75% OF AVAILABLE BANDWIDTH

[mohammedrafiq]

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,

[kwillacey]

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]

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]

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]

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).