Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide

UDLD Overview

The Cisco-proprietary UDLD protocol allows the devices connected through fiber optic or copper (for example, Category 5 cabling) Ethernet cables that are connected to the LAN ports to monitor the physical configuration of the cables and detect whether a unidirectional link exists. When a unidirectional link is detected, the UDLD shuts down the affected LAN port and alerts the corresponding user, because unidirectional links 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. In Layer 1, auto negotiation takes care of physical signaling and fault detection. UDLD performs tasks that auto negotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected LAN ports. When you enable both auto negotiation and UDLD, the 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 the traffic transmitted by a local device over a link is received by a 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, the link does not stay up as long as the auto negotiation is active. In such a scenario, the logical link is undetermined, and the UDLD does not take any action. If both the fibers are working normally in Layer 1, the UDLD in Layer 2 determines whether those fibers are connected correctly and whether the traffic is flowing bidirectionally between the correct neighbors. This check cannot be performed by auto negotiation because auto negotiation operates in Layer 1.

The Cisco ASR 1000 Series Aggregation Services Routers periodically transmit the UDLD packets to the neighbor devices on LAN ports where UDLD is enabled. If the packets are echoed back within a specific timeframe 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 for the protocol to successfully identify and disable the unidirectional links.

Note By default, the UDLD is disabled on all ports to avoid sending unnecessary traffic.

Figure 9-1 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 9-1 Unidirectional Link

Configuring the UDLD Aggressive Mode

Configure the UDLD aggressive mode only on the point-to-point link between the network devices that support the UDLD aggressive mode. With UDLD aggressive mode enabled, a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving the UDLD packets. The UDLD tries to re-establish the connection with the neighbor; the port is disabled after eight failed retries.

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 the UDLD aggressive mode is enabled, the UDLD will error disable the ports on the link to prevent the traffic from being discarded under the following scenarios:

• One side of a link has a port (either Tx and Rx) stuck.
• One side of a link remains up while the other side of the link has gone down.

Default UDLD Configuration

Table 9-1 shows the default UDLD configuration.

Enabling UDLD Globally

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

SUMMARY STEPS

1. enable

2. configure terminal

3. udld { enable | aggressive }

DETAILED STEPS

Enabling UDLD on Individual LAN Interfaces

To enable the UDLD on individual LAN interfaces, perform this task:

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type slot/port

4. udld port [aggressive]

DETAILED STEPS

Disabling UDLD on Fiber-Optic LAN Interfaces

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

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type slot/port

4. udld port disable

DETAILED STEPS

Configuring the UDLD Probe Message Interval

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

SUMMARY STEPS

1. enable

2. configure terminal

3. udld message time interval

DETAILED STEPS

Resetting the Disabled LAN Interfaces Manually

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

SUMMARY STEPS

1. enable

2. udld reset

DETAILED STEPS

Resetting the Disabled LAN Interfaces Automatically

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

SUMMARY STEPS

1. enable

2. configure terminal

3. udld recovery

4. udld recovery interval interval

DETAILED STEPS

Debugging UDLD

To enable the debugging of an UDLD activity, perform this task:

SUMMARY STEPS

1. enable

2. debug udld {events | packets | registries}

DETAILED STEPS

Example: Verifying a UDLD Configuration

The following example show how to use the show command to verify an UDLD configuration:

Sample Output for the show udld interface-id Command

Example: Verifying Information About Neighbors

The following example shows how to view the information pertaining to neighbors:

Sample Output for the show udld neighbors Command

Example: Displaying all the UDLD Interface Statuses

The follwing example shows how to display all the UDLD interface statuses:

Sample Output for the show udld Command

Related Documents

Standards

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