Congestion-Avoidance Concepts and Random Early Detection (RED) 475

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

Congestion-Avoidance Concepts and Random Early

Detection (RED)

1Describe the function of the congestion window in TCP, and how it is changed as a result of packet loss.

2Identify the two TCP windowing mechanisms, and describe when each is used.

3Describe the process of TCP slow start, and when it occurs.

4Describe the process of TCP congestion avoidance, and when it occurs.

5Describe the meaning of the term “global synchronization,” and discuss what causes it.

6Define the meaning of the term “tail drop.”

7Define the meaning of the term “TCP starvation.”

8Does RED compare the actual queue depth or the average queue depth to queue thresholds when deciding whether it should discard a packet? Why this one, and not the other?

9Describe how RED uses actual queue depth to calculate average queue depth. Do not list the formula, but just describe the general idea.

10Assume the RED minimum threshold is 20, the maximum threshold is 40, and the mark probability denominator is 10. What must be true for RED to discard all new packets?

11Assume the RED minimum threshold is 20, the maximum threshold is 40, and the mark probability denominator is 10. What must be true for RED to discard 5 percent of all new packets?

Flow-Based WRED (FRED) 477

29List the queuing tools that can enable FRED for use with some or all queues, effectively enabling FRED concurrently with the queuing tool.

30Suppose that an interface has five active flows, with Flow 1 consuming 20 queue entries, and a maximum queue size of 40. Describe the terms “maximum per-flow queue depth,” and give an example of how it is calculated with this example. Use default values for any information not stated in the question.

31Taking as many defaults as possible, list the configuration commands needed to configure precedence-based FRED on interface S1/1.

32Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based FRED on interface S1/1.

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

DQOS Exam Topics

•Explain the need for link-efficiency tools.

•Explain available LFI techniques including MLP interleaving and FR fragmentation using FRF.11 Annex-C or FRF.12.

•Explain Real Time Protocol header compression (cRTP) as a tool for improving link efficiency.

•Configure and monitor various LFI methods and cRTP.

QoS Exam Topics

•Describe payload compression and payload compression algorithms available on Cisco routers.

•Describe header compression and header compression algorithms available on Cisco routers.

•Configure Cisco IOS header compression mechanisms.

•Describe link fragmentation and interleaving (LFI).

•Configure link fragmentation and interleaving (LFI) mechanisms on Cisco IOS routers.

C H A P T E R 7

Link-Efficiency Tools

Most WAN links are leased from a service provider, with one of the variables affecting the pricing being the bandwidth on the link. For instance, the distance and the bandwidth, or clock rate, on the link affect leased lines. Frame Relay service providers base their prices in part based on the access rate of the access links into the Frame Relay network, and the committed information rate (CIR) of the various virtual circuits (VCs).

If the offered load on the network consistently exceeds the bandwidth or clock rate of the link the traffic must flow across, unpredictable behavior can result. For example, queues consistently fill, causing more delay, jitter, and drops. If the offered load far exceeds the clock rate for a period of time, most data applications slow down significantly, with voice and video streams possibly even becoming unusable. Depending on how you configure quality of service (QoS) in the network, some traffic types may perform as expected, but with a likely result of allowing some other traffic types to degrade even more quickly, because most QoS tools by design end up favoring one type of traffic over another.

This chapter covers two classes of QoS tools that directly impact the usage of the bandwidth in a network—compression tools and link fragmentation and interleaving (LFI) tools. Compression tools compress the number of bytes in a packet so that fewer bytes need to be sent over a link.

LFI tools directly impact serialization delays—and serialization delay is impacted by actual link bandwidth. The slower the link, the longer it takes to serialize a packet. If a small packet must wait on a large packet to be serialized onto a link, the small packet may experience too much delay, particularly on slow-speed links. LFI tools reduce the delay experienced by a short packet by breaking larger packets into smaller pieces, and by transmitting the original small packets in between the fragments of the original larger packets. Smaller packets get better service, and in many cases, smaller packets are part of a delay-sensitive application, such as Voice over IP (VoIP).