# How do i calculate Normal Burst Size (in bytes) on an interface.

Endorsed Question
Jul 8th, 2008

I want to limit a customer's traffic to 512kpbs on the fa0/1.2 subinterface of my router. the command for that is 'rate-limit {input | output} bps burst-normal burst-max

conform-action action exceed-action action'

Can anyone help me with the formulae to calc the normal burst size, and an idea of what the burst-max can be

1 vote
Cisco Endorsed by emcaaman

Hi,

My configs go in this way for 512K

512000 63000 63000

Bits Bytes Bytes

As per cisco.

bps: -Average rate in bits per second. The value must be in increments of

8 kbps.

burst-normal: Normal burst size in bytes. The minimum value is bps divided by 2000.

burst-max:- Excess burst size in bytes.

If you want to set up the BW for input and output on the interface for 512 K.

The bps should be in bits value. so thats the first value.

Burst normal would be how much max can the link burts to same 512 so the value above in in bytes.

Burts max:- how much is the max bw after which the packets can be dropped its again 512k. Value in bytes.

So i feel it goes this way. apply the values and test the in and out BW and let me know if you dont get the results limited to 512 Kb.

Hope this helps,

Regards,

Pravin

Overall Rating: 5 (2 ratings)

## Replies

I have a rule of thumb that allows for 1/4 of BW.

512000bps =>128000b or 16000Byte burst.

HTH

Sam

tdrais Tue, 07/08/2008 - 05:38

This is what you will find in the cisco doc

Policing Traffic with CAR

CAR embodies a rate-limiting feature for policing traffic. When policing traffic with CAR, Cisco recommends the following values for the normal and extended burst parameters:

normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds

extended burst = 2 * normal burst

Now why they choose these numbers is unclear. These seem kinda big to me but they may be choosing a value that is safe for most conditions.

The only bad thing about making this too high is that you waste memory in your router. Larger values also tend to make your utilization of your underlining circuit more bursty but it depends on your situation if this will cause any issues.

In general you will need more buffers (ie larger burst) when the ratelimit you are tryng to obtain is a lot lower than than the physical speed of the interface.

It is farily easy to see when you set it too low because the show rate command will show packet drop but the transfer rate will be below the limit you configured.

Pravin Phadte Tue, 07/08/2008 - 06:13

Hi,

My configs go in this way for 512K

512000 63000 63000

Bits Bytes Bytes

As per cisco.

bps: -Average rate in bits per second. The value must be in increments of

8 kbps.

burst-normal: Normal burst size in bytes. The minimum value is bps divided by 2000.

burst-max:- Excess burst size in bytes.

If you want to set up the BW for input and output on the interface for 512 K.

The bps should be in bits value. so thats the first value.

Burst normal would be how much max can the link burts to same 512 so the value above in in bytes.

Burts max:- how much is the max bw after which the packets can be dropped its again 512k. Value in bytes.

So i feel it goes this way. apply the values and test the in and out BW and let me know if you dont get the results limited to 512 Kb.

Hope this helps,

Regards,

Pravin

Thanks pravin

i eventually used this formula: N burst = (configured rate * 1.5)/8, as used in cisco one of cisco's examples on the subject.