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:

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:

Thanks a lot Rene. Excellent post !!!

Thanks for this clear and concise lesson, much appreciated

3.200 50ms: How did you get the 3.200?

Thanks.

Hi, Rene

What do you mean the physical access rate? For me it is not clear, could give an explanation on this?

Hi Rene,

That was a great bunch of posts. Just for the benefit of students, can you please state this?

Bc and Be - are they supposed to be expressed in bits, bytes ? vary with different families/models of Cisco devices?

Thanks,

Ayyappan