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75% OF AVAILABLE BANDWIDTH

mohammedrafiq
Level 1
Level 1

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,

4 Replies 4

kwillacey
Level 3
Level 3

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.

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
Level 10
Level 10

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

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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