This issue is applicable with 100g + hold down timer configuration OR 400g + hold down timer configuration along with interface flaps. Due to aggressive interface flaps we get into the credit drain issue.

When this issue happens, with repeated link flaps on MPC10E line-cards for 100GE interfaces, traffic stops egressing the affected interface and report syslog messages:

"XQSS_CMERROR_DSTAT_INT_REG_DROP0_QDEPTH_UNDRN" or "mqss_wo_coreif_conn_credits_wait_for_init_value" during link down event.
 

[Oct 23 12:50:56.541 LOG: Info] smic_mac_process_linkstate: Enabling MAC Rx for ifd et-5/0/2
[Oct 23 12:50:56.541 LOG: Info] MQSS(0): QUADPORT2: Cleared Ethernet MAC Local Fault Delta Event for Port 0 (et-5/0/2)
[Oct 23 12:50:56.542 LOG: Info] ifd_process_flaps_common: IFD et-5/0/2 - pfe link UP processing is complete 
[Oct 23 13:30:05.055 LOG: Info] MQSS(0): QUADPORT2: Detected Ethernet MAC Local Fault Delta Event for Port 0 (et-5/0/2)
[Oct 23 13:30:05.268 LOG: Info] ifp et-5/0/2 ifd_mdown: 15025051606 ms
[Oct 23 13:30:06.269 LOG: Err] mqss_wo_coreif_conn_credits_wait_for_init_value: Timeout occured while waiting for available credits value to become initial 
value - conn_num 20, credits 0, init_credits 16
[Oct 23 13:30:06.269 LOG: Err] ztchip_mqss_wanio_stream_out_disable: Waiting for available credits value to become initial value for WO connection failed - 
status 29, wan_port_group 0, conn_num 20
[Oct 23 13:30:06.269 LOG: Err] mqss_stream_out_disable_wanio_zt_yt: Performing egress PHY stream disable operations for WANIO failed - status 29, instance 0
, stream_num 1100, conn_num 20, mac_type 2, mac_set 2, mac_port_num 0, port_speed 5
[Oct 23 13:30:06.269 LOG: Err] mqss_stream_out_disable_wanio: Performing egress PHY stream disable operations for WANIO failed - status 29, stream_num 1100
[Oct 23 13:30:06.269 LOG: Err] mqss_stream_out_disable: Performing egress PHY stream disable operations for WANIO failed - status 29, stream_num 1100
[Oct 23 13:30:06.269 LOG: Err] mqss_ifd_link_up_down_handler: Disabling PHY stream for egress side failed - status 29, instance 0, phy_stream 1100
[Oct 23 13:30:06.269 LOG: Info] pfe_ifd_link_updown: IFD et-5/0/2 - pfe link DOWN processing is complete
[Oct 23 13:30:06.269 LOG: Err] pfe_ifd_link_updown: Handling IFD link DOWN failed - status 29, ifd et-5/0/2

Short Term Solution = With a JSU fix of PRs  1642584 + 1688972 and Hold Down timer = 1 sec down 30 sec up can help.
Long Term Solution = Move to junos which has fix for this master PR 1719682. 

To recover instantly from the issue, reboot the FPC.