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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.

Per-Hop Behavior Queue Design Principles

The goal of convergence in the network is to enable voice, video, and data applications to seamlessly coexist in the network by providing each with appropriate QoS service expectations and guarantees.

When real-time applications are the only ones that consume link bandwidth, non-real–time applications’ performance can be significantly degraded. Extensive testing results show that there is significant performance impact on non-real–time applications when more than one-third of the links is used by real-time applications as part of a strict-priority queue. Thus, it is recommended that no more than a third of link bandwidth be used for strict-priority queuing. This principle prevents non-real–time applications from being dropped out of their required QoS recommendations. In other words, it is recommended that no more than 33 percent of the bandwidth be used for the expedite forwarding (EF) queue. It is also important to note that this 33 percent design principle is simply a best practices design recommendation and not necessarily a mandatory rule.

It is recommended that a minimum of one queue be provisioned for assured forwarding per-hop behavior (AF PHB), but up to four subclasses can be defined within the AF class: AF1x, AF2x, AF3x, and AF4x. Each queue belonging to the specified AF subclass must have a bandwidth guarantee that corresponds to the application requirements of that traffic subclass.

The default forwarding (DF) class consists of all traffic that is not explicitly defined in other queues. If an enterprise is using many applications, it is important to have adequate space for those traffic types. It is recommended that typically 25 percent of link bandwidth be used for this service class. Figure 16-3 illustrates an example of bandwidth allocation leveraging these recommended best practices.

Figure 16-3

Figure 16-3 Bandwidth Allocation Example

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