Shaping with Burst up to interface speed

One of the QoS topics that CCIE R&S students have to master is shaping and how calculating 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:

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:

With a CIR rate of 512 Kbps, 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:

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:

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