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Traffic shaping in Cisco IOS

Recently I’ve received numerous questions about the traffic shaping functionality of Cisco routers (and how it can be used to create artificial queues). Obviously it was time to write a long-overdue article on traffic shaping basics, the Cisco IOS algorithms and queuing structures.

Read the traffic shaping article in CT3 wiki

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  1. Ivan,
    I just want to say that this is one of the _BEST_ brief explanations I have ever read!

    Keep in that way!!!

  2. Hi Ivan,

    I've read your article at and that is a great explanation about the operation of traffic shaping. I still have some questions and I hope you can make it clear to me:

    1/ Assume Bc = 8000 bits and a 1500-byte (12000-bit) packet comes to the router. Will the router put the whole packet into shaping queue, or will it send the first 8000 bits of the packet and waiting for the next Tc to send the remaining 4000 bits?

    2/ Are the shaping queue and the software (output) queue unrelated queues? For example, we configure the router to use CBWFQ for the shaping queue, and WFQ for the software queue. Assume shaping is active and TX Ring is full, the packet will be put into one of the class queues of the shaping queue, and then requeued into one of the dynamic flow queues of the software queue. Is that right?

    Your idea would be greatly appreciated !

    1. #1 - A router never slices the packets while shaping them. The whole packet goes into the shaping queue until enough tokens accumulate to let it pass.

      #2 - Yes, they're unrelated. What you can or cannot configure is a different matter (and I haven't tracked that for years).


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