packet droping inside LLQ

[jouk]

Hello,

I have the following policy-map configured on the serial interface:

policy-map VOICE_UP

  class VOICE

   priority 26

  class TELEMETRY

   priority 110

  class USER1

   shape average 1536000

   bandwidth 670

  class USER2

   shape average 1536000

   bandwidth 670

  class class-default

   fair-queue

!

interface Serial4/0

bandwidth 1536

max-reserved-bandwidth 100

service-policy output VOICE_UP

load-interval 30

According this policy-map I have four type of traffic - voice, telemetry, user1 and user2. Telemetry traffic has very low bitrate, but information transfered in this traffic is very important. That is the reason why I'm make corresponding class using 'priority' command.

The problem:

When I use 'sh policy-map interface serial 4/0 | inc 30 sec' command to monitor policy-map on interface, I see drop rate in output of this command. At the same time, I know that the capacity of channel is not exceeded.

Example:

#sh policy-map interface serial 4/0 | inc 30 sec

      30 second offered rate 3000 bps, drop rate 0 bps

      30 second offered rate 5000 bps, drop rate 2000 bps

      30 second offered rate 491000 bps, drop rate 0 bps

      30 second offered rate 252000 bps, drop rate 0 bps

      30 second offered rate 0 bps, drop rate 0 bps

Why this drop rate occur ?

[sean_evershed]

Hi, when you specify the priority command for a class, it takes a  bandwidth argument that gives maximum bandwidth. In the event of congestion, policing is used to drop packets when the bandwidth is exceeded.

Therefore you may need to assign more bandwidth to the Telemetry class.

[jouk]

I agree with you. But I think that congestion is not occur , isn't it ? As you can see from configuration above the total channel capacity is 1536 kpbs and used amount of bandwidth is about 751 kbps (as shown by the 'sh policy-map interface serial 4/0 | inc 30 sec' command). So what is the reason to drop the packets ?

[jouk]

Does any one know why packet droping occur inside of configured policy-map shown at my first post?

Thank you.