CBWFQ question

Unanswered Question
Jun 15th, 2007

Following is quoted from Cisco QOS training material talking about CBWFQ.

It says "the bandwidth assigned to the packets of a class

determines the order in which packets are sent. " How come the bandwidth determines the order of the packets being sent?

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For CBWFQ, the weight for a packet belonging to a specific class is derived from the bandwidth that you

assigned to the class when you configured it. Therefore, the bandwidth assigned to the packets of a class

determines the order in which packets are sent. All packets are serviced fairly based on weight.

I have this problem too.
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Brandon Buffin Fri, 06/15/2007 - 13:10

With CBWFQ, unlike WFQ, preference is given based on packet classification. The CBWFQ algorithm determines this preference based on the bandwidth command given to each class in the class map. So, in general, packets in a class with a higher bandwidth will go before packets from a lower bandwidth class because that queue is being serviced more often. This will always be the case for a priority queue.

Hope this helps.

Brandon

ciscoforum Tue, 06/19/2007 - 07:00

Thanks. If that's the case then the lower bandwidth class always has to wait for the higher bandwidth class packets to be sent out. It will creates delay for the lower bandwidth traffic, right?

Brandon Buffin Tue, 06/19/2007 - 07:07

This is true of a priority queue which is configured using the "priority" command as opposed to the "bandwidth" command. In this case, packets in the priority queue will always be sent before other packets. With CBWFQ, the higher bandwidth queue will simply be serviced more often. In some cases, this means that these packets will be sent before others, but not always. In either case, this does create delay for packets in a lower bandwidth class or packets not in the priority queue. When certain packets are given priority treatment either by way of the priority queue or by way of assigning them to a higher bandwidth class, other packets suffer.

Hope this helps. If so, please rate the post.

Brandon

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