This article explains why packets may be discarded due to an invalid TX cookie on T Series devices, and suggests the best way to deal with the problem.

Packet drops are observed with an incrementing RL-dropped queue counter.​

Egress queues: 8 supported, 8 in use
Queue: 0, Forwarding classes: DEFAULT
  Queued:
    Packets              :        14272483091674            2281932444 pps
    Bytes                :        12770250291548           46354791928 bps
  Transmitted:
    Packets              :        12470269453482            2281932561 pps
    Bytes                :        10912040025143           46355271160 bps
    Tail-dropped packets : Not Available  
    RL-dropped packets   :                     0                     0 pps
    RL-dropped bytes     :                     0                     0 bps
    RED-dropped packets  :         1802213637413                     0 pps
    RED-dropped bytes    :         1858209867557                     0 bps
Queue: 1, Forwarding classes: REALTIME
  Queued:
    Packets              :         3885485415277                    48 pps
    Bytes                :         4124724019160                 32560 bps
  Transmitted:
    Packets              :         2095265893992                    48 pps
    Bytes                :         2264602776937                 32560 bps
    Tail-dropped packets : Not Available  
    RL-dropped packets   :    277749388874789659                    19 pps
    RL-dropped bytes     :    273264582547602772                 38216 bps
    RED-dropped packets  :         1790219521285                     0 pps
    RED-dropped bytes    :         1860121242223                     0 bps
    RED-dropped bytes    :         1778548196865  

The following logs are also observed:

%PFE-3: (FPC Slot 5, PIC Slot 3) IQ2(5/3): Link [0] - 30399344448995098 packets discarded due to invalid TX Cookie.
%PFE-3: (FPC Slot 5, PIC Slot 3) IQ2(5/3): Link [0] - 31525244355837722 packets discarded due to invalid TX Cookie.
%PFE-3: (FPC Slot 5, PIC Slot 3) IQ2(5/3): Link [0] - 37154743890050842 packets discarded due to invalid TX Cookie.
%PFE-3: (FPC Slot 5, PIC Slot 3) IQ2(5/3): Link [0] - 41658343517421338 packets discarded due to invalid TX Cookie.
%PFE-3: (FPC Slot 5, PIC Slot 3) IQ2(5/3): Link [0] - 42784243424263962 packets discarded due to invalid TX Cookie.
%PFE-3: (FPC Slot 5, PIC Slot 3) IQ2(5/3): Link [0] - 43910143331106586 packets discarded due to invalid TX Cookie.

This is related to the IQ2 PIC. The Packet Forwarding Engine (PFE) appends an 8-byte cookie to the packets that are transmitted out of the port. This TX cookie contains various fields, including Service ID and Exit Port.

Using the Service ID, a lookup is done to derive the EZchip scheduler queue and using the Exit Port, the outgoing interface is determined. If the Exit Port is incorrect in the received TX cookie, then the “Invalid TX cookie” counter gets incremented and the packet is dropped. This problem is seen to be due to a corrupted TX cookie received from the PFE, or the IQ2 itself could be corrupting the received cookie.

If this is a one-time occurrence, rebooting or re-seating the PIC in which the interface packet drops are observed should resolve the issue. You need to continue monitoring the queue drops. Refer to request chassis pic for details on performing a soft reboot of the PIC.

However, if the queue drops are observed even after rebooting or re-seating the PIC, you need to create an RMA for the faulty PIC in which the drops are observed. Contact Support for further assistance.