Shaping with Burst up to interface speed

One of the QoS topics that CCIE R&S students have to master is shaping and how to calculate the burst size. In this short lesson I want to explain how to calculate the burst size so that you can allow bursting up to the physical interface rate after a period of inactivity. Let’s take a look at an example:

router two pvcs

Above we have a router with two PVCs. The physical AR (Access Rate) of this interface is 1536 Kbps. The PVC on top has a CIR rate of 512 Kbps and the one at the bottom has a CIR of 64 Kbps. Let’s say we have the following requirements:

  • Each PVC has to be shaped to the CIR rate.
  • After a period of inactivity, both PVCs should be able to burst up to the physical access rate.
  • Tc should be 50 Ms.

So how do we calculate this? Let’s first calculate the Bc for the first PVC that has a CIR of 512 Kbps:

512.000 bits 1000 ms
51.200 bits 100 ms
25.600 50 ms

With a CIR rate of 512 Kbps it means we can send 512.000 bits in 1000 ms. In 50 ms we will be able to send 25.600 bits. Now we have to calculate the number of Be bits so that we can burst up to the AR rate. The physical access rate is 1536 Kbps:

1536.000 bits 1000 ms
153.600 bits 100 ms
76.800 bits 50 ms

So with a Tc of 50 milliseconds we have to send 76.800 bits to get up to the physical access rate. So what value should we configure for our Be?

The Bc and Be combined should be 76.800 bits to get to the physical access rate:

76.800 bits – 25.600 bits (Bc) = 51.200 bits

Set your Bc to 25.600 bits and your Be to 51.200 bits and you’ll be able to burst up to the physical access rate.

Now let’s calculate this for the 64 Kbps link, first the Bc:

64.000 bits 1000 ms
6.400 bits 100 ms
3.200 bits 50 ms

So the Bc is 3.200 bits. Now we can calculate the Be:

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Forum Replies

  1. Please help me out to implement QoS over ADSL link. I want to prioritize my voice traffic and assign 15% of BW.
    Please check the config of my usual WAN link not sure what would be on ADSL.

    class-map match-any Client-Signal-Class
     match precedence 3
     match ip dscp af31
     match ip dscp cs3
    class-map match-any Client-VOIP-Class
     match precedence 5
     match ip dscp ef
    policy-map 50Mb-VOIP-PMAP
     class Client-VOIP-Class
      priority percent 10
     class Client-Signal-Class
      bandwidth percent 5
     class class-default
    policy-map 50Mb-VOIP
     class class-default
    ... Continue reading in our forum

  2. Hello Prashant,

    Both policing and shaping have a common goal: to “rate-limit” exceeding traffic.

    How they do it is different though. A shaper will “buffer” the traffic while a policer “drops” the traffic.

    A practical example could be an ISP router that is connected to multiple customer routers. Let’s say that the routers are connected with GigabitEthernet links but the customers are only paying for a 100/100 Mbit connection.

    The ISP will then use a policer to drop all incoming traffic that exceeds 100 Mbit.

    On the customer end, you probably don’t want your traf

    ... Continue reading in our forum

  3. Hello Jose

    Yes indeed this can be the case. You must coordinate with the ISP so that your shaping policy on the edge of your network doesn’t send more traffic than the ISP is configured to handle especially if the ISP implements a more strict policy than your edge router. Otherwise, as you describe in your post, traffic can and will be dropped by the ISP.

    I hope this has been helpful!


  4. Thank you Laz!
    Sorry I have another question; I’m confused about what is the value I should use for BC, what exactly has to be in consideration to decide this value? In some documentation I found BC=(CIR/8)*1,5 but I guess that it can’t be a static rule, I think it should be diferent for UDP voice than TCP for example. Could you help me to clear this doubt?

  5. Hello Jose

    There’s no single answer to your question. You are correct in that the Bc that you will use will depend on the traffic that you want to shape. As mentioned in the lesson, Bc depends upon the Tc and upon the actual Committed Information Rate or the required shaped speed of the link, like so:

    Bc = Tc * CIR

    And Tc will depend on the kind of traffic that will be shaped. If you’re shaping voice, Tc should be around 10 ms. So Bc = 10 ms * CIR. Now if your CIR is very large like 5Mbps, then Bc will be 50 kilobits, but if your CIR is relatively small li

    ... Continue reading in our forum

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