# QOS Burst Rates

Sep 3rd, 2010

Hi all,

I have been working on our companies QOS policies for the best part of a week. I think I have everything present and correct in terms of a base policy.

We have a few links that require some policing, I am having some trouble with the burst rates; I think I am on the right track, however I am worried I may not be.

If I want to police a specific class and we are limiting the rate to 10000000 bits per second, the equation below doesn't make much sense to me:

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

Do I take my configured rate of 10000000 multiply by 8000(1 byte) then divide by 8(8 bits) and multiply by 1.5? Giving me 384000000?

This to me makes no sense, am I reading the equation wrong?

Overall Rating: 0 (0 ratings)

## Replies

Lei Tian Fri, 09/03/2010 - 06:00
• Cisco Employee,

Hi,

The equation is bc=cir/8 * 1.5. On your case, cir=10Mbit/s, so the bc=15Mbit or 1875Kbytes. You might want lower the time interval, 1.5 second seems too hight.

Regards,

Lei Tian

jamieparr Fri, 09/03/2010 - 06:06

Thanks Lei Tian,

The equation is a lot simpler than I thought. I am using 1.5 seconds as Cisco recommends this as the RTT. Is this not the case? Is there a method for determining RTT?

Thanks,

Jamie

jamieparr Mon, 09/06/2010 - 01:01

Can anyone help with the RTT? I cannot find any Cisco documentation on calculating this.

Thanks

Lei Tian Mon, 09/06/2010 - 04:06
• Cisco Employee,

Hi Jamie,

Sorry, I didn't see you asked another question. The RTR or TC is determined based on how bursty will you tolerate this type of traffic. A cir 10M with 1.5 second as the TC would allow temporary burst to 100Mb/s. I would not recommend use 1.5 second to calculate the BC, unless you know this type of traffic is very bursty by nature.

A rule of thumb is to use the platform default value, or use 200 ms if you have to configure the bc.