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Traffic shaping - excessive drops?

I am shaping my outbound router (2921) link to 50Mbs.  I have my QOS policy nested inside my shaping policy and I have noticed (what I think) is a high amount of packet drops in one of my queues.

Below is the output of 2 show policy-map interface g0/0 run back to back.  As you can see I lost around 21 packets in just a few seconds, yet my link is only using around 7Mbs.  My counters were cleared around 3 hours before I took this sample.  I have my 2 QOS policies listed after the output.  Any ideas why I am seeing all these packet drops?  My QoS understanding isn't exactly expert level yet, but it seems like something isn't working correctly.  Any tips would be appreciated.  Thanks in advance.

SAMPLE 1

GigabitEthernet0/0

  Service-policy output: SHAPE

    Class-map: class-default (match-any)

      31573394 packets, 23655469220 bytes

      30 second offered rate 7131000 bps, drop rate 13000 bps

      Match: any

      Queueing

      queue limit 256 packets

      (queue depth/total drops/no-buffer drops) 0/55237/0

      (pkts output/bytes output) 31518160/23588134456

      shape (average) cir 50000000, bc 200000, be 200000

      target shape rate 50000000

      Service-policy : WAN_QOS

        queue stats for all priority classes:

          queue limit 64 packets

          (queue depth/total drops/no-buffer drops) 0/0/0

          (pkts output/bytes output) 120258/23706082

        Class-map: EF (match-all)

          120258 packets, 23706082 bytes

          30 second offered rate 0 bps, drop rate 0 bps

          Match:  dscp ef (46)

          Priority: 25% (12500 kbps), burst bytes 312500, b/w exceed drops: 0

        Class-map: AF41 (match-any)

          20181 packets, 2770959 bytes

          30 second offered rate 2000 bps, drop rate 0 bps

          Match:  dscp af41 (34)

            20181 packets, 2770959 bytes

            30 second rate 2000 bps

          Match: access-group 120

            0 packets, 0 bytes

            30 second rate 0 bps

          Queueing

          queue limit 64 packets

          (queue depth/total drops/no-buffer drops) 0/0/0

          (pkts output/bytes output) 20181/2770959

          bandwidth 40% (20000 kbps)

          QoS Set

            dscp af41

              Packets marked 20181

        Class-map: AF21 (match-all)

          28562372 packets, 22381305435 bytes

          30 second offered rate 4721000 bps, drop rate 13000 bps

          Match:  dscp af21 (18)

          Queueing

          queue limit 64 packets

          (queue depth/total drops/no-buffer drops) 0/55091/0

          (pkts output/bytes output) 28507286/22314221299

          bandwidth 34% (17000 kbps)

        Class-map: class-default (match-any)

          2870584 packets, 1247686802 bytes

          30 second offered rate 2410000 bps, drop rate 0 bps

          Match: any

          queue limit 256 packets

          (queue depth/total drops/no-buffer drops) 0/146/0

          (pkts output/bytes output) 2870438/1247436290

          QoS Set

            dscp default

              Packets marked 2869359

            Exp-weight-constant: 9 (1/512)

            Mean queue depth: 0 packets

            class     Transmitted       Random drop      Tail drop          Minimum        Maximum     Mark

                      pkts/bytes     pkts/bytes       pkts/bytes          thresh         thresh     prob

            0         2870438/1247436290    134/64490         12/984               20            40  1/10

            1               0/0               0/0              0/0                 22            40  1/10

            2               0/0               0/0              0/0                 24            40  1/10

            3               0/0               0/0              0/0                 26            40  1/10

            4               0/0               0/0              0/0                 28            40  1/10

            5               0/0               0/0              0/0                 30            40  1/10

            6               0/0               0/0              0/0                 32            40  1/10

            7               0/0               0/0              0/0                 34            40  1/10

SAMPLE 2

GigabitEthernet0/0

  Service-policy output: SHAPE

    Class-map: class-default (match-any)

      31589736 packets, 23665225208 bytes

      30 second offered rate 7131000 bps, drop rate 13000 bps

      Match: any

      Queueing

      queue limit 256 packets

      (queue depth/total drops/no-buffer drops) 0/55258/0

      (pkts output/bytes output) 31534478/23597868640

      shape (average) cir 50000000, bc 200000, be 200000

      target shape rate 50000000

      Service-policy : WAN_QOS

        queue stats for all priority classes:

          queue limit 64 packets

          (queue depth/total drops/no-buffer drops) 0/0/0

          (pkts output/bytes output) 120258/23706082

        Class-map: EF (match-all)

          120258 packets, 23706082 bytes

          30 second offered rate 0 bps, drop rate 0 bps

          Match:  dscp ef (46)

          Priority: 25% (12500 kbps), burst bytes 312500, b/w exceed drops: 0

        Class-map: AF41 (match-any)

          20185 packets, 2771873 bytes

          30 second offered rate 2000 bps, drop rate 0 bps

          Match:  dscp af41 (34)

            20185 packets, 2771873 bytes

            30 second rate 2000 bps

          Match: access-group 120

            0 packets, 0 bytes

            30 second rate 0 bps

          Queueing

          queue limit 64 packets

          (queue depth/total drops/no-buffer drops) 0/0/0

          (pkts output/bytes output) 20185/2771873

          bandwidth 40% (20000 kbps)

          QoS Set

            dscp af41

              Packets marked 20185

        Class-map: AF21 (match-all)

          28576958 packets, 22389554081 bytes

          30 second offered rate 4721000 bps, drop rate 13000 bps

          Match:  dscp af21 (18)

          Queueing

          queue limit 64 packets

          (queue depth/total drops/no-buffer drops) 0/55112/0

          (pkts output/bytes output) 28521846/22322447962

          bandwidth 34% (17000 kbps)

        Class-map: class-default (match-any)

          2872335 packets, 1249193172 bytes

          30 second offered rate 2410000 bps, drop rate 0 bps

          Match: any

          queue limit 256 packets

          (queue depth/total drops/no-buffer drops) 0/146/0

          (pkts output/bytes output) 2872189/1248942723

          QoS Set

            dscp default

              Packets marked 2871109

            Exp-weight-constant: 9 (1/512)

            Mean queue depth: 0 packets

            class     Transmitted       Random drop      Tail drop          Minimum        Maximum     Mark

                      pkts/bytes     pkts/bytes       pkts/bytes          thresh         thresh     prob

            0         2872189/1248942723    134/64490         12/984               20            40  1/10

            1               0/0               0/0              0/0                 22            40  1/10

            2               0/0               0/0              0/0                 24            40  1/10

            3               0/0               0/0              0/0                 26            40  1/10

            4               0/0               0/0              0/0                 28            40  1/10

            5               0/0               0/0              0/0                 30            40  1/10

            6               0/0               0/0              0/0                 32            40  1/10

            7               0/0               0/0              0/0                 34            40  1/10

POLICY MAPS

show policy-map

  Policy Map WAN_QOS

    Class EF

      priority 25 (%)

    Class AF41

      bandwidth 40 (%)

      set dscp af41

    Class AF21

      bandwidth 34 (%)

    Class class-default

      set ip dscp default

      queue-limit 256 packets

       packet-based wred, exponential weight 9

      class    min-threshold    max-threshold    mark-probablity

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

      0       -                -                1/10

      1       -                -                1/10

      2       -                -                1/10

      3       -                -                1/10

      4       -                -                1/10

      5       -                -                1/10

      6       -                -                1/10

      7       -                -                1/10

  Policy Map SHAPE

    Class class-default

      Average Rate Traffic Shaping

      cir 50000000 (bps)

      queue-limit 256 packets

      service-policy WAN_QOS


Everyone's tags (2)
1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Bronze

Traffic shaping - excessive drops?

The drops seem to be happening with Class AF21. What type of traffic matches such DSCP value in your network?

You may have to increase the queue-limit under the class to something greater than 64 until the drops stop.

I don't think the shaper is causing the issue but if you want to isolate, you can apply the child policy-map to the interface and see if drops continue.

Regards,

Edison

3 REPLIES
Hall of Fame Super Bronze

Traffic shaping - excessive drops?

The drops seem to be happening with Class AF21. What type of traffic matches such DSCP value in your network?

You may have to increase the queue-limit under the class to something greater than 64 until the drops stop.

I don't think the shaper is causing the issue but if you want to isolate, you can apply the child policy-map to the interface and see if drops continue.

Regards,

Edison

New Member

Re: Traffic shaping - excessive drops?

I am putting the majority of my business WAN traffic in AF21 and setting my internet and a few bandwidth hogging servers in class-default.  I increased my queue-limit on my AF21 class to 256 and I have already lost 2715 packets in 15 minutes.


I guess I dont understand why a drop-rate is being applied when the link isn't full, or the queue isn't even hitting it's minimum bandwidth limit of 17Mbs.  Should I maybe increase my Bc value to something higher than the default 4ms?

---EDIT---

After the initial loss of packets, packet loss slowed down to almost nothing, so increasing the queue limit seems to have helped. 

Hall of Fame Super Bronze

Re: Traffic shaping - excessive drops?

Packets that match that class will be placed in queues before they are dequeued. If the numbers of queues aren't enough, you will get packet drops. This behavior will occur even if you aren't reaching the maximum available throughput of the link and often seen under bursty traffic conditions.

As you stated, the situation has been resolved so please mark the thread accordingly.

Regards,

Edison

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