Configuring UDLD

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Prerequisites for UDLD

•When configuring the mode (normal or aggressive), make sure that the same mode is configured on both sides of the link.

Restrictions for UDLD

•UDLD is not supported on ATM ports.

•A UDLD-capable port cannot detect a unidirectional link if it is connected to a UDLD-incapable port of another switch.

•Loop guard works only on point-to-point links. We recommend that each end of the link has a directly connected device that is running STP.

Information About UDLD

UDLD

UniDirectional Link Detection (UDLD) is a Layer 2 protocol that enables 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. All connected devices must support UDLD for the protocol to successfully identify and disable unidirectional links. When UDLD detects a unidirectional link, it disables the affected port and alerts you. Unidirectional links can cause a variety of problems, including spanning-tree topology loops.

Modes of Operation

UDLD supports two modes of operation: normal (the default) and aggressive. In normal mode, UDLD can detect unidirectional links due to misconnected ports on fiber-optic connections. In aggressive mode, UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to misconnected ports on fiber-optic links.

In normal and aggressive modes, UDLD works with the Layer 1 mechanisms to learn 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 ports. When you enable both autonegotiation 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 traffic sent by a local device is received by its neighbor but traffic from the neighbor is not received by the local device.

In normal mode, UDLD detects a unidirectional link when fiber strands in a fiber-optic port are misconnected and the Layer 1 mechanisms do not detect this misconnection. If the ports are connected correctly but the traffic is one way, UDLD does not detect the unidirectional link because the Layer 1 mechanism, which is supposed to detect this condition, does not do so. In this case, the logical link is considered undetermined, and UDLD does not disable the port.

When UDLD is in normal mode, if one of the fiber strands in a pair is disconnected, as long as autonegotiation is active, the link does not stay up because the Layer 1 mechanisms detects a physical problem with the link. In this case, UDLD does not take any action and the logical link is considered undetermined.

In aggressive mode, UDLD detects a unidirectional link by using the previous detection methods. UDLD in aggressive mode can also detect a unidirectional link on a point-to-point link on which no failure between the two devices is allowed. It can also detect a unidirectional link when one of these problems exists:

•On fiber-optic or twisted-pair links, one of the ports cannot send or receive traffic.

•On fiber-optic or twisted-pair links, one of the ports is down while the other is up.

•One of the fiber strands in the cable is disconnected.

In these cases, UDLD disables the affected port.

In a point-to-point link, UDLD hello packets can be considered as a heart beat whose presence guarantees the health of the link. Conversely, the loss of the heart beat means that the link must be shut down if it is not possible to reestablish a bidirectional link.

If both fiber strands in a cable are working normally from a Layer 1 perspective, UDLD in aggressive mode detects whether those fiber strands are connected correctly and whether traffic is flowing bidirectionally between the correct neighbors. This check cannot be performed by autonegotiation because autonegotiation operates at Layer 1.

Methods to Detect Unidirectional Links

UDLD operates by using two methods:

•Neighbor database maintenance

UDLD learns about other UDLD-capable neighbors by periodically sending a hello packet (also called an advertisement or probe) on every active port to keep each device informed about its neighbors.

When the switch receives a hello message, it caches the information until the age time (hold time or time-to-live) expires. If the switch receives a new hello message before an older cache entry ages, the switch replaces the older entry with the new one.

Whenever a port is disabled and UDLD is running, whenever UDLD is disabled on a port, or whenever the switch is reset, UDLD clears all existing cache entries for the ports affected by the configuration change. UDLD sends at least one message to inform the neighbors to flush the part of their caches affected by the status change. The message is intended to keep the caches synchronized.

•Event-driven detection and echoing

UDLD relies on echoing as its detection mechanism. Whenever a UDLD device learns about a new neighbor or receives a resynchronization request from an out-of-sync neighbor, it restarts the detection window on its side of the connection and sends echo messages in reply. Because this behavior is the same on all UDLD neighbors, the sender of the echoes expects to receive an echo in reply.

If the detection window ends and no valid reply message is received, the link might shut down, depending on the UDLD mode. When UDLD is in normal mode, the link might be considered undetermined and might not be shut down. When UDLD is in aggressive mode, the link is considered unidirectional, and the port is disabled.

If UDLD in normal mode is in the advertisement or in the detection phase and all the neighbor cache entries are aged out, UDLD restarts the link-up sequence to resynchronize with any potentially out-of-sync neighbors.

If you enable aggressive mode when all the neighbors of a port have aged out either in the advertisement or in the detection phase, UDLD restarts the link-up sequence to resynchronize with any potentially out-of-sync neighbor. UDLD shuts down the port if, after the fast train of messages, the link state is still undetermined.

Figure 30-1 UDLD Detection of a Unidirectional Link

Default UDLD Settings

Table 30-1 Default UDLD Settings

Feature
Default Setting

UDLD global enable state

Globally disabled

UDLD per-port enable state for fiber-optic media

Disabled on all Ethernet fiber-optic ports

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

Disabled on all Ethernet 10/100 and 1000BASE-TX ports

UDLD aggressive mode

Disabled


How to Configure UDLD

Enabling UDLD Globally

Follow these steps to enable UDLD in the aggressive or normal mode and to set the configurable message timer on all fiber-optic ports on the switch:

 
Command
Purpose

Step 1 

configure terminal

Enters global configuration mode.

Step 2 

udld {aggressive | enable | message time message-timer-interval}

Specifies the UDLD mode of operation:

•aggressive—Enables UDLD in aggressive mode on all fiber-optic ports.

•enable—Enables UDLD in normal mode on all fiber-optic ports on the switch. UDLD is disabled by default.

An individual interface configuration overrides the setting of the udld enable global configuration command.

For more information about aggressive and normal modes, see the "Modes of Operation" section.

•message time message-timer-interval—Configures the period of time between UDLD probe messages on ports that are in the advertisement phase and are detected to be bidirectional. The range is from 1 to 90 seconds.

Note This command affects fiber-optic ports only. Use the udld interface configuration command to enable UDLD on other port types. For more information, see the "Enabling UDLD on an Interface" section.

Step 3 

end

Returns to privileged EXEC mode.

Enabling UDLD on an Interface

 
Command
Purpose

Step 1 

configure terminal

Enters global configuration mode.

Step 2 

interface interface-id

Specifies the port to be enabled for UDLD, and enters interface configuration mode.

Step 3 

udld port [aggressive]

UDLD is disabled by default.

•udld port—Enables UDLD in normal mode on the specified port.

•udld port aggressive—Enables UDLD in aggressive mode on the specified port.

Note Use the no udld port interface configuration command to disable UDLD on a specified fiber-optic port.

For more information about aggressive and normal modes, see the "Modes of Operation" section.

Step 4 

end

Returns to privileged EXEC mode.

Setting and Resetting UDLD Parameters

 
Command
Purpose

Step 1 

configure terminal

Enters global configuration mode.

Step 2 

udld reset

(Optional) Resets all ports disabled by UDLD.

Step 3 

no udld {aggressive | enable}

(Optional) Disables the UDLD ports.

Step 4 

udld {aggressive | enable}

(Optional) Reenables the disabled ports.

Step 5 

errdisable recovery cause udld

(Optional) Enables the timer to automatically recover from the UDLD error-disabled state.

Step 6 

errdisable recovery interval interval

(Optional) Specifies the time to recover from the UDLD error-disabled state.

Step 7 

interface interface-id

Enters interface configuration mode.

Step 8 

no udld port

(Optional) Disables the UDLD fiber-optic port.

Step 9 

udld port [aggressive]

(Optional) Re-enables the disabled fiber-optic port.

Step 10 

shutdown

(Optional) Disables an interface port.

Step 11 

no shutdown

(Optional) Restarts a disabled port.

Step 12 

show udld

(Optional) Verifies your entries.

Maintaining and Monitoring UDLD

Command
Purpose

show udld [interface-id]

Displays UDLD status.


Additional References

The following sections provide references related to switch administration:

Related Documents

Related Topic
Document Title

Cisco IE 2000 commands

Cisco IE 2000 Switch Command Reference, Release 15.0(2)EC

Cisco IOS basic commands

Cisco IOS Configuration Fundamentals Command Reference


Standards

Standards
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

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MIBs

MIBs
MIBs Link

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To locate and download MIBs using Cisco IOS XR software, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: http://cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml


RFCs

RFCs
Title

No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

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