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Preparing for the CCNP ONT Exam: IP Quality of Service

Chapter Description

Amir Ranjbar provides essential background, definitions, and concepts for you to start learning IP quality of service and preparing for the CCNP ONT exam.

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, the information in this section is a convenient way to review the day before the exam.

In a converged enterprise network, four major issues affect the performance and perceived quality of applications:

  • Available bandwidth
  • End-to-end delay
  • Variation of delay (jitter)
  • Packet loss

Lack of sufficient bandwidth, high end-to-end delay, high variation in delay, and excessive packet loss lower the quality of applications.

QoS is the ability of the network to provide better or "special" service to a set of users or applications or both to the detriment of other users or applications or both. You can use several QoS features, tools, and technologies to accomplish the QoS goals. Classification, marking, congestion avoidance, congestion management, compression, shaping, and policing are examples of QoS tools available in Cisco IOS. The three steps of implementing QoS in an enterprise network are as follows:

Step 1Identify the network traffic and its requirements
Step 2Define traffic classes
Step 3Define a QoS policy for each traffic class

The main QoS models of today are as follows:

  • Best-effort—The best-effort model requires no QoS configuration and mechanisms; therefore, it is easy and scalable, but it provides no Differentiated Service to different application types.
  • IntServ—IntServ provides guaranteed service (Hard QoS). It uses signaling to reserve and guarantee resources for each traffic flow below it. RSVP is the common signaling protocol for resource reservation signaling on IP networks. Per-flow signaling and monitoring escalate the overhead of the IntServ model and make it nonscalable.
  • DiffServ—DiffServ is the most modern of the three models. It requires traffic classification and marking and providing differentiated service to each traffic class based on its marking. DiffServ is scalable, but its drawback is that it requires implementation of complex QoS features on network devices throughout the network.

Network administrators have four methods at their disposal to implement QoS on their network's Cisco devices:

  • Cisco IOS CLI—Configuring QoS using Cisco IOS CLI is the most complex and time-consuming method. It requires that you learn different syntax for each QoS mechanism.
  • MQC—MQC is a modular command-line interface that is common across different Cisco platforms, and it separates the task of defining different traffic classes from the task of defining QoS policies.
  • Cisco AutoQoS—Because AutoQoS automatically generates QoS commands on your router or switch, it is the simplest and fastest method among the four QoS implementation methods. However, should you need to fine-tune the AutoQoS configuration results, you must use MQC (or CLI) to do so. Fine-tuning of the commands that AutoQoS generates is seldom necessary.
  • Cisco Router and Security Device Manager (SDM) QoS Wizard—Cisco SDM offers several wizards for implementing services, such as IPsec, VPN, and proactive management through performance monitoring, in addition to the QoS Wizard. Cisco SDM QoS Wizard allows you to remotely configure and monitor your Cisco routers without using the CLI. The SDM GUI makes it simple for you to implement QoS services, features, and policies.

Table 2-3 compares Cisco IOS CLI, MQC, AutoQoS, and SDM with respect to how easy they are to use, whether they allow you to fine-tune their results, how time consuming they are, and how modular they are.

Table 2-3. Comparing QoS Implementation Methods






Ease of use

Most difficult

Easier than legacy CLI



Ability to fine-tune

Yes (OK)

Very well



Time consuming to implement

Most time consuming (longest)

Moderate time consumed (average)

Least time consuming

Very little time consumed (short)


Weakest (poor)

Very modular (excellent)

Very modular (excellent)


MQC is the recommended and the most powerful method for implementing QoS. It is modular, it promotes re-use of written code, and it facilitates consistency of QoS configurations among your Cisco devices. MQC also reduces the chances for errors and conflicts, while allowing you to take advantage of the latest features and mechanisms offered by your version of Cisco IOS.