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Cisco AVVID Network Infrastructure Enterprise Quality of Service Design

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Chapter 1 Overview

What is the Quality of Service Toolset?

Mission-Critical Data

Recommendation:

Gold class or mission-critical--DSCP AF21-23, IP Precedence 2, CoS 2; Silver

class--DSCP AF11-AF13, IP Precedence 1, CoS 1

Although the variety of factors that determine the importance of application traffic make it difficult to
apply a formula to the provisioning of data, guidelines can be set for the classification of mission-critical
traffic. Delay-sensitive applications that are critical to the operation of an enterprise, should be placed
in the "gold-data" category and assigned a DSCP PHB ranging from AF21 to AF23 to protect them from
drops. Less delay-sensitive applications that are important, but not critical, to the operation of an
enterprise, should be placed in the "silver-data" category and assigned a higher drop preference of AF11
to AF13.

If gold-data traffic is assigned AF21-AF23 and silver-data is assigned AF11-AF13, then these traffic
flows will automatically be backwardly compatible with the IP Precedence model. Gold-data will be
recognized as IP Precedence 2 and silver-data will be recognized as IP Precedence 1.

Less-Than-Best-Effort Data

Recommendation:

DSCP 2-6, IP Precedence 0, CoS 0

As mentioned before, non-critical, bandwidth-intensive data traffic should be assigned to the class
known as "less-than-best-effort." This traffic is delay-insensitive and should be given the least
preference of any of the classes and, as such, should be dropped sooner than any other traffic. Therefore,
less-than-best-effort traffic should be assigned a DSCP of 2-6 (decimal). This assignment is backwardly
compatible with an IP Precedence value of 0.

Best-Effort Data

Recommendation:

DSCP BE, IP Precedence 0, CoS 0

All other traffic should be placed in the "best-effort" category. This includes all non-interactive traffic,
regardless of importance.Best-effort traffic should be assigned a DSCP of BE. This assignment is
backwardly compatible with an IP Precedence value of 0.

Scheduling Tools

Scheduling tools refer to the set of tools that determine how a frame/packet exits a node. Whenever
packets enter a device faster than they can exit it (as with speed mismatches) then a point of congestion,
or a bottleneck, can occur. Devices have buffers that allow for scheduling higher-priority packets to exit
sooner than lower priority ones, which is commonly called queueing. Queueing algorithms are activated
only when a devices is experiencing congestion and are deactivated when the congestion clears.

Figure 1-9

shows a generic example of queueing.