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[MX] Rate limit drop is displayed as RED drop on Coarse grain card (non-EQ DPC)

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In the implementation of rate limit on Coarse grain card (non-EQ DPCE), when rate limit is configured, we see RED drops for traffic beyond the configured CIR.


During a test, we sent 1024 Bype packets to VPLS instance. The traffic is sent to realtime queue, which is strict high and rate limit to 80M. However, we see RED drops when traffic is over 80M. The following was also observed:

  1. Running the same test on Fine a grain card (DPCE 40x 1GE X EQ) with the same config/traffic, the FG card can show drops in rate limit queue.
  2. When changing the scheduler config from percent (percent 8) to raw number (80M), the result is the same.
  3. Trying different versions 18.2 and 15.1, the issue is the same.
  4. Trying different packet size and also mixed size packets, the issue is the same

On a non-queuing DPC, by design, the rate-limited statistics are not displayed under RL-stats (under 'show interfaces xxxx queue' command). The rate-limit drops are indeed displayed as RED drops on a non-queuing DPC.

Queue: 1, Forwarding classes: REALTIME
    Packets              :             867461310                  9607 pps
    Bytes                :          891750226680              79012080 bps
    Packets              :             803778081                  8897 pps
    Bytes                :          826283867268              73168928 bps
    Tail-dropped packets :                     0                     0 pps
    RL-dropped packets   :                     0                     0 pps
    RL-dropped bytes     :                     0                     0 bps
    RED-dropped packets  :              63683229                   712 pps
     Low                 :              63683229                   712 pps
     Medium-low          :                     0                     0 pps
     Medium-high         :                     0                     0 pps
     High                :                     0                     0 pps
    RED-dropped bytes    :           65466359412               5855488 bps
     Low                 :           65466359412               5855488 bps
     Medium-low          :                     0                     0 bps
     Medium-high         :                     0                     0 bps
     High                :                     0                     0 bps

Related config:

set class-of-service interfaces ge-1/0/7 scheduler-map sched-map1
set class-of-service scheduler-maps sched-map1 forwarding-class REALTIME scheduler sched_realtime_80M
set class-of-service schedulers sched_realtime_80M transmit-rate percent 8
set class-of-service schedulers sched_realtime_80M transmit-rate rate-limit
set class-of-service schedulers sched_realtime_80M priority strict-high

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