Classification and Marking for Cisco DQOS and QOS Exams

This chapter covers the following exam topics specific to the DQOS and QoS exams:

DQOS Exam Topics

• Explain the reason for classification and marking.

• Explain the difference between classification and marking.

• Explain class of service, IP precedence, and DiffServ code points.

• Configure QoS policy using Modular QoS CLI.

• Explain the role of network-based application recognition (NBAR).

• Classify and mark traffic.

QoS Exam Objectives

• Describe policy-based routing and how it can be used to classify and mark IP packets.

• Configure the policy-based routing mechanism on Cisco routers.

• List other mechanisms that also support classification and marking capabilities (committed access rate, class-based marking).

• Describe the Modular QoS CLI (MQC) concept and its structure.

• Describe Modular QoS CLI classification options.

• Configure the Modular QoS CLI to perform classification.

• Describe network-based application recognition (NBAR).

• Describe Modular QoS CLI policy options.

• Configure the Modular QoS CLI to perform service policies.

QoS classification tools categorize packets by examining the contents of the frame, cell, and packet headers; whereas marking tools allow the QoS tool to change the packet headers for easier classification. Many QoS tools rely on a classification function to determine to which traffic the tool applies. To place voice and data traffic in separate queues, for example, you must use some form of classification to differentiate the two types of traffic and place the identified traffic in the proper queue. Marking provides a way for QoS tools to change bits in the packet header to indicate the level of service this packet should receive from other QoS tools. For instance, you can use marking tools to change the marking in voice packets to ensure that a classification tool can differentiate a voice packet from a data packet. Without the marking feature, the frame, packet, or cell remains unchanged.

Marking involves placing a value into one of the small number of well-defined frame, packet, or cell header fields specifically designed for QoS marking. By marking a packet, other QoS functions can perform classification based on the marked field inside a header. Marking simplifies the network's QoS design, it simplifies configuration of other QoS tools, and it reduces the overhead required by each of the other QoS tools to classify the packets.

Although classification and marking tools do not directly affect the bandwidth, delay, jitter, or loss experienced by traffic in the network, classification and marking tools are the building blocks for all other QoS tools. With these tools, all traffic on the network is identified for the next QoS tool to act upon.

The concepts that apply to all classification and marking are covered in the first section of this chapter, including the terminology, fields used, and the meaning behind each of the available marked fields. Following that, each of the classification and marking tools is covered, with example configurations, show, and debug commands.

"Do I Know This Already?" Quiz Questions

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 8-question quiz, derived from the major sections in "Foundation Topics" portion of the chapter, helps you determine how to spend your limited study time.

Table 3-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 3-1 "Do I Know This Already?" Foundation Topics Section-to-Question Mapping

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:

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

• 7 or 8 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.

Classification and Marking Concepts Questions

1. Describe the difference between classification and marking.

2. Describe, in general, how a queuing feature could take advantage of the work performed by a classification and marking feature.

3. Which of the following QoS marking fields are carried inside an 802.1Q header: QoS, CoS, DE, ToS byte, User Priority, ToS bits, CLP, Precedence, QoS Group, DSCP, MPLS Experimental, or DS?

4. Which of the following QoS marking fields are carried inside an IP header: QoS, CoS, DE, ToS byte, User Priority, ToS bits, CLP, Precedence, QoS Group, DSCP, MPLS Experimental, or DS?

CAR, PBR, and CB Marking Questions

5. Define the meaning of MQC, and spell out what the acronym stands for.

6. What configuration command lists the marking details when configuring CB marking? What configuration mode must you use to configure the command? What commands must you issue to place the configuration mode user into that mode?

7. What configuration command lists the marking details when configuring CAR? What configuration mode must you use to configure the command? What commands must you issue to place the configuration mode user into that mode?

8. What configuration command lists the classification details when configuring PBR? What configuration mode must you use to configure the command? What commands must you issue to place the configuration mode user into that mode?

Foundation Topics

The contents of the "Foundation Topics" section of this chapter, and most of the rest of the chapters in this book, follow the same overall flow. Each chapter describes a type of category of QoS tool. Each "Foundation Topics" section begins with coverage of the concepts behind these tools. Then, each tool is examined, with coverage of how each tool works like the other tools, and how it works differently than the other tools. So, most of the core concepts are explained in the first part of the chapter; some of the concepts may be explained in the section about a specific tool, however, particularly if the concepts apply only to that tool.

The second part of the chapter covers several classification and marking tools: class-based marking (CB marking), committed access rate (CAR), policy-based routing (PBR), and dial peers. For each tool, the pertinent configuration, show, and debug commands are also covered.