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Two-Rate, Three-Color Policer (Two Buckets)

The third main option for CB Policing uses two separate policing rates. The lower rate is the previously discussed committed information rate (CIR), and the higher, second rate is called the peak information rate (PIR). Packets that fall under the CIR conform to the traffic contract. Packets that exceed the CIR, but fall below PIR, are considered to exceed the contract. Finally, packets beyond the PIR are considered to violate the contract.The key difference between the single-rate and dual-rate three-color policer is that the dual-rate method essentially allows sustained excess bursting. With a single-rate, three-color policer, an excess burst exists, but the burst is sustained only until the Be bucket empties. A period of relatively low activity has to occur to refill the Be bucket. With the dual-rate method, the Be bucket does not rely on spillage when filling the Bc bucket, as depicted in Figure 16-7. (Note that these buckets are sometimes called the CIR and PIR buckets with dual-rate policing.)

Figure 16-7 Refilling CIR and PIR Dual Token Buckets

 
The refilling of the two buckets based on two different rates is very important. For example, imagine you set a CIR of 128 kbps (16 kilobytes/second), and a PIR of 256 kbps (32 kBps). If 0.1 second passed before the next packet arrived, then the CIR bucket would be replenished with 1600 tokens (1/10 of 1 second

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