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The port status is "errdisable" due to a UniDirectional Link Detection condition



A port may go into errdisable status due to a UniDirectional Link Detection (UDLD) condition. The errdisable status indicates that the port was automatically disabled by the switch Operating System (OS) software due to an error condition encountered on the port.

To determine if a port is in errdisable status, issue the show port command. For example, to check the status on port 3/2, issue the show port 3/2 command. This is a sample command output:

Cat5500> (enable) show port 3/2
Port  Name               Status     Vlan       Level  Duplex Speed Type
----- ------------------ ---------- ---------- ------ ------ ----- ------------
3/2                      errdisable 1          normal   auto  auto 10/100BaseTX

The switch sends a message to the console that describes why the port was disabled when it puts a port in the errdisable state. If syslog is configured, the message is available on the syslog server as well.

Another way to determine the reason for the errdisable status is to issue the show errdisable-timeout command. This command is available in CatOS 5.4(1) and later versions. This is a sample command output:

Cat5500> (enable) show errdisable-timeout
ErrDisable Reason   Timeout Status
------------------- --------------
bpdu-guard          disable
channel-misconfig   disable
duplex-mismatch     disable
udld                enable
other               disable

Interval: 30 seconds

Port ErrDisable Reason
---- -----------------
3/2   udld

Note: UDLD works by exchanging protocol packets between the neighboring devices. Both devices on the link must support UDLD and have it enabled on respective ports.

Each switch port configured for UDLD sends UDLD protocol packets that contain the port device (or port ID) and the neighbor device (or port IDs) seen by UDLD on that port. Neighboring ports must see their own device or port ID (echo) in the packets received from the other side.

If the port does not see its own device or port ID in the incoming UDLD packets for a specific duration of time, the link is considered unidirectional. Therefore, the respective port is disabled and a message similar to this is printed on the console:

UDLD-3-DISABLE: Unidirectional link detected on port 3/2. Port disabled. (CatOS)
PM-SP-4-ERR_DISABLE: udld error detected on Gi4/1, putting Gi4/1 in err-disable state. (Cisco IOS system software)

The port shutdown by UDLD remains disabled until it is manually re-enabled, or until the errdisable timeout expires (if configured). These are possible reasons for this message:

  • The link is up on both sides, but packets are only received by one side. This may be a hardware or a software issue.    
  • Wiring mistakes occur when receive and transmit fibers are not connected to the same port on the remote side. 

    In this case, check to ensure wiring is set up properly. 

  • In aggressive mode, if the link state of the port was determined to be bi-directional and the UDLD information times out while the link on the port is still up, UDLD tries to re-establish the state of the port. If not successful, the port is put into errdisable state. This may indicate that a port is stuck (On one side, a port neither transmits nor receives, but the link is up on both sides).    

Once the cause of the errdisable status has been found and corrected, issue the set port enable command to re-enable the port. For example, to re-enable port 3/2, the set port enable 3/2 command must be issued.

However, if the set port enable command is issued without the cause of the errdisable status being corrected, the port eventually goes back to the errdisable status.

For more information on the errdisable port status, refer to these documents:

For more information on UDLD operation, configuration, and commands, refer to these documents:

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