Question on rate-limit command

Discussion in 'Cisco' started by John, Jul 4, 2003.

  1. John

    John Guest

    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?

    --
    ___________
    John Holmes
     
    John, Jul 4, 2003
    #1
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  2. > 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.
     
    Chris Bisazza, Jul 6, 2003
    #2
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  3. John

    mysticwave

    Joined:
    Jul 23, 2007
    Messages:
    1
    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:hmm2:
     
    mysticwave, Jul 23, 2007
    #3
  4. John

    agros14

    Joined:
    Apr 3, 2008
    Messages:
    2
    CAR values

    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
     
    agros14, Apr 22, 2009
    #4
  5. John

    Aladdin.Hakim

    Joined:
    Feb 21, 2010
    Messages:
    1
    Location:
    Jordan
    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 ???
     
    Aladdin.Hakim, Feb 21, 2010
    #5
  6. John

    area0

    Joined:
    Apr 8, 2010
    Messages:
    1
    Yes,you are right my friend Alaa but inorder to be more accuracy it will be 38.4 Mbps

    Hafez Bakry,,,
     
    area0, Apr 8, 2010
    #6
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