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This publication describes how to configure the UniDirectional Link Detection (UDLD) protocol in Release 12.1(2)E and later of the Cisco IOS for the Catalyst 6000 Family Switches product.
This publication 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 interface 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 mechanisms 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 interfaces. 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 from a Layer 1 perspective, then UDLD at Layer 2 determines whether those fibers are connected correctly and whether traffic is flowing bidirectionally between the right neighbors. This check cannot be performed by autonegotiation, because autonegotiation operates at Layer 1.
The switch periodically transmits UDLD packets to neighbor devices on interfaces with UDLD enabled. 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 interface 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 23-1 shows an example of a unidirectional link condition. Switch B successfully receives traffic from Switch A on the interface. However, Switch A does not receive traffic from Switch B on the same interface. UDLD detects the problem and disables the interface.
Table 23-2 shows the default UDLD configuration.
| Feature | Default Value |
|---|---|
UDLD global enable state | Globally disabled |
UDLD per-interface enable state for fiber-optic media | Enabled on all Ethernet fiber-optic interfaces |
UDLD per-interface enable state for twisted-pair (copper) media | Disabled on all Ethernet 10/100 and 1000BaseTX interfaces |
These sections describe how to configure UDLD:
To enable UDLD globally on all fiber-optic interfaces on the switch, perform this task:
| Command | Purpose |
|---|---|
Router(config)# [no] udld enable | Enable UDLD globally on fiber-optic interfaces on the switch. Use the no udld enable command to globally disable UDLD on fiber-optic interfaces. Note---This command only configures fiber-optic interfaces. Individual interface configuration overrides the setting of this command. |
To enable UDLD on individual interfaces, perform this task:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config-if)# udld enable | Enable UDLD on a specific interface. On a fiber-optic interface, this command overrides the udld enable global configuration command setting. | ||
| Router# show udld interface | Verify the configuration. |
To disable UDLD on individual non-fiber-optic interfaces, perform this task:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config-if)# no udld enable | On non-fiber-optic interfaces, the no udld enable command disables UDLD. Note---On fiber-optic interfaces, the no udld enable command reverts the interface configuration to the udld enable global configuration command setting. | ||
| Router# show udld interface | Verify the configuration. |
To disable UDLD on individual fiber-optic interfaces, perform this task:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config-if)# udld disable | Use the udld disable command to disable UDLD on a fiber-optic interface. Use the no udld disable command to revert to the udld enable global configuration command setting. Note---This command is not supported on non-fiber-optic interfaces. | ||
| Router# show udld interface | Verify the configuration. |
To reset all interfaces that have been shutdown by UDLD, perform this task:
| Command | Purpose |
|---|---|
Router(config)# udld reset | Use the udld reset command to reset all interfaces that have been shutdown by UDLD. |
The following documents are available for the Catalyst 6000 family switches running the Cisco IOS for the Catalyst 6000 Family of Switches product:
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/ios127xe/index.htm
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Posted: Fri Jul 7 08:37:08 PDT 2000
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