Velocity Reviews > Question on rate-limit command

# Question on rate-limit command

John
Guest
Posts: n/a

 07-04-2003
I am having some difficulty understanding how to determine the effect of
the burst amounts on peak bandwidth. For an easy math example:

rate-limit output 8000000 1000000 1000000 conform-action transmit
exceed-action drop

This will presumably limit the interface to 8Mbits/sec. I am allowing a normal
burst of 1Mbytes/sec (or 8Mbits/sec) and set max burst the same as normal
burst to remove it from the scenario.

My question is, to calculate peak allowed bandwidth do I add the normal
burst to the configured rate which gives a bursted allowance of
16Mbits/sec or does it allow 8Mbits/sec with no burst beyond since my
burst rate is the same as the configured rate? My intent is to put a hard
limit on a circuit that cannot be exceeded even briefly.

I have read the Cisco material on the subject and know what they
consider the optimal settings but they do not state definitively that
traffic will not exceed the configured nominal rate. Has anyone been able
to do any testing to determine whether burst rate settings allow traffic
to exceed the configured rate?

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John Holmes
http://www.velocityreviews.com/forums/(E-Mail Removed)

Chris Bisazza
Guest
Posts: n/a

 07-06-2003
> rate-limit output 8000000 1000000 1000000 conform-action transmit
> exceed-action drop
>
> This will presumably limit the interface to 8Mbits/sec. I am allowing a

normal
> burst of 1Mbytes/sec (or 8Mbits/sec) and set max burst the same as normal
> burst to remove it from the scenario.

I think you're misinterpreting this.

You are rate-limiting to 8 Mbps, correct. The burst, however, has nothing to
do with this. It is a measure of how that 8 Mbps limit is applied. By
specifying 1 MB, you have configured the router to allow a 1 MB
uninterrupted stream on the rate-limited interface before actually
rate-limiting the output. When this stream of data is through, then whatever
parameters you have specified are applied to any traffic over and above the
8 Mbps rate.

Remember that when packets arrive, they do so at line speed. The burst
parameters make more sense if you look at them as a timeslot. In your
example, you set this timeslot to one second by specifying the amount of
data that would arrive in a second.

The max burst is the maximum amount of traffic that can pass in a given
timeslot (as specified by the burst) no matter what. If a connection is
idle, then the first timeslot will be allowed this amount of traffic.

> My question is, to calculate peak allowed bandwidth do I add the normal
> burst to the configured rate which gives a bursted allowance of
> 16Mbits/sec or does it allow 8Mbits/sec with no burst beyond since my
> burst rate is the same as the configured rate? My intent is to put a hard
> limit on a circuit that cannot be exceeded even briefly.

Rate-limit cannot work like that, because data inherently consists of
packet-size bursts at line speed. The only way you can achieve what you ask
for over here is by using a serial link and specifying a clockrate of the
data speed that you want.

Rate limiting, on the other hand, allows you to set limits on data
throughput in a bursty environment such as a typical network.

> I have read the Cisco material on the subject and know what they
> consider the optimal settings but they do not state definitively that
> traffic will not exceed the configured nominal rate. Has anyone been able
> to do any testing to determine whether burst rate settings allow traffic
> to exceed the configured rate?

I think I answered that above.

Hope that helps,
Chris.

mysticwave
Junior Member
Join Date: Jul 2007
Posts: 1

 07-23-2007
Hello all, newbie here, I googled 'rate-limit' and found this forum. Greetings!

I read the above reply on the question, but still dont quite understand the 'time-slot' metaphor and hope someone can shed some light.

We have rate-limit command on a 3825 Router as follows:
rate-limit output 2000000 100000 200000 conform-action transmit exceed-action drop

Using the metaphor, the 'time-slot' here is 100000bytes (0.8Mbits) at a rate of 2Mbps. And the maximum allowed size of the time-slot is 200000bytes (1.6Mbits).??

It doesnt matter what burst size you have specified, as long as 2Mbps is the rate-limit, you should get 2Mbps datarate?

However, on performing a normal file transfer, the data rate is observed (using NetPerSec) to be at an average of 1.7Mbps.

Confused

agros14
Junior Member
Join Date: Apr 2008
Posts: 2

 04-22-2009
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:

Burst-normal = configured rate * 1/8 * 1.5 seconds (1/8 for convert bit to byte)

Burst-max = Burst-normal * 2

For Example
rate-limit output 496000 93000 186000 conform-action transmit exceed-action drop

Junior Member
Join Date: Feb 2010
Location: Jordan
Posts: 1

 02-21-2010
Quote:
 Originally Posted by agros14 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: Burst-normal = configured rate * 1/8 * 1.5 seconds (1/8 for convert bit to byte) Burst-max = Burst-normal * 2 For Example rate-limit output 496000 93000 186000 conform-action transmit exceed-action drop I hope this can help you out
i have a rate limit to 200 Mbps on a Giga interface. and accourding the mentioned formula the burst will be 37 Mbps !!!! does it make any sense ???

area0
Junior Member
Join Date: Apr 2010
Posts: 1

 04-08-2010
Yes,you are right my friend Alaa but inorder to be more accuracy it will be 38.4 Mbps

Hafez Bakry,,,