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## Forum Replies

1. Hi Davis,

It depends what you want to achieve.

For example, the ISP will probably use policing since they don’t want to waste resources to buffer customer packets exceeding traffic.

The customer however probably doesn’t want packet drops so they will configure shaping to match the rate that the ISP polices at.

Also keep in mind that shaping adds delay so it’s not a good idea to use this for realtime traffic like VoIP.

Rene

Policing is typically implemented by ISPs to limit the traffic of their customers so it will depend on the traffic contract that they sell you.

The single rate two color policer might be a bit unfair since it doesn’t allow bursting. With a CIR of 128kbps then you’ll only be able to reach 128kbps if you keep sending traffic non-stop.

The single rate three color policer allows bursting. Since data traffic is “bursty” by nature, this is probably a bit more fair to use. When your connection is idle, you can accumulate up to the Be and spend your Bc+Be

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3. Hi Rene,

How actually we calculate the BC and BE base on the CIR. What is the standard we should set?

Is the calculation below correct?

``````CAR (Police) formula:
Bc = CIR / 8 * Tc           (Default Tc = 1.5 seconds)
Be = 2 * Bc
``````

Davis

4. Hi Davis,
How Be is calculated depends on what type of policer is being used.

``````Single Rate, Two Color Policer
Bc = CIR/32
Be = 0
``````
``````Single Rate, Three Color Policer
Bc = CIR/32
Be = Bc
``````
``````Two Rate, Three Color Policer
Bc = CIR/32
Be = PIR/32 (Peak Information Rate)
``````

5. Hi Chris,

With a policer, each token represents a single byte so that’s how you know the size of the bucket. When you police at 128000 bps, that’s a bucket that can store 128000 / 8 = 16000 bytes.

Rene

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