Chapter 2. QoS Tools and Architectures

QoS Exam Topics

This chapter covers the following exam topics specific to the QOS exam:

• List and explain the models for providing Quality of Service on a network

• Explain the purpose and function of the DiffServ model

• Describe the basic format of and explain the purpose of the DSCP field in the IP header

• Define and explain the different per hop behaviors used in DSCP

• Explain the interoperability between DSCP-based and IP-precedence-based devices in a network

• Given a list of QoS actions, correctly match the QoS actions to mechanisms for implementing QoS and identify where in a network the different QoS mechanisms are commonly used

To build a house, you need tools, you need materials, you need labor, and you need architectural plans. To build a network using quality of service (QoS), you need tools, labor, and an architecture. This chapter lists the various IOS QoS tools and explains the two predominant QoS architectures: integrated services (IntServ) and differentiated services (DiffServ).

Chapter 1, "QoS Overview," covered various types of QoS tools. There are several different categories of QoS tools in Cisco IOS Software. This chapter begins by listing the tools in each categoryat least the ones covered on the QOS exam. This first section also includes a brief introduction to concepts behind each type of tool. All the tools listed here get further treatment in later chapters of the book.

As a tool for learning, the second section of this chapter explains the basics of flow-based QoS tools and Class-Based QoS tools. Taking a few minutes to think about these concepts, and why they make sense, is useful before looking at the two formalized QoS modelsnamely DiffServ and IntServ.

Next, this chapter then examines the DiffServ architecture in detail. DiffServ attempts to provide Internet-scale QoS, which is a lofty goal indeed! DiffServ uses a Class-Based approach to differentiate between packets, which scales somewhat better than its predecessor, IntServ. Whether DiffServ succeeds in this goal remains to be seen; however, many of the concepts can be helpful with any QoS implementation.

Finally, the chapter ends with a short discussion on IntServ and Best Effort architectures. IntServ uses the Resource Reservation Protocol (RSVP) to reserve bandwidth for individual flows in the network. Best Effort is actually just a plan in which the network makes no real effort to give one type of packet better service than any other.

This chapter concludes the introductory materials in this book; the remainder of this book delves into the details of the various QoS tools.