Handbook

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Quality of Service
Introduction
Quality of Service features allow you to allocate network resources to mission-critical applications at the
expense of applications that are less sensitive to such factors as time delays or network congestion. You
can configure your network to prioritize specific types of traffic, ensuring that each type receives the
appropriate Quality of Service (QoS) level.
The following topics are discussed in this section:
Quality of Service Overview
Using ACL Filters
Using DSCP Values to Provide QoS
Using 802.1p Priorities to Provide QoS
Queuing and Scheduling
Overview
QoS helps you allocate guaranteed bandwidth to the critical applications, and limit bandwidth for less
critical applications. Applications such as video and voice must have a certain amount of bandwidth to
work correctly; using QoS, you can provide that bandwidth when necessary. Traffic for applications that
are sensitive to timing out or cannot tolerate delay can be assigned to a high-priority queue.
By assigning QoS levels to traffic flows on your network, you can ensure that network resources are
allocated where they are needed most. QoS features allow you to prioritize network traffic, thereby
providing better service for selected applications.
The following figure shows the basic QoS model used by this switch.
Figure 11 QoS model
The switch uses the Differentiated Services (DiffServ) architecture to provide QoS functions. DiffServ is
described in IETF RFCs 2474 and 2475.
With DiffServ, you can establish policies to direct traffic. A policy is a traffic-controlling mechanism that
monitors the characteristics of the traffic, (for example, its source, destination, and protocol) and
performs a controlling action on the traffic when certain characteristics are matched.
The switch can classify traffic by reading the IEEE 802.1p priority value, or by using filters to match specific
criteria. When network traffic attributes match those specified in a traffic pattern, the policy instructs the
switch to perform specified actions on each packet that passes through it. The packets are assigned to
different Class of Service (COS) queues and scheduled for transmission.
The basic switch QoS model works as follows:
Classify traffic:
Read 802.1p Priority
Match ACL filter parameters
Meter traffic:
Define bandwidth and burst parameters
Select actions to perform on in-profile and out-of-profile traffic
Perform actions:
Drop packets
Pass packets
Mark DSCP or 802.1p Priority
Set COS queue (with or without re-marking)