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QoS Design Principles and Best Practices

Chapter Description

In this sample chapter from Designing for Cisco Network Service Architectures (ARCH) Foundation Learning Guide: CCDP ARCH 300-320, 4th Edition, the authors cover some best practice QoS design principles and QoS strategy models that are used to implement the numerous QoS tools we have at our disposal. Remember that usually, more than one solution fits the given QoS requirements, so simplifying the models leveraged can significantly accelerate and ensure proper QoS deployment.

QoS Strategy Models

Before applying any QoS tools, organizations need to define the strategy and goals for different applications running in their network. This will result in defining a certain number of traffic classes to meet the end-to-end QoS objectives of an organization. Three basic QoS strategy models can be deployed, depending on the granularity of applications running within an organization’s network:

  • 4-Class QoS Strategy Model

  • 8-Class QoS Strategy Model

  • 12-Class QoS Strategy Model

Although the more classes you define, the more specific and granular traffic treatment will be per application, the selection of a certain strategy model must be based on application requirements coupled with the WAN provider QoS model (if there is any WANs with QoS). The following sections provide a detailed view into each of these QoS strategy models.

4-Class QoS Strategy

The 4-class QoS strategy model is the simplest of the three models (in terms of QoS polices) and typically accounts for telephony, signaling, transactional/mission-critical, and best-effort data. When businesses deploy telephony applications in their network, three classes of traffic are typically required (telephony, signaling, and default/best effort).

Typically, the fourth class is the Assured Forwarding (AF) class. The AF class is used for transactional and mission-critical data applications such as SQL databases. The AF class can also be used for multimedia conferencing, multimedia streaming, and bulk data applications.

The 4-class QoS strategy model, as shown in Figure 16-5, is an example of where an organization has deployed IP telephony. In addition to separating telephony, signaling, and default/best-effort traffic, the organization has defined one mission-critical transactional data class.

Figure 16-5

Figure 16-5 The 4-Class QoS Strategy Model

The four traffic classes of QoS markings and guarantees are as follows:

  • Voice (Real time): Marked with EF and provisioned to leverage up to one-third of link bandwidth

  • Signaling: Marked with CS3 and provisioned to leverage a minimum of 7 percent of link bandwidth

  • Mission-critical data (Transactional Data): Marked with AF31 and provisioned to leverage 35 percent of link bandwidth

  • Default (best-effort data): Marked with DF and provisioned to take advantage of 25 percent of link bandwidth

Voice and signaling guarantees must be selected based on the volume of voice calls and the VoIP codec that is used through the given link. Mission-critical data is selected based on the decision of the director of each company department who has given info about critical business application needs to the networking team.

8-Class QoS Strategy

The 8-class QoS strategy model builds upon the 4-class model and includes the following additional classes:

  • Multimedia conferencing

  • Multimedia streaming

  • Network control

  • Scavenger

The two additional multimedia traffic types in this model are multimedia conferencing and multimedia streaming. The explicitly defined network control traffic class is used for applications such as network routing protocol updates or network infrastructure control traffic such as OAM. The 8-class QoS strategy model is illustrated in Figure 16-6.

Figure 16-6

Figure 16-6 The 8-Class QoS Strategy Model

As can be seen from Figure 16-6, the recommendations for each traffic class in this model are as follows:

  • Voice: Marked with EF and limited to 10 percent of link bandwidth in a strict-priority queue

  • Multimedia conferencing (Interactive video): Marked with AF41 or sometimes as EF and limited to 23 percent of link bandwidth in a strict-priority queue

  • Multimedia streaming: Marked with AF31 and guaranteed 10 percent of link bandwidth with WRED enabled

  • Network control: Marked with CS6 and guaranteed 5 percent of link bandwidth

  • Signaling: Marked with CS3 and provisioned with minimum of 2 percent of link bandwidth

  • Transactional data: Marked with AF21 and provisioned with 24 percent of link bandwidth with WRED enabled

  • Default (best-effort data): Marked with DF and provisioned with 25 percent of link bandwidth

  • Scavenger: Marked with CS1 and provisioned with a maximum of 1 percent of link bandwidth

12-Class QoS Strategy

The 12-class QoS strategy model builds upon the 8-class model and includes the following additional classes:

  • Real-time Interactive

  • Broadcast Video

  • Management/OAM

  • Bulk Data

The 12-class QoS strategy model represents Cisco’s interpretation of the RFC 4594 recommendation and, as previously noted, incorporates a slight modification by swapping the markings used for signaling and broadcast video. The 12-class QoS strategy model is illustrated in Figure 16-7.

Figure 16-7

Figure 16-7 The 12-Class QoS Strategy Model

As can be seen from Figure 16-7, the recommendations for each traffic class in this model are as follows:

  • Voice: Marked with EF and limited to 10 percent of link bandwidth in a strict-priority queue

  • Broadcast video: Marked with CS5 or sometimes as EF and limited to 10 percent of link bandwidth in a strict-priority queue

  • Real-time interactive: Marked with CS4 or sometimes as EF and limited to 13 percent of link bandwidth in a strict-priority queue

  • Multimedia conferencing: Marked with AF41 or sometimes as EF and limited to 10 percent of link bandwidth in a strict-priority queue

  • Multimedia streaming: Marked with AF31 and guaranteed 10 percent of link bandwidth with WRED enabled

  • Network control: Marked with CS6 and provisioned as guaranteed bandwidth 2 percent of link bandwidth

  • Signaling: Marked with CS3 and provisioned with a minimum of 2 percent of link bandwidth

  • Management/OAM: Marked with CS2 and provisioned with a minimum of 3 percent of link bandwidth

  • Transactional data: Marked with AF21 and provisioned with 10 percent of link bandwidth with WRED enabled

  • Bulk data: Marked with AF11 and provisioned with 4 percent of link bandwidth with WRED enabled

  • Default (best-effort data): Marked with DF and provisioned with 25 percent of link bandwidth

  • Scavenger: Marked with CS1 and provisioned with a maximum of 1 percent of link bandwidth

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