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John
Guest
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 Posted: Fri Jul 04, 2003 12:03 pm Post subject: Question on rate-limit command |
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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
jwholmes@earthlink.net |
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Chris Bisazza
Guest
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| Posted: Sun Jul 06, 2003 1:22 pm Post subject: Re: Question on rate-limit command |
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| Quote: | 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.
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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.
| Quote: | 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.
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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.
| Quote: | 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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I think I answered that above.
Hope that helps,
Chris. |
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