Chapter 5. Congestion Management

QoS Exam Topics

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

• List and explain the different queuing algorithms

• Explain the components of hardware and software queuing systems on Cisco routers and how they are affected by tuning and congestion

• Describe the benefits and drawbacks of using WFQ to implement QoS

• Explain the purpose and features of Class-Based WFQ (CBWFQ)

• Explain the purpose and features of Low Latency Queuing (LLQ)

• Identify the Cisco IOS commands required to configure and monitor LLQ on a Cisco router

Most people understand the basic concepts of queuing, because most of us experience queuing every day. We wait in line to pay for groceries, we wait for a bank teller, we wait to get into a ride at an amusement park, and so on. So, most of the queuing concepts inside this chapter are intuitive.

Cisco uses the term "congestion management" to refer to queuing systems in their products. This chapter begins with coverage of some queuing concepts inside Cisco routers, including the distinction between hardware and software queues, and where software queues can be used. (Queuing inside LAN switches, particularly the Cisco 2950 series switches, is covered in Chapter 9, "LAN QoS.")

Following that, the second and third sections of the three major sections in this chapter cover specifics about several Queuing mechanisms available inside Cisco routers. The first of these two sections covers only concepts about several older Queuing tools. The final section covers both concepts and configuration for the three major queuing mechanisms covered by the current QoS exam, namely WFQ, CBWFQ, and LLQ.