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Syslog message: XQSS.*Qdepth underrun error in Drop engine 0



Article ID: KB31690 KB Last Updated: 12 Oct 2021Version: 2.0

The "Qdepth underrun error in Drop engine 0" message reports an EACHIP software state issue related to buffer management.

This is a Troubleshooting Article for a PFE ASIC Syslog Event.
To view other documented syslog events related to XMCHIP, XLCHIP, MQCHIP, LUCHIP, EACHIP, and PECHIP, see KB31893 - Index of Articles for Troubleshooting PFE ASIC Syslog Events.



When a "Qdepth underrun error in Drop engine 0" event occurs, a message similar to the following is reported:

Jul 25 15:55:42.199 router0: %PFE-3: fpc8 Cmerror Op Set: XQSS(0): XQ-SS[0]: Qdepth underrun error in Drop engine 0, ERR_LOG: Q_num: 0 Dequeue byte count: 0

  • Traffic impact or permanent impact on packet forwarding may occur if errors are seen repeatedly
  • Alarm will be raised



This is due to a transient software issue in the EACHIP once during packet dequeue event the 32-bit queue depth value under flowed.


Perform these steps to determine the cause and resolve the problem (if any).  Continue through each step until the problem is resolved.

  1. Collect the show command output.

    Capture the output to a file (in case you have to open a technical support case). To do this, configure each SSH client/terminal emulator to log your session.

    show log messages
    show log chassisd
    start shell network pfe <fpc#>
    show nvram
    show syslog messages

  2. Analyze the show command output.

In the 'show log messages', review the events that occurred at or just before the appearance of the "Qdepth underrun error in Drop engine 0" message. Frequently these events help identify the cause.

  • No RMA is required

  • FPC restart during a maintenance window should clear this error​

  • Contact your technical support representative if this issue is seen repeatedly

This article is indexed in KB31893 - primary Index of Articles for Troubleshooting PFE ASIC Syslog Events; tag EACHIPTSG

Tip: When looking at an event in the logs, it is important to focus on the first error message in a collection of syslog messages. The first error message is usually the cause of all the follow-on error messages. The follow-on collateral damage error messages can be ignored.


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