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Deploying Basic Quality of Service Features


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Deploying Basic Quality of Service Features



The Case for QoS



Queuing in a Router



Priority Queuing



Custom Queuing



Understanding IP Precedence



Weighted Fair Queuing




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Enhanced IP Services for Cisco Networks

From: Enhanced IP Services for Cisco Networks
Author: Donn Lee
Publisher: Cisco Press (53)
More Information

4. Deploying Basic Quality of Service Features

An effective network provides more than just connectivity between endpoints. As you add applications and leverage the convenience of networking, your network must be intelligent enough to recognize and prioritize mission-critical and delay-sensitive traffic. This ability to deliver data based on such policies as importance and time is called the quality of service (QoS) capability of the network.

This chapter explains basic QoS principles and the configuration of IOS queuing services that deliver QoS. These are the main topics of this chapter:

  • The Case for QoS

  • Queuing in a Router

  • Priority Queuing

  • Custom Queuing

  • Understanding IP Precedence

  • Weighted Fair Queuing

The Case for QoS

The network serves a wide range of applications for your organization—many more applications than the mere file and printer sharing of the past. In addition to standard IP traffic such as web (HTTP), File Transfer Protocol (FTP), Telnet, and Simple Mail Transfer Protocol (SMTP), networks are carrying:

  • Real-time, mission-critical corporate data—financial transactions, customer orders, warehouse and shipping records, manufacturing statistics and control data, research and development CAD/code files, and so on

  • Delay-sensitive data—interactive applications such as server and mainframe logins, packetized voice, video conferences, and data collaboration such as electronic whiteboard and client GUI sharing

  • Bulk data transfers—system backups, overnight data synchronization and delivery

  • Unknown data—Uncontrolled or unknown traffic such as user-initiated applications (networked games and shareware, for example)

Despite the broad range of users and applications, many networks give equal access to all users and the same delivery priority to all applications. They do not inspect or care about the contents of the data.

With an ever-increasing number of new applications on the network, the idea of equal priority to all applications becomes a problem. The problem is that applications vary by how they use the network, how they behave during network congestion, and how important they are to your organization.

Not all applications are equal. Applications that are mission-critical to your organization deserve preferential and speedy delivery. Some applications may be delay-sensitive and need low delay through the network in order to function properly. Yet other applications may be low priority and should yield network resources to high-priority applications.

After all, your network is a valuable resource that is shared and finite in capacity.

A network that can vary performance based on application type is said to have classes, or qualities of service. A QoS is a grade of performance the network provides and differentiates from other grades of performance in the network. A high QoS provides faster delivery and less delay than a low QoS. By assigning applications to different QoS levels, you can vary the performance of applications to reflect your organization's objectives.


The actual number of QoS levels you can use depends on the underlying network and queuing technology. Because the focus of this book is on IOS, the sections that follow in this chapter and Chapter 5, “Deploying Advanced Quality of Service Features,” explain the QoS levels available in IOS services.

Organizations consider many factors when defining the QoS for an application. The most common criteria are as follows:

  • Mission-critical versus non-mission-critical— Does this application directly affect my organization's profits and sales? How will my customers perceive the delays in this application, and what is the impact? Does this application affect how quickly I can bring my product to market?

  • Delay-sensitive versus delay-insensitive— How easily does the user of the application perceive delay? Even if an application is not mission-critical, it may require minimal delay because a human is interacting with it in real time. Login applications such as Telnet are delay-sensitive because typing is difficult when there is a perceptible delay between keystrokes and the display of those keystrokes. On the other hand, non-interactive applications (such as file system backup or FTP) may be triggered and completed with minimal or no user intervention.

    Applications such as voice and video over IP are delay-sensitive. These applications need consistent, predictable bandwidth and low delay; otherwise, the transmission may appear garbled or choppy. This does not necessarily mean that all users of voice and video should be given high priority, as it might be desirable to set policies (in conjunction with QoS) to limit these applications to certain users.

  • Political versus apolitical— Whose data is this? What users should get better service from the network?

Cisco offers several IOS services for delivering QoS. As we cover each service in the following sections, keep in mind that each service has its own behavior, purpose, and place in the network. It is not enough to know how to configure these services, but rather, to know how they work, when they might be needed, and where they should be placed in the network. This is covered in the following sections as well as in Chapter 5.


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