C H A P T E R 4
Congestion Management
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. Queuing concepts in real life are quite similar to queuing in routers and switches. This chapter begins with a more formalized introduction to queuing, to clarify what most people already intuitively know. This chapter covers such issues as how many queues exist, how routers choose to place packets into each queue, what to do when the queue is full, and so on.
Following the discussion of concepts, the text explains the concepts and configurations related to the queuing tools covered on the DQOS exam. In Appendix B, “Topics on the CCIP QoS Exam,” you can find coverage of some other queuing tools that happen to be covered on the QoS exam. Each tool can be compared with the other tools based on the same details covered in the concepts section of the chapter—how to classify a packet into a queue, what to do if the queue is full, and so on. The discussion about queuing tools also examines the details particular to each tool, the configuration, and show commands related to each tool.
When Cisco does make the next revision of these two exams, I am guessing that a few queuing tools will be dropped from the QoS exam. However, that’s just a guess, so make sure to check www.cisco.com for changes to the exams, and www.ciscopress.com/ 1587200589 for the latest advice from the authors on a good study plan in case the exams have changed.
“Do I Know This Already?” Quiz
The purpose of the “Do I Know This Already?” quiz is to help you decide if 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 16-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 4-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.
“Do I Know This Already?” Quiz 235
6Describe the WFQ scheduler process. Include at least the concept behind any formulas, if not the specific formula.
7You previously disabled WFQ on interface S0/0. List the minimum number of commands required to enable WFQ on S0/0.
8What commands list statistical information about the performance of WFQ?
CBWFQ and LLQ Questions
9Describe the CBWFQ scheduler process, both inside a single queue and among all queues.
10Describe how LLQ allows for low latency while still giving good service to other queues.
11Compare and contrast IP RTP Priority and LLQ. In particular, mention what other queuing tools can be used concurrently with each, how each classifies packets, and which is recommended by Cisco.
12Compare and contrast the CBWFQ command that configures the guaranteed bandwidth for a class with the command that enables LLQ for a class.
Comparing Queuing Options Questions
13Which of the following queuing tools allows for WRED inside a single queue? First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ); and IP RTP Priority.
14Which of the following queuing tools can always service a particular queue first, even when other queues have packets waiting? First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ); and IP RTP Priority.
15Which of the following queuing tools allows for a percentage bandwidth to be assigned to each queue? First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ); and IP RTP Priority.
16Which queuing tools could be configured to provide the lowest possible latency for voice traffic? Of these, which does Cisco recommend as the best option for voice queuing today?