optic 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 fiber-optic 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 fiber-optic links. If the packets are not echoed back to the sender, the link is flagged as unidirectional and the port is shut down. All devices connected through fiber-optic ports must support UDLD in order for the protocol to successfully identify and disable unidirectional links.
Figure 25-1 shows an example of a unidirectional link condition. Switch B successfully receives traffic from Switch A on the fiber-optic port. However, Switch A does not receive traffic from Switch B on the same port. UDLD detects the problem and disables the port.
UDLD requires the following hardware and software:
Table 25-1 shows the default UDLD configuration.
| Feature | Default Value |
|---|---|
UDLD global enable state | Globally disabled |
UDLD per-port enable state | Enabled on all Ethernet ports using fiber-optic media |
These sections describe how to configure 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 |
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)
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]] |
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)
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]] |
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)
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 |
This example shows how to disable UDLD globally on the switch:
Console> (enable) set udld disable UDLD disabled globally Console> (enable)
To display the UDLD enable state, perform this task in privileged mode:
| 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]] |
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 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.
Posted: Mon Jul 19 12:55:24 PDT 1999
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