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BCMSN Exam Certification Guide

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396 Chapter 16: Quality of Service Overview

Foundation Summary

The Foundation Summary is a collection of information that provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary can help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final preparation before the exam, this summarized information is a convenient way to review the day before the exam.

Packet delivery can be affected by the following conditions:




QoS can be implemented as several models:

Best effort—Delivery occurs on a “first-come, first-served” basis with no special handling.

IntServ where the necessary bandwidth and handling is reserved from sender to receiver before the traffic flow can begin.

DiffServ where special handling of traffic is performed by each switch or router independently, on a per-hop basis. No reservations are necessary; each packet is examined and handled according to local QoS policies.

QoS can be quantified by one of the following indicators:

CoS—A 3-bit value (0-7) that is carried along with packets in a trunk

IP Precedence—A 3-bit value (0-7) that is carried within each packet’s IP header

DSCP—A 6-bit value (0-63) that is carried within each packet’s IP header

The DSCP codepoint is made up of the following two parts:

Class Selector—A 3-bit value that specifies a class of service

Drop Precedence—A 3-bit value that specifies how likely a packet is to be dropped (when necessary)

DSCP codepoint names specify a per-hop behavior (AF or EF), along with the Class Selector (1-4 for AF) and Drop Precedence (1-3 for AF). Codepoint name EF is the best service available for user traffic and has no class or drop precedence notation, although it is actually Class 5.

Foundation Summary 397

The following “building blocks” can perform QoS within a Catalyst switch:

Ingress queueing—Inbound packets are placed in specific ingress port queues and serviced accordingly.

Classification—Packets are identified or classified so that the proper QoS policies can be applied to them.

Trust—A switch can be configured to trust or not trust inbound QoS information from another source.

Marking—The QoS information of classified packets can be manipulated to obtain different, desired values.

Policer—The volume of classified traffic can be held within desired limits or can be dropped completely.

Scheduling—Packets are placed into specific egress queues based on QoS information. These queues are serviced accordingly. Weighted Round Robin (WRR) is normally used for queue scheduling.

Congestion avoidance—A port queue is protected from congestion proactively. This is normally done by Weighted Random Early Detection (WRED), where packets are randomly dropped to ease the demands for queue space.

Switch port queues are described by a queue type notation: xpyqzt, where x is the number of p strict priority queues; y is the number of q standard queues; and z is the number of t WRED thresholds.

398 Chapter 16: Quality of Service Overview


The questions and scenarios in this book are more difficult than what you should experience on the actual exam. The questions do not attempt to cover more breadth or depth than the exam; however, they are designed to make sure that you know the answers. Rather than allowing you to derive the answers from clues hidden inside the questions themselves, the questions challenge your understanding and recall of the subject. Hopefully, these questions will help limit the number of exam questions on which you narrow your choices to two options and then guess.

The answers to these questions can be found in Appendix A.

1.What are some of the problems that QoS can help relieve?

2.Which protocol reserves network resources in the IntServ QoS model?

3.What protocol maintains the DiffServ QoS model?

4.What range of values can the CoS field contain?

5.How is the CoS information passed from LAN switch to LAN switch?

6.How is the IP Precedence field related to the DSCP field?

7.Put the following DSCP codepoints in order of increasing service quality:






8.If a packet contains the DSCP codepoint name “AF31,” what would the IP Precedence value be?

9.If a switch port is configured as “untrusted,” what is the resulting CoS value of incoming packets?

10.Should there be a trust boundary at every switch, where each overwrites QoS information? Explain why or why not.

11.What method is used for scheduling in Catalyst switches?

Q&A 399

12.Name one method that is used for congestion avoidance.

13.When are packets dropped from the strict priority queue on an interface?

14.What is the disadvantage of using the tail drop method to avoid congestion?

15.If WRED drops packets at random, is that bad?

16.A switch port has a queue type 1p1q4t. What does this mean?

17.If a switch port has a queue type 1p2q2t, what is the strict priority queue’s queue number?

This chapter covers the following topics that you need to master for the CCNP BCMSN exam:

Applying QoS Trust—This section discusses how the DiffServ per-hop behavior begins, by choosing whether to trust QoS information from other sources.

Defining a QoS Policy—The DiffServ model is implemented through policies that can be defined and applied on a switch.

Tuning Egress Scheduling—This section explains how you can configure a switch to correctly and efficiently buffer packets into queues before forwarding. Scheduling also involves the manner that packets are pulled from the queues as they are forwarded.

Using Congestion Avoidance—This section discusses the configuration that allows a switch to avoid conditions that cause egress queues to fill and overflow.

Verifying and Troubleshooting QoS—This section provides a brief summary of the commands that verify the configuration and operation of DiffServ QoS.

C H A P T E R 17

DiffServ QoS Configuration

Chapter 16 covered the basic theory and functionality behind quality of service (QoS) operations on a Catalyst switch. This chapter extends the QoS topic by explaining how you can configure switches to implement DiffServ QoS. Remember that DiffServ is a per-hop behavior, so every switch along the traffic path must be configured to support equivalent QoS policies.

Catalyst switches have a rich, powerful QoS feature set. With power comes versatility, and with versatility comes many commands and confusion. This chapter presents the many QoS features and functions in a logical sequence. Some of the more esoteric features and commands have been omitted from this chapter because they are not covered in the BCMSN course and are too specific for the most common environments.

“Do I Know This Already?” Quiz

The purpose of the “Do I Know This Already?” quiz is to help you decide what parts of this chapter to use. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.

This 12-question quiz, derived from the major sections in the “Foundation Topics” portion of the chapter, helps you determine how to spend your limited study time.

Table 17-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

Table 17-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

Foundation Topics Section

Questions Covered in This Section



Applying QoS Trust




Defining a QoS Policy




Tuning Egress Scheduling




Using Congestion Avoidance




402 Chapter 17: DiffServ QoS Configuration

CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong. Giving yourself credit for an answer you correctly guess skews your selfassessment results and might give you a false sense of security.

1.By default, how does a switch port treat incoming QoS information?

a.It is trusted.

b.It is not trusted.

c.It is just passed along with the packet.

d.It is overridden according to the type of traffic.

2.What command can configure an interface to trust only the inbound DSCP information?

a.mls qos trust dscp

b.no mls qos trust

c.no mls qos untrusted dscp

d.mls trust qos dscp

3.Which of the following commands defines what traffic will be identified for a QoS policy?




d.mls trust

4.What classification method can match traffic flows that use dynamic UDP or TCP port numbers?

a.Standard IP access list

b.Extended IP access list



5.Which configuration command can contain actions to take on classified traffic?




d.mls trust

“Do I Know This Already?” Quiz 403

6.Which one of the following is an action that cannot be taken on classified traffic?

a.Mark with new QoS information.

b.Trust inbound QoS information.

c.Assign to an outbound interface.

d.Police it to control the bandwidth used.

7.What command can apply a QoS policy to an interface?

a.access-group policy-name

b.policy-group policy-name

c.mls qos policy policy-name

d.service-policy policy-name

8.How many QoS policy maps can be applied to an interface?

a.One, controlling both inbound and outbound traffic.

b.Two; one in each inbound or outbound direction.

c.As many as are configured.

d.None; policy maps cannot be applied to interfaces.

9.Which command configures WRR to service queue 2 16 times more than queue 1, for a 1p2q2t interface?

a.wrr-queue bandwidth 1 16

b.wrr-queue bandwidth 16 255

c.A and B

d.Neither A nor B

10.Which command configures WRR to service the strict-priority queue 64 times more than standard queue 1, for a 1p2q2t interface?

a.wrr-queue bandwidth 1 4 64

b.wrr-queue bandwidth 2 8 128

c.wrr-queue bandwidth 4 16 255

d.None of the above

404Chapter 17: DiffServ QoS Configuration

11.Which command enables WRED on interface queue 2?

a.wred 2

b.wrr-queue random-detect 2

c.wred-queue random-detect 2

d.wred-queue 2

12.A switch port has a minimum WRED threshold configured at 25 percent. If the queue stays below 25 percent, what can happen to the queued packets?

a.They can be dropped at random.

b.They can never be dropped.

c.They will always be dropped.

d.The minimum threshold must always be at 0 percent.

The answers to the “Do I Know This Already?” quiz are found in Appendix A, “Answers to Chapter ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows:

10 or less overall score—Read the entire chapter. This includes the “Foundation Topics,” “Foundation Summary,” and “Q&A” sections.

11 or 12 overall score—If you want more review on these topics, skip to the “Foundation Summary” section, and then go to the “Q&A” section at the end of the chapter. Otherwise, move to Chapter 18, “IP Telephony.”

Foundation Topics 405

Foundation Topics

Before any QoS features can be configured and used on a Catalyst switch, QoS itself must be enabled. By default, QoS is disabled, so any of its features must be explicitly configured according to the unique policies needed.

Does this mean that the switch offers no QoS at all? Not really; having QoS disabled simply means that only “best effort” packet delivery is in use. For example, switch port queues use the tail-drop method during times of congestion. Although this might not be ideal, it does take care of the basic packet forwarding needs.

To enable QoS on a switch, use the following global configuration command:

Switch(config)# mls qos

Also, keep in mind that Catalyst switches implement the DiffServ per-hop behavior by basing QoS operations on an internal Differentiated Services Code Point (DSCP) value. Refer to Figure 17-1 to see how the internal DSCP is used as a packet moves through the switch at these locations:

1.Packets entering the switch are assigned an internal DSCP value that is derived only from the QoS parameter that is trusted on the inbound switch port. The trusted parameter is mapped into DSCP values.

2.As the packets are internally switched, each packet’s internal DSCP value (as well as other QoS parameters) can be adjusted or acted upon.

3.As packets are queued at an engress port, the internal DSCP value is converted to a CoS (class of service) value, which is then used to determine the egress queueing or scheduling. The DSCP values are mapped into CoS values.

Although the various QoS parameter maps add to the complexity of configuring QoS on a Catalyst switch, the maps are a simple concept to remember. They are explained in more detail at the appropriate points in this chapter.

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