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DQOS Exam Certification Guide - Cisco press.pdf
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C H A P T E R 10


“Why would I go through the hassle of configuring and managing QoS on my LAN, which is already overprovisioned? If I have to, I will just add more bandwidth!”

This is a common statement. Conventional wisdom dictates, “If you have enough bandwidth, you do not need QoS. QoS is only used for WAN links that do not have enough bandwidth. LANs have plenty of bandwidth, so you are safe.”

Although this sounds reasonable, it is not entirely accurate. Bandwidth is only one of the factors that need to be taken into consideration for a network that supports real-time applications.

This chapter discusses the capabilities of Cisco Catalyst LAN switches and examines some recommended configurations designed to prioritized real-time applications across your local-area network (LAN).

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

Table 10-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

688 Chapter 10: LAN QoS

Table 10-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping


Foundation Topics Section




Covering These Questions








LAN QoS Concepts

1 to 6







Catalyst 6500

7 to 10







Catalyst 4500/4000

11 to 14







Catalyst 3550/3524

15 to 18






All questions


1 to 18






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:

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

15 or 16 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.

1What is instantaneous buffer overrun?

2What is meant by the term “1p1q4t?”

3What packet or frame marking(s) can a Layer 2 switch use to prioritize traffic?

4What must be done for a Layer 2 switch to properly classify a packet received from a host across the WAN?

5What is a drop threshold?

6What is a trust boundary, and where should it be set?

7Explain why the first packet in a flow may not retain the proper CoS values when a Catalyst 6500 is configured with a PFC.

8What is the difference between Hybrid mode and Native mode in a 6500 series switch?

9What queue and threshold is recommended for call control traffic on a Catalyst 6500?

“Do I Know This Already?” Quiz 689

10Which series of Ethernet line cards are preferred in the Catalyst 6500 series for QoS? Why?

11Which supervisor engine is preferred on the Catalyst 4500/4000 series? Why?

12What trust state are the ports of a 4500 with a Supervisor III in when QoS is enabled?

13In which queue number does the priority queue reside on a Catalyst 4500 or 4000 with a Supervisor III Engine?

14What keyword(s) is used to enable the priority queue on a Catalyst 4500 or 4000 with a Supervisor III module?

15In which queue number does the priority queue reside on a Catalyst 3550?

16What command enables QoS on the Catalyst 3550?

17Name the four methods a Catalyst 3550 can use to classify traffic.

18What command enables QoS on the Catalyst 3524?