This article describe the reason for syslog message XQSS_CMERROR_PR_INT_REG_FEOP_LT_DQS_QC.

Syslog message XQSS_CMERROR_PR_INT_REG_FEOP_LT_DQS_QC reports a software issue in the ZTCHIP when processing is requested for an empty queue. Logs similar to those below are seen during the issue period:
Mar 6 21:27:22 MX960-re0 : %PFE-3: fpc3 MQSS(0): WO: Packet Error - Error Packets 5, Connection 10Mar 6 21:27:22 MX960-re0 : %PFE-3: fpc3 ZT[0:0]: MCIF Rx: Checksum error detected on WO response - Chunk Address 0x1107c73Mar 6 21:27:26 MX960-re0 : %PFE-3: fpc3 MQSS(0): WO: Packet Error - Error Packets 1, Connection 41Mar 6 21:27:26 MX960-re0 : %PFE-3: fpc3 ZT[0:0]: MCIF Rx: Checksum error detected on WO response - Chunk Address 0x1107c73
Mar 6 21:27:49 MX960-re0 bbesmgd: %USER-4-SMD_RSMON_RTT_EXCEEDED: RTT exceeded threshold for FPC 3, throttle_phase_count 1
Mar 6 21:27:50 MX960-re0 : %PFE-3: fpc3 platform_zt_module_get_state_ack_cb: module id:27 action 2 error-code 230067 ack 0 timed out
Mar 6 21:27:50 MX960-re0 : %PFE-5: fpc3 Performing action cmalarm for error /fpc/3/pfe/0/cm/0/XQSS(0)/0/XQSS_CMERROR_DSTAT_INT_REG_DROP0_QDEPTH_UNDRN (0x230067) in module: XQSS(0) with scope: pfe category: functional level: major, oc_category: unknown oc_category
Mar 6 21:27:50 MX960-re0 : %PFE-5: fpc3 Performing action log for error /fpc/3/pfe/0/cm/0/XQSS(0)/0/XQSS_CMERROR_SCHED_QL4_INT_REG_DQU_QSUM_UDR (0x230063) in module: XQSS(0) with scope: pfe category: functional level: major, oc_category: unknown oc_category
Mar 6 21:27:50 MX960-re0 : %PFE-5: fpc3 Performing action get-state for error /fpc/3/pfe/0/cm/0/XQSS(0)/0/XQSS_CMERROR_SCHED_QL4_INT_REG_DQU_QSUM_UDR (0x230063) in module: XQSS(0) with scope: pfe category: functional level: major, oc_category: unknown oc_category
Mar 6 21:27:50 MX960-re0 : %PFE-5: fpc3 Performing action cmalarm for error /fpc/3/pfe/0/cm/0/XQSS(0)/0/XQSS_CMERROR_SCHED_QL4_INT_REG_DQU_QSUM_UDR (0x230063) in module: XQSS(0) with scope: pfe category: functional level: major, oc_category: unknown oc_category
Mar 6 21:27:50 MX960-re0 : %PFE-5: fpc3 CMError: /fpc/3/pfe/0/cm/0/XQSS(0)/0/XQSS_CMERROR_PR_INT_REG_FEOP_LT_DQS_QC (0x23012f), scope: pfe, category: functional, severity: major, module: XQSS(0), type: XQSS FEOP LT DQS QC, oc_category: unknown oc_category
Mar 6 21:27:50 MX960-re0 : %PFE-5: fpc3 CMError: /fpc/3/pfe/0/cm/0/XQSS(0)/0/XQSS_CMERROR_CPQW_ERR_INT_FSET_FAST_DEQ_DRY_ERR (0x2300e1), scope: pfe, category: functional, severity: major, module: XQSS(0), type: XQSS CPQW FAST DEQ DRY, oc_category: unknown oc_category
Mar 6 21:27:50 MX960-re0 : %PFE-5: fpc3 Performing action log for error /fpc/3/pfe/0/cm/0/XQSS(0)/0/XQSS_CMERROR_CPQW_ERR_INT_FSET_FAST_DEQ_DRY_ERR (0x2300e1) in module: XQSS(0) with scope: pfe category: functional level: major, oc_category: unknown oc_category

This is due to a transient software issue in the ZTCHIP. The ZTCHIP software state is usually reset internally to correct this condition.  Usually, this error is a chain reaction to an original/underlying issue, so always check the preceding logs. If not self-corrected, it could lead to a wedge condition where the affected PFE will be disabled resulting in service impact 

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

• Collect the show command output

show log messages
show log chassisd
start shell network pfe <fpc#.0>
show nvram
show syslog messages
show cmerror modules brief ---> check for XQSS id [In current example error is in "XQSS(0)"] 

For instance:

FMPC3(MX960-re0 vty)# sh cmerror module brief  

-------------------------------------------------------------------------

Module Name       Active Errors PFE   Callback  ModuleData

                     Specific Function       

-------------------------------------------------------------------------

17    XQSS(0)     0       Yes   0x149140800 0x3750258664

show cmerror module <id> ---> replace id with XQSS module id

• Analyze the show command output.
• In the 'show log messages', review the events that occurred at or just before the appearance of the "XQSS_CMERROR_CPQW_ERR_INT_FSET_FAST_DEQ_DRY_ERR" message. Frequently, these events help identify the cause.
  • No RMA is required
  • Collect debugging information using  KB31922 - shell script collecting data if packet forwarding at Packet Forwarding Engine is compromised  [juniper.net]
  • ​ If the error is seen repeatedly, an FPC restart  during a maintenance window should clear this error
  • If this error is preceded by XQSS_CMERROR_SCHED_QL4_INT_REG_DQU_QSUM_UDR error check out PR1793375 for fix
  • Contact your technical support representative if this issue is seen repeatedly.

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.