Software Configuration Guide, Cisco IOS Release 15.2(2)E (Catalyst 2960, 2960-S, 2960-SF and 2960-Plus Switches)
Configuring UniDirectional Link Detection
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Configuring UniDirectional Link Detection

Configuring UniDirectional Link Detection

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Restrictions for Configuring UDLD

The following are restrictions for configuring UniDirectional Link Detection (UDLD):

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

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

  • UDLD is not supported on ATM ports.


Caution


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

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.

Normal Mode

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 normal mode, when UDLD 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.

Aggressive Mode

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.

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.

The following figure shows an example of a unidirectional link condition.

Figure 1. UDLD Detection of a Unidirectional Link



Methods to Detect Unidirectional Links

UDLD operates by using two methods:

  • Neighbor database maintenance

  • Event-driven detection and echoing

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 operation. 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.

UDLD Reset Options

If an interface becomes disabled by UDLD, you can use one of the following options to reset UDLD:

  • The udld reset interface configuration command.

  • The shutdown interface configuration command followed by the no shutdown interface configuration command restarts the disabled port.

  • The no udld {aggressive | enable} global configuration command followed by the udld {aggressive | enable} global configuration command reenables the disabled ports.

  • The no udld port interface configuration command followed by the udld port [aggressive] interface configuration command reenables the disabled fiber-optic port.

  • The errdisable recovery cause udld global configuration command enables the timer to automatically recover from the UDLD error-disabled state, and the errdisable recovery interval interval global configuration command specifies the time to recover from the UDLD error-disabled state.

Default UDLD Configuration

Table 1 Default UDLD Configuration

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.

SUMMARY STEPS

    1.    configure terminal

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

    3.    end


DETAILED STEPS
     Command or ActionPurpose
    Step 1configure terminal


    Example:
    
    Switch# configure terminal
    
    
     

    Enters global configuration mode.

     
    Step 2udld {aggressive | enable | message time message-timer-interval}


    Example:
    
    Switch(config)# udld enable 
    message time 10
    
    
     

    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.

    • 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; the default value is 15.

      Note   

      This command affects fiber-optic ports only. Use the udld interface configuration command to enable UDLD on other port types.

    Use the no form of this command, to disable UDLD.

     
    Step 3end


    Example:
    
    Switch(config)# end
    
    
     

    Returns to privileged EXEC mode.

     

    Enabling UDLD on an Interface

    Follow these steps either to enable UDLD in the aggressive or normal mode or to disable UDLD on a port.

    SUMMARY STEPS

      1.    configure terminal

      2.    interface interface-id

      3.    udld port [aggressive]

      4.    end


    DETAILED STEPS
       Command or ActionPurpose
      Step 1configure terminal


      Example:
      
      Switch# configure terminal
      
      
       

      Enters global configuration mode.

       
      Step 2interface interface-id


      Example:
      
      Switch(config)# interface 
      gigabitethernet 1/0/1
      
      
       

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

       
      Step 3udld port [aggressive]


      Example:
      
      Switch(config-if)# udld port aggressive
      
      
       

      UDLD is disabled by default.

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

      • udld port aggressive—(Optional) 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.

       
      Step 4end


      Example:
      
      Switch(config-if)# end
      
      
       

      Returns to privileged EXEC mode.

       

      Monitoring and Maintaining UDLD

      Command Purpose

      show udld [interface-id | neighbors]

      Displays the UDLD status for the specified port or for all ports.

      Additional References for UDLD

      Related Documents

      Related Topic Document Title

      Layer 2 command reference

      Command Reference, Cisco IOS Release 15.2(2)E (Catalyst 2960, 2960-S, 2960-SF and 2960-Plus Switches)

      Error Message Decoder

      Description Link

      To help you research and resolve system error messages in this release, use the Error Message Decoder tool.

      https:/​/​www.cisco.com/​cgi-bin/​Support/​Errordecoder/​index.cgi

      Standards and RFCs

      Standard/RFC Title
      None

      MIBs

      MIB MIBs Link

      All supported MIBs for this release.

      To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

      http:/​/​www.cisco.com/​go/​mibs

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