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 CD-ROM nearer to the end of your preparation, for practice with the actual exam format.

Congestion-Avoidance Concepts and Random Early Detection (RED)

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

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

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

4.
Describe the meaning of the term "global synchronization," and discuss what causes it.

5.
Define the meaning of the term "tail drop."

6.
Define the meaning of the term "TCP starvation."

7.
Does 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?

8.
Describe how RED uses actual queue depth to calculate average queue depth. Do not list the formula, but just describe the general idea.

9.
Assume 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?

10.
Assume 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?

11.
Define how RED uses the mark probability denominator. Give one example.

12.
Define the term "exponential weighting constant." If the value is lowered compared to the default setting of 9, how does RED behave differently?

13.
Define the term "WRED Profile."

14.
Explain how you can tune how fast or slow that WRED changes the calculated average queue depth over time.

Weighted RED (WRED)

15.
Spell out the words represented by the initials RED, WRED, and FRED.

16.
List the three WRED terms that name the separate states in which WRED discards no packets, a percentage of packets, and all packets.

17.
List the queuing tools that can be concurrently supported on an interface when WRED has been enabled directly on a serial interface, assuming no retrictions on the particular model of router.

18.
Identify the most important difference between RED operation and WRED operation.

19.
Describe how WRED "weights" packets.

20.
List the queuing tools that can enable WRED for use with some or all of their queues, effectively enabling WRED concurrently with the queuing tool, assuming no retrictions on the particular model of router.

21.
What command enables you to look at WRED drop statistics when WRED is configured inside an MQC class?

22.
Taking as many defaults as possible, list the configuration commands needed to configure precedence-based WRED on interface S1/1.

23.
Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based WRED on interface S1/1.

24.
Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based WRED inside class class1, inside policy map my-policy. (You can assume that the CBWFQ configuration has already been completed, and you just entered global configuration mode. Assume that you need just to enable WRED in class class1.)

25.
List the command needed to set the minimum threshold to 25, the maximum threshold to 50, and the mark probability denominator to 4, for precedence 2.

26.
What show command lists detailed statistics about random drops on interface S1/1?

Explicit Congestion Notification

27.
For a single WRED profile, WRED can be either dropping no packets, randomly choosing packets to discard, or dropping all packets. For which of these three states does ECN impact WRED's discard actions? How does it change what WRED does to the packets?

28.
Identify the bits in the IP header used with ECN, by name and location.

29.
Imagine a router on which WRED and ECN are enabled, and WRED decides to randomly discard a packet. What must be true in order for WRED to discard the packet, instead of using ECN logic to mark and forward the packet? Explain the role of any other devices besides the router.

30.
Imagine a router on which WRED and ECN are enabled, and WRED decides to randomly discard a packet. What must be true in order for WRED to use ECN logic to mark and forward the packet, instead of discarding the packet? Explain the role of any other devices besides the router.

31.
Imagine a policy map with WRED already configured for class class-web. What additional command is required to also enable ECN for the packets in that class?