4-14 Traffic Policing and Line Rate ConfigurationWhen configuring traffic policing and line rate, go to these sections for information you are interested in:z Traffic Policing and Line Rate Overviewz Configuring Traffic Policingz Configuring the Line Ratez Displaying and Maintaining Traffic Policing, GTS, and Line RateTraffic Policing and Line Rate OverviewWithout limits on user traffic, a network can be overwhelmed very easily. To help assign networkresources such as bandwidth efficiently to improve network performance and hence user satisfaction,QoS technologies such as traffic policing and rate limit were introduced. For example, you canconfigure a flow to use only the resources committed to it in a certain time range, thus avoiding networkcongestion caused by burst traffic.Traffic policing and line rate limit traffic rate and resource usage according to traffic specifications. Oncea particular traffic exceeds its specifications such as bandwidth assigned to it, it is policed to ensure thatit is under the specifications. Generally, token buckets are used to evaluate traffic specifications.Traffic Evaluation and Token BucketsToken bucket featuresA token bucket is analogous to a container holding a certain number of tokens. The system puts tokensinto the bucket at a set rate. When the token bucket is full, the extra tokens overflows.Evaluating traffic with the token bucketThe evaluation of traffic specifications is based on whether the number of tokens in the bucket can meetthe need of packet forwarding. Generally, one token is associated with a 1-bit forwarding authority. If thenumber of tokens in the bucket is enough for forwarding the packets, the traffic conforms to thespecification and is called conforming traffic; otherwise, the traffic does not conform to the specificationand 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. It isusually set to the committed information rate (CIR).z Burst size or the capacity of the token bucket. It is the maximum traffic size that is permitted in eachburst. It is usually set to the committed burst size (CBS). The set burst size must be greater thanthe maximum packet size.Evaluation is performed for each arriving packet. In each evaluation, if the number of tokens in thebucket is enough, the traffic conforms to the specification and the tokens for forwarding the packet aretaken away; if the number of tokens in the bucket is not enough, the traffic is excessive.
