specification, and the traffic is called "conforming traffic." Otherwise, the traffic does not conform to the specification, and the traffic is called "excess traffic."

A token bucket has the following configurable parameters:

•Mean rate—Rate at which tokens are put into the bucket, or the permitted average rate of traffic. It is usually set to the committed information rate (CIR).

•Burst size—The capacity of the token bucket, or the maximum traffic size permitted in each burst. It is usually set to the committed burst size (CBS). The set burst size must be greater than the maximum packet size.

One evaluation is performed on each arriving packet. In each evaluation, if the number of tokens in the bucket is enough, the traffic conforms to the specification and the tokens for forwarding the packet are taken away. If the number of tokens in the bucket is not enough, it means that too many tokens have been used and the traffic is excessive.

Working mechanism of rate limit

With rate limit configured on an interface, all packets to be sent through the interface are firstly handled by the token bucket of rate limit. If the token bucket has enough tokens, packets can be forwarded. Otherwise, packets are put into QoS queues for congestion management. In this way, the traffic passing the physical interface is controlled.

Figure 461 Rate limit implementation

With a token bucket used for traffic control, when the token bucket has tokens, the bursty packets can be transmitted. When no tokens are available, packets cannot be transmitted until new tokens are generated in the token bucket. In this way, the traffic rate is restricted to the rate for generating tokens, the traffic rate is limited, and bursty traffic is allowed.

Priority mapping

Concepts

When a packet enters a network, it is marked with a certain priority to indicate its scheduling weight or forwarding priority. Then, the intermediate nodes in the network process the packet according to the priority.

When a packet enters a device, the device assigns to the packet a set of predefined parameters (including the 802.1p priority, DSCP values, and local precedence).

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