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DQOS Exam Certification Guide - Cisco press.pdf
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C H A P T E R 6

Congestion Avoidance Through

Drop Policies

Quality of service (QoS) congestion-avoidance tools help prevent congestion before it occurs. These tools monitor queue depth, and before the queue fills, they drop some packets. The computers sending the packets might reduce the frequency of sending packets in reaction to the packet loss, particularly if the application sending the data uses TCP. In the moments after the congestion-avoidance tool discards packets, congestion is reduced, because less traffic is sent into the network.

Cisco congestion-avoidance tools rely on the behavior of TCP to reduce congestion. TCP flows do slow down after packet loss. By discarding some TCP packets before congestion gets bad, congestion-avoidance tools may actually reduce the overall number of packets dropped, and reduce congestion, thereby indirectly reducing delay and jitter.

The first section of this chapter begins with a review of TCP and a description of how TCP slows down after packet loss. Following that, several of the problems that can be solved using congestion-avoidance tools are described, namely tail drop, global synchronization, and TCP starvation. The concepts section of this chapter ends with coverage of Random Early Detection (RED), which defines several algorithms, which are the basis of IOS congestion-avoidance tools.

Weighted RED (WRED) and Flow-Based WRED (FRED) are the two congestionavoidance tools available in IOS. These are covered in sequence with the differences between the two highlighted at the end of the 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 6-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

Use Table 6-1 to record your score.

426 Chapter 6: Congestion Avoidance Through Drop Policies

Table 6-1

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












Foundation Topics Section Covering These Questions








Congestion-Avoidance Concepts and RED

1 to 4








5 to 8








9 to 12






All questions


1 to 12





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 “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.

Congestion-Avoidance Concepts and RED Questions

1Describe the process of TCP slow start and discuss when it occurs.

2Describe the meaning of the term “global synchronization,” and discuss what causes it.

3Define the meaning of the term “tail drop.”

4Does RED compare the actual queue depth or the average queue depth to queue thresholds when deciding whether it should discard a packet? Why this one, and not the other?

WRED Questions

5List the queuing tools that can enable WRED for use with some or all of their queues, effectively enabling WRED concurrently with the queuing tool.

6Describe how WRED “weights” packets.

“Do I Know This Already?” Quiz 427

7Taking as many defaults as possible, list the configuration commands needed to configure precedence-based WRED on interface S1/1.

8Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based WRED inside class class1, inside policy map my-policy. (You can assume that the CBWFQ configuration has already been completed, and you just entered global configuration mode. Assume that you need just to enable WRED in class class1.)

FRED Questions

9Identify the most significant difference between FRED operation and WRED operation.

10List the three categories of flows defined by FRED, and identify which category has its packets discarded most aggressively.

11Taking as many defaults as possible, list the configuration commands needed to configure precedence-based FRED on interface S1/1.

12Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based FRED on interface S1/1.