C H A P T E R 5

Traffic Policing and Shaping

Traffic policing allows devices in one network to enforce a traffic contract. Traffic contracts define how much data one network can send into another, typically expressed as a committed information rate (CIR) and a committed burst (Bc). Policing measures the flow of data, and discards packets that exceed the traffic contract.

Similarly, traffic shaping allows devices to conform to a traffic contract. In cases where packets that exceed the traffic contract might be discarded, the sending device may choose just to slow down its sending rate, so that the packets are not discarded. The process of sending the traffic more slowly than it could be sent, to conform to a traffic contract, is called shaping.

In short, policing typically drops out-of-contract traffic, whereas shaping typically delays it.

Shaping and policing share several concepts and mechanisms. Both need to measure the rate at which data is sent or received, and take action when the rate exceeds the contract. Often when policing is used for packets entering a network, shaping is also used on devices sending into that network. Although shaping and policing are not always used in the same networks, there are more similarities than differences, so both are covered in this single chapter.

“Do I Know This Already?” Quiz

The purpose of the “Do I Know This Already?” quiz is to help you decide whether you really need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 12-question quiz, derived from the major sections in “Foundation Topics” section of this chapter, helps you determine how to spend your limited study time.

Table 5-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

Table 5-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

320 Chapter 5: Traffic Policing and Shaping

CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, mark this question wrong for purposes of the self-assessment. Giving yourself credit for an answer you correctly guess skews your self-assessment results and might provide you with a false sense of security.

You can find the answers to the “Do I Know This Already?” quiz in Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows:

•10 or less overall score—Read the entire chapter. This includes the “Foundation Topics,” the “Foundation Summary,” and the “Q&A” sections.

•11 or 12 overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the “Q&A” section. Otherwise, move to the next chapter.

Shaping and Policing Concepts Questions

1Explain the points during the process of a single router receiving and forwarding traffic at which shaping and policing can be enabled on a router.

2Compare and contrast the actions that shaping and policing take when a packet exceeds a traffic contract.

3If a router has a shaping tool configured, with a shaping rate of 256 kbps, and a Bc of 16,000 bits, what Tc value does the shaping tool use?

4Define the terms Tc, Bc, Be, and CIR.

Policing with CAR and CB Policer Questions

5List the command, with the correct syntax, that sets a policed rate of 512 kbps, a Bc of 1 second’s worth of traffic, and a Be of an additional 0.5 second’s worth of traffic, when using CAR. Do not assume any defaults; explicitly set the values in the command. You can choose any other settings needed for the command.

6Which policing tools allow for three categories of actions to take?

7Explain the concept behind re-marking policed packets versus discarding the packets.

8CB policing has been configured under subinterface s0/0.1. What show command lists statistics for CB policing behavior just for that subinterface?

“Do I Know This Already?” Quiz 321

Shaping with FRTS, GTS, DTS, and CB Shaping

9Along with the class-map, policy-map, and service-policy commands, CB shaping requires one specific command that actually sets values used for the shaping function. List the command, with the correct syntax, that sets a shaped rate of 128 kbps, a Bc of 8000, and a Be of 8000, when using CB shaping. Do not assume any defaults; explicitly set the values in the command.

10Compare and contrast the use of the class-map and map-class commands in terms of how each is used by FRTS and CB shaping.

11DTS has been configured under subinterface s0/0.1. What show command displays statistics for DTS behavior just for that subinterface?

12Which of the traffic-shaping tools can be enabled on each VC on a Frame Relay multipoint subinterface?