Priority Queuing and Custom Queuing 313

Q&A

As mentioned in the Introduction, you have two choices for review questions. The questions that follow next give you a more difficult challenge than the exam itself by using an open-ended question format. By reviewing now with this more difficult question format, you can exercise your memory better, and prove your conceptual and factual knowledge of this chapter. You can find the answers to these questions in Appendix A.

The second option for practice questions is to use the CD included with this book. It includes a testing engine and more than 200 multiple-choice questions. You should use this CD nearer to the end of your preparation, for practice with the actual exam format. You can even customize the CD exam to include, or not include, the topics that are only on the CCIP QoS.

Conceptual Questions

1Describe the benefits of having a single FIFO output queue.

2Explain the effects of changing a single FIFO queue’s length to twice its original value. Include comments about how the change affects bandwidth, delay, jitter, and loss.

3Explain the purpose of a TX Ring and TX Queue in a Cisco router.

4Explain how a long TX Ring might affect the behavior of a queuing tool.

5Describe the command output that identifies the length of the TX Ring or TX Queue, and whether the length was automatically lowered by IOS.

6Explain under what circumstances the TX Ring, interface output queues, and subinterface output queues both fill and drain, and to where they drain.

Priority Queuing and Custom Queuing

7Assume a queuing tool has been enabled on interface S0/0. Describe the circumstances under which the queuing tool would actually be used.

8Explain the circumstances under which it would be useful to enable a queuing tool on a subinterface.

9Describe the classification feature of Priority Queuing, including the list of items that can be examined for classification decisions.

10Describe the classification feature of Custom Queuing, including the list of items that can be examined for classification decisions.

11List the classification options available to Custom Queuing that are not also available to Priority Queuing.

314 Chapter 4: Congestion Management

12Describe the process and end result of the scheduling feature of Priority Queuing.

13Describe the process and end result of the scheduling feature of Custom Queuing.

14List the maximum number of queues used by Priority Queuing and Custom Queuing.

WFQ

15Characterize the effect the WFQ scheduler has on different types of flows.

16Describe the WFQ scheduler process. Include at least the concept behind any formulas, if not the specific formula.

17You previously disabled WFQ on interface S0/0. List the minimum number of commands required to enable WFQ on S0/0.

18What commands list statistical information about the performance of WFQ?

19Define what comprises a flow in relation to WFQ.

20You just bought and installed a new 3600 series router. Before adding any configuration to the router, you go ahead and plug in the new T1 Frame Relay access link to interface S0/0. List the minimum number of commands required to enable WFQ on S0/0.

CBWFQ, LLQ, IP RTP Priority

21Describe the CBWFQ scheduler process, both inside a single queue and among all queues.

22Describe how LLQ allows for low latency while still giving good service to other queues.

23Compare 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.

24Compare and contrast the CBWFQ command that configures the guaranteed bandwidth for a class with the command that enables LLQ for a class.

25Describe the CBWFQ classification options. List at least five fields that can be matched without using an ACL.

26Name the two CBWFQ global configuration commands that define classification options, and then the per-hop behaviors, respectively. Also list the command that enables CBWFQ on an interface.

27List the command used to configure IP RTP Priority on a serial link.

28Characterize the type of traffic that can be queued using both IP RTP priority and LLQ. List specific port numbers and IP addresses as applicable, and describe the type of traffic.

CBWFQ, LLQ, IP RTP Priority 315

29Examine the following configuration (see Example 4-10). Which of the five policy maps would certainly enable LLQ for voice payload traffic, based only of the information in the configuration?

Example 4-10 Exhibit for CBWFQ Configuration Questions

!

class-map match-all class1 match ip rtp 16384 16383 class-map match-all class2 match access-group 101 class-map match-all class3 match ip rtp 16384 32767 class-map match-all class4

match ip dscp ef class-map match-all class4

match access-group 102

!

policy-map pmap1 class class1 priority 60 policy-map pmap2 class class2 priority 60 policy-map pmap3 class class3 priority 60 policy-map pmap4 class class4 priority 60 policy-map pmap5 class class5 priority 60

!

interface Serial0/0 service-policy output ?????

!

access-list 101 permit udp any gt 16383 any gt 16383

access-list 102 permit udp any range 16383 32767 any range 16383 32767

!

30Using the same exhibit as in the preceding example, describe what must also be true for pmap4 to queue voice payload traffic successfully, and only voice payload traffic, in a lowlatency queue.

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

DQOS Exam Topics

•Describe the difference between policing and shaping and how each ones relates to QoS.

•Describe the various mechanisms for policing, when to apply each, and how to configure them.

•Identify the various types of traffic shaping, their differences, and how to apply each.

•Configure the different types of traffic shaping.

QoS Exam Objectives

•Describe shaping and policing mechanisms available in Cisco IOS.

•Describe the benefits and drawbacks of traffic shaping and policing mechanisms.

•Describe Generic Traffic Shaping (GTS) on Cisco routers.

•Configure GTS on Cisco routers.

•Describe Frame Relay Traffic Shaping (FRTS) and its differences from GTS.

•Configure Frame Relay Traffic Shaping (FRTS) on Cisco routers.

•Describe the Committed Access Rate (CAR) mechanism including its benefits and drawbacks.

•Configure Committed Access Rate (CAR) on Cisco Routers.