rate-limit an interface causes extreme slowness

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May 1st, 2009
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I have applied rate-limit to an interface to limit available bandwidth to 30mb and it is causing issues with FTP. I applied this command:

rate-limit input 30000000 15000 15000 conform-action transmit exceed-action drop

for int and out.

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John Blakley Fri, 05/01/2009 - 11:01
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Take a look at this link:


It suggests that this isn't uncommon (compliments of Joseph). You may want to look at a policy-map and class-maps to limit your traffic in/out and see if that helps.



Joseph W. Doherty Fri, 05/01/2009 - 11:32
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Your burst sizes are likely too small. (I.e. 10 packets at 1,500 bytes.)

"Testing of TCP traffic suggests that the chosen normal and extended burst values should be on the order of several seconds worth of traffic at the configured average rate. That is, if the average rate is 10 Mbps, then a normal burst size of 10 to 20 Mbps and an Excess Burst size of 20 to 40 Mbps would be appropriate.

We recommend 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

With the listed choices for parameters, extensive test results have shown CAR to achieve the configured rate. If the burst values are too low, then the achieved rate is often much lower than the configured rate. "

From: http://www.cisco.com/en/US/docs/ios/12_1/qos/configuration/guide/qcdpolsh.html

mjhagen Fri, 05/01/2009 - 13:30
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would you have a recommendation on what those values would be

Joseph W. Doherty Fri, 05/01/2009 - 16:46
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For optimal TCP performance, you could try sizing for BDP (bandwidth delay product).

cisco_lad2004 Sun, 05/03/2009 - 01:16
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I am using this in production:

Class internet-in

set dscp default

police 29 mbps 1000 kbyte conform-action transmit exceed-action drop

I take configured rate/4 then divide by 8 again to get byte value , then round it.

while adjusting my burst size, I was testing the effect and impact on network to ensure the theoretical calculation is giving the desired experience.




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