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Reg: Help of QOS

Hi Guys,

Could you please advise how to configure the shaping on the cisco router. I know about the first field but I am not sure how to calculate the second and third field in the command. For example

Shape 1000000 ? ?

The 1mb is the cir and the second field is the bc (the interface speed) and the third field is excess burst (be) in bytes. Am I right ?

If yes could you please let me know how to calculate them. I have seen on cisco as well as other sites but it is so confusing.

Tks & Regards


Re: Reg: Help of QOS


A simple explanation on simple shaping:-

Tc - Time interval, measured in milliseconds, over which the commited burst (bc) can be sent. With many shapiing tools, Tc = Bc/CIR.

Bc - Commited burst size, measured in bits. This is the amount of traffic that can be sent during the Tc interval. Typicall defined in the traffic contract.

CIR - Committed information rate, in bits per second, which defines the rate of a VC according to the buisness contract with the provider.

Shaped rate - the rate, in bits per second, to which a particular configuration wants to shape the traffic. It may or may not be set to the CIR.

Be - Excess burst size, in bits. This is the number of bits beyond the Bc that can be sent after a period of inactivity.

Shaping:- Tc=Bc/shaping rate

You can of course have other falvours of shaping:-

Class-Based Shaping

Shaping for voice using LLQ and a Small Tc

Shaping by banwidth percent

Class-based shaping to a Peak Rate

Frame-relay Traffic Shaping


Re: Reg: Help of QOS

Shaping buffers pkts in a queue & sends Bc pkts every Tc interval. So even if you specify a CIR value, Bc & Be values will be calculated automatically by the IOS.

You have to be more specific about Bc & Be in policing, where if a couple of Tc interval passes without data then they will be accumulated & sent the next time pkt of bigger size comes to the queue.

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