CBWFQ: shape AND bandwidth?

Unanswered Question
Aug 12th, 2010

i came to an example where there is a following config:

policy-map TEST

  class class1

     bandwidth 128

     shape average 256

  class class-default

     fair-queue

i remember that bandwidth set the minimum bandwidth, while police/shape set the maximum bandwidth for the class.

in this example, i know that the policy reserve 128 for class1.

if the BW exceed 128, the traffic goes to class-default and never "hit" the shape command....

i cant seem to figure how the shape command help here. as far as i know, bandwidth and shape/police should have the same value.

otherwise i really cant understand this configuration.

any1 have a clue?

I have this problem too.
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Giuseppe Larosa Sun, 08/15/2010 - 07:30

Hello Arnon,

>> if the BW exceed 128, the traffic goes to class-default and never "hit" the shape command....

this is not correct, traffic classified in the class is in the class this is not a policer but a policy-map

This configuration provides a minimum of 128 kbps when the link is full to a maximum of 256 kbps to IP flows classified in this traffic class

without the shape command if no traffic is present on other traffic classes, the only upper limit is the interface speed (true or the result of a shaper in hierarchical QoS)

setting shape command to the same value of bandwidth makes minimum= maximum that is removes elasticity from CBWFQ

Hope to help

Giuseppe

arnon l Sun, 08/15/2010 - 10:28

>> if the BW exceed 128, the traffic goes to class-default and never "hit" the shape command...

actually it is correct. i a class exceed its given BW, the rest of the traffic is handled by class default.

i found the answer yesterday....

the shaper was configured for cases where there is no congestion (thus bandwidth command does not take effect) and we want to limit the class.

Giuseppe Larosa Mon, 08/16/2010 - 14:28

Hello Arnon,

>> actually it is correct. i a class exceed its given BW, the rest of the traffic is handled by class default.

not in my experience at least on high end devices like C7500 or 12000 I never see this to happen

and also according to Documentation

>> The default class of the policy map (commonly known as the class-default class) is the class to which traffic is directed if that traffic does not satisfy the match criteria of other classes whose policy is defined in the policy map

http://www.cisco.com/en/US/docs/ios/qos/configuration/guide/config_wfq_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1075725

Hope to help

Giuseppe

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