2-9Figure 2-8 Evaluate traffic with the token bucketTokenbucketPackets droppedPacketclassificationPackets to be sentthrough this interfacePackets sentTokens are put into thebucket at the set rateThe evaluation for the traffic specification is based on whether the number of tokens in the bucket canmeet the need of packet forwarding. If the number of tokens in the bucket is enough to forward thepackets (generally, one token is associated with a 1-bit forwarding authority), the traffic conforms to thespecification, and the traffic is called conforming traffic; otherwise, the traffic does not conform to thespecification, and the traffic is called excess traffic.A token bucket has the following configurable parameters:z Mean rate: At which tokens are put into the bucket, namely, the permitted average rate of traffic. Itis usually set to the committed information rate (CIR).z Burst size: the capacity of the token bucket, namely, the maximum traffic size that is permitted ineach burst. It is usually set to the committed burst size (CBS). The set burst size must be greaterthan the maximum packet size.One evaluation is performed on each arriving packet. In each evaluation, if the number of tokens in thebucket is enough, the traffic conforms to the specification and the corresponding tokens for forwardingthe packet are taken away; if the number of tokens in the bucket is not enough, it means that too manytokens have been used and the traffic is excessive.The working mechanism of line rateWith line rate configured on an interface, all packets to be sent through the interface are firstly handledby the token bucket of line rate. If there are enough tokens in the token bucket, packets can beforwarded; otherwise, packets are put into QoS queues for congestion management. In this way, thetraffic passing the physical interface is controlled.
