Enterprise QoS Solution Reference Network Design Guide

Chapter 1 Quality of Service Design Overview
Version 3.3
How is QoS Optimally Deployed within the Enterprise?
Cisco IOS software allows the abstraction (and thus configuration) of multiple strict priority LLQs. In
such a multiple LLQ context, this design principle would apply to the sum of all LLQs to be within
one-third of link capacity.
Note This strict priority queuing rule (limit to 33 percent) is simply a best practice design recommendation
and is not a mandate. There may be cases where specific business objectives cannot be met while holding
to this recommendation. In such cases, enterprises must provision according to their detailed
requirements and constraints. However, it is important to recognize the tradeoffs involved with
over-provisioning strict priority traffic and its negative performance impact on non-realtime-application
response times.
Whenever a Scavenger queuing class is enabled, it should be assigned a minimal amount of bandwidth.
On some platforms, queuing distinctions between Bulk Data and Scavenger traffic flows cannot be made
because queuing assignments are determined by CoS values and these applications share the same CoS
value of 1. In such cases you can assign the Scavenger/Bulk queuing class a bandwidth percentage of 5.
If you can uniquely assign Scavenger and Bulk Data to different queues, then you should assign the
Scavenger queue a bandwidth percentage of 1.
The Realtime, Best Effort and Scavenger queuing best practice principles are shown in Figure 1-7.
Figure 1-7 Realtime, Best Effort and Scavenger Queuing Rules
Best Effort > 25%
Scavenger/Bulk <
5%
Because platforms support a variety of queuing structures, configure consistent queuing policies
according to platform capabilities to ensure consistent PHBs.
For example, on a platform that only supports four queues with CoS-based admission (such as a Catalyst
switch) a basic queuing policy could be as follows:
Realtime (33%)
Critical Data
Best Effort Data (25%)
Scavenger/Bulk (5%)
Critical Data
Realtime < 33%
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