CB Policing with Be

CB policing uses a simpler algorithm than CAR when a Be value has been configured. CB policing categorizes packets into three groups: conform, exceed, and violate. By using three categories, a very simple and very useful algorithm can be used.

Just like with CAR, to understand how CB policing works is to understand how the buckets are filled and drained. CB policing continues to replenish Bucket1 when a packet arrives. If Bucket1 is full, the extra, or spilled tokens, replenish Bucket2. If Bucket2 fills, the excess tokens spill onto the ground and are wasted. Figure 5-16 shows the basic process:

Figure 5-16 Refilling Dual Token Buckets with CB Policing

Refill Bytes Upon Arrival of Packet, per Formula:

(New_packet_arrival_time - previous_packet_arrival_time) * Policed_rate / 8

Spillage Falls into Bucket2

Spillage Falls into Bucket2

Spillage

Spillage

With Bc and Be configured, CB policing uses dual token buckets, and the algorithm is simple:

1 If the number of bytes in the packet is less than or equal to (<=) the number of tokens in Bucket1, the packet conforms. CB policing removes tokens from Bucket1 equal to the number of bytes in the packet, and performs the action for packets that conform to the contract.

2 If the packet does not conform, and the number of bytes in the packet is less than or equal to (<=) the number of tokens in Bucket2, the packet exceeds. CB policing removes tokens from Bucket2 equal to the number of bytes in the packet, and performs the action for packets that exceed the contract.

3 If the packet neither conforms nor exceeds, it violates the traffic contract. CB policing does not remove tokens from either bucket, and performs the action for packets that violate the contract.

Essentially, packets that fit within Bc conform, those that require the extra bytes allowed by Be exceed, and those that go beyond even Be are considered to violate the traffic contract.

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