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DQOS Exam Certification Guide - Cisco press.pdf
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322 Chapter 5: Traffic Policing and Shaping

Foundation Topics

Traffic shaping solves some of the most important issues relating to quality of service (QoS) in networks today. Even when policing is not also used, traffic shaping solves a category of delay and loss problems called egress blocking, which can occur in all multiaccess WANs, such as Frame Relay and ATM networks. Traffic shaping is covered extensively in CCNP and CCIE exams and labs, so the coverage in this chapter will help you with other exams as well.

Policing solves specific problems relating to network capacity and traffic engineering. Suppose, for example, that an Internet service provider (ISP) engineers their network to effectively forward packets at rate x. Suppose further that the Sales department sells enough access so that the customers all together pay for x capacity. However, the customers can collectively send 10x into the ISP’s network, so everyone suffers. Policing just gives a network engineer the ability to “enforce the law” by discarding excess traffic, much like a real policeman just enforces the law of the local community. Policing can also prevent a single customer from taking too much bandwidth, even if the provider has enough capacity to handle the extra traffic.

This chapter first explains the core concepts of traffic shaping and policing. Following that, each of four traffic-shaping tools—GTS, CB shaping, DTS, and FRTS—are covered, including additional concepts specific to each tool. The two traffic-policing tools, CAR and CB policing, are covered at the end of the chapter.

Traffic-Policing and Traffic-Shaping Concepts

Traffic shaping and traffic policing both measure the rate at which data is sent or received. Policing discards excess packets, so that the overall policed rate is not exceeded. Shaping enqueues the excess packets, which are then drained from the queue at the shaping rate. In either case, both policing and shaping prevent the traffic from exceeding the bit rate defined to the policer or shaper.

This section covers concepts related to both shaping and policing. It starts with some of the motivations for using shaping and policing. One classic reason to choose to shape occurs when the device at the other end of the link is policing. Suppose, for instance, that R1 sits in an enterprise, and R2 is inside an ISP. R1 sends packets to R2, and R2 polices traffic, discarding any traffic beyond x bits per second (bps). The ISP might have chosen to police at R2 to protect the network from accepting too much traffic. R1 could be configured to shape the traffic it sends to R2 to the same rate as the policer at R2, instead of having the excess packets discarded by R2. Other less-obvious reasons for both shaping and policing exist. The upcoming section discusses these.

This section also discusses the mechanisms shaping and policing use to perform their functions. For instance, both policers and shapers must measure bit rates. To measure a rate, a number of bits or bytes over a time period must be observed and calculated. To keep the process simple,