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 - Master Index of Articles for Troubleshooting PFE ASIC Syslog Events [juniper.net] .

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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.

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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.

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   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
exit

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 [juniper.net] ; tag EACHIPTSG