Kevin Dorrell, CCIE #20765

24 Mar 2008

QoS study week

Filed under: Frame Relay, QoS — dorreke @ 12:33

This week, I want to concentrate some effort on understanding QoS, particularly shaping and policing.  I am fed up with losing 2 to 8 points every time I do a mock lab because I have not got a firm grasp of the subject.  Sometimes I muddle through.  I know the commands, and usually I can match up the parameters in the requirements with the parameters of some command or other, but I am not confident.

I read the chapter in the Wendell Odom book this morning.  It all seemed to make sense, but it left me wondering about two points:

  1. If you are traffic-shaping, and you have a frame in the queue that is longer than BC+BE, and the bucket overflows at BC+BE, how do you ever get enough tokens in the bucket to transmit it?
  2. If CB policing replenishes the bucket(s) with a prorata number of tokens according to how long ago was the last frame transmitted, if there is a long silence so the last frame transmitted was a very long time ago, then is the bucket replenished with an enormous number of tokens.  Doesn’t that make the traffic very very bursty?
  3. On page 566, he states “FRTS cannot classify traffic in order to shape a subset of traffic on a particular VC.”  But you can specify a service-policy within a map-class.  And in the corresponding policy map, you can specify policing under the class-default.  So what happens if you try and specify other classes in the policy map?

No doubt these things will become clearer as I read more.


1 Comment »

  1. If the frame is longer than the amount of tokens, then the frame is queued or dropped. It is queued if there is enough buffer and transmitted during the next tc. The be is only used for the first tc thus you get bc+be. After the first interval you send bc.

    Here are some good references:

    I would be interested in seeing what you come up with in your review.

    Comment by Doug — 25 Mar 2008 @ 02:22

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