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## QoS policing on interfaces

Hi all

I have been designing a QoS policy for our campus. I have put together what I believe to be good practice, however I have a couple of questions.

The first is if I set my policing values for my 100mb access ports and i apply my policy to a 1gb uplink port to our core, will this not cause the 1gb port to operate at a reduced rate?

Second question, is 10% of the rate a fair value to use for the burst rate? For example 'police 5000000 312500 exceed-act....'

Any help greatly appreciated.

Thanks

Jamie

Cisco Employee

## Re: QoS policing on interfaces

"The first is if I set my policing values for my 100mb access ports and i  apply my policy to a 1gb uplink port to our core, will this not cause  the 1gb port to operate at a reduced rate?"

Could you please elaborate more on the above statement? Whether or not the link would operate at a reduce rate depends on the agggregate throughput leaving the gig port and whether or not they exceed the configured busrt Value.

"Second question, is 10% of the rate a fair value to use for the burst rate? For example 'police 5000000 312500 exceed-act....'"

For most scenarios the following formula will give the best burst value:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801c8c4b.shtml

"To determine the burst parameter, use this equation:

• Burst = (Rate [bps]) * 0.00025 [sec/interval])           or (maximum packet size [bits]), whichever is           greater.

For example, if you want to calculate the minimum burst value needed to       sustain a rate of 1 Mbps on an Ethernet network, the rate is defined as 1 Mbps       and the maximum Ethernet packet size is 1518 bytes. The equation is:

• Burst = (1,000,000 bps * 0.00025) or (1518 bytes           * 8 bits/byte) = 250 or 12144."

You may want to use a different burst value only when there is a specific requirement on your network (like an application) that requires the special tuning.

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