75% OF AVAILABLE BANDWIDTH

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Jan 19th, 2009
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Hi<

Please can someone veify that by default you can only use 75% of available bandwidth of your physical interface, i mean 750M of 1G, otherwise you configure max-bandwith command to utilize 100%, but then it degrade the performance ?


Regards,

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kwillacey Mon, 01/19/2009 - 07:26
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I configured some QoS the other day and used more than 75% of the bandwidth and got an error so I would say by default you can't. I would not say to use 100% I would say use as much as you need so as to not take any chances. I have actually changed one to 95% and there was no degrade in performance.

mohammedrafiq Mon, 01/19/2009 - 07:34
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Thanks for the reply,

I would say it depend on packet size as well, wether its 64 or 1518.

The formula for Ethernet throughput is as follows:

- Frame rate (fps) = network speed ÷ ((frame size + 8 + 12) x 8)

- Data throughput (Mbps) = frame rate x frame size x 8


Data throughput is therefore dependent on packet size, where frame overhead has a bigger impact on smaller data loads.


With a 1518 packet size:

- Frame rate = (100 x 10e6) ÷ ((1,518 + 8 + 12) x 8) = 8,127 fps

- Data throughput = 8,127 × 1,518 × 8 = 98.69 Mbps


With a 64 packet size:

- Frame rate = (100 x 10E6) ÷ ((64 + 8 + 12) x 8) = 148,809 fps

- Data throughput = 148,809 × 64 × 8 = 76.19 Mbps



Regards,


milan.kulik Mon, 01/19/2009 - 07:54
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Hi,


I'd rephrase your sentence:

You can only reserve 75% of available bandwidth to your CBWFQ classes with the bandwidth command by default leaving the 25% for the default class plus control plane protocols.


If you need to reserve more, you can use max-bandwith command to increase this limit.

But you should know very well the risk then.


BR,

Milan

Joseph W. Doherty Mon, 01/19/2009 - 08:09
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As Milan correctly notes, on most platforms that support CBWFQ, you can define explicit class bandwidth reservations that sum up to 75% (unless the default is changed), however any class can still use 100% of the link (assuming it's not otherwise limited).

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