Need help with setting burst size using traffic policing

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Oct 4th, 2007
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I am trying to determine the burst size when using traffic policing. I have read several documents on it but am having difficulty understanding how to determine the burst size and max burst size. I would like to rate limit a specific VLan to 300Mbs inbound on a 6509 running native 12.1(13)E9. Is there a simple formula I can use to determine this or could someone direct me to a link that depicts how to determine burst size. I know it is not "rocket science" but I am having difficulty understanding how to do it.

Thanks much

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htarra Wed, 10/10/2007 - 12:46
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To determine the burst parameter, you can use this equation:

Burst = (Rate [bps]) * 0.00025 [sec/interval]) or (maximum packet size[bits]), whichever is greater.

For example, if you want to calculate the minimum burst value needed to sustain a rate of 5 Mbps on an Ethernet network, the rate is defined as 5 Mbps and the maximum Ethernet packet size is 1518 bytes. The equation is: Burst = (5,000,000 bps * 0.00025) or (1518 bytes * 8 bits/byte) = 1250 or 12144.

The larger result is 12144, which you round to 13 kbps.

Kevin Dorrell Wed, 10/10/2007 - 17:28
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One thing to bear in mind is that the burst is expresed differently for shaping than for policing.

For shaping, it is expressed in bits, as described above, and in:

For policing, it is expressed in bytes, so you need 1/8 of these values:

I wish it was more consistent.

Kevin Dorrell


Hi Kevin,

I'm debating with the same issue, but my case is slightly different.

We're policing on the egress interfaces of a 4507 switch limiting each interface for a certain amount of bandwidth (from 512Kb to 20Mb), the problem we're facing is setting the right burst size.

We tried different figures and tested the outcome, which came opposite to what's documented in Cisco configuration guides.

For some silly reason if you keep the same burst size but you change the packet size you won't get the same result!

The question now is how can we set the burst regardless of the packet size to always let the switch police to the configured value?



Ps: the below document proved to be not accurate

Kevin Dorrell Fri, 10/12/2007 - 00:10
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That is interesting what you have observed. When you have characterized and understood the real-life behavior, could you post your findings here please so we can all share in the experience?

I sounds like a bug to me. When you configure with a '?' for each parameter, what units does it give for the burst size?


Kevin Dorrell


Hi Kevin,

I'm attaching the Excel file showing the different figures. The other interesting point is the values showed in Column I. It looks like on the way back the value is always equal to the CIR despite we're not policing on the egress!

The test layout is as below:

Laptop (packet generator) ---->4500----->router



Ps: Also it's noticeable that when the frame size is small, the value observed on the router is higher than the policed CIR (for example, in row 3, we're policing to 128k but 141000bps are reaching the router!)


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