Allied Telesis AT-S62 - Manual (Page 256)

Chapter 16: Quality of Service
Section II: Advanced Operations 256
Classifiers
Classifiers are used to identify a particular traffic flow, and range from
general to specific. (See Chapter 14, Classifiers on page 219 for more
information.) Note that a single classifier should not be used in different
flows that will end up, via traffic classes, assigned to the same policy. A
classifier should only be used once per policy. Traffic is matched in the
order of classifiers. For example, if a flow group has classifiers 1, 3, 2 and
5, that is the order in which the packets are matched.
Flow Groups
Flow groups are used to group similar traffic flows together, and allow
more specific QoS controls to be used, in preference to those specified
by the traffic class. Flow groups consist of a small set of QoS parameters
and a group of classifiers. Once a flow group has been added to a traffic
class it cannot be added to another traffic class. A traffic class may have
many flow groups. Traffic is matched in the order of the flow groups. For
example, if a traffic class has flow groups 1, 3, 2 and 5, this is the order in
which the packets are matched.
QoS controls at the flow group level provide a QoS hierarchy. Non-
default flow group settings are always used, but if no setting is specified
for a flow group, the flow group uses the settings for the traffic class to
which it belongs. For example, you can use a traffic class to limit the
bandwidth available to web and FTP traffic combined. Within that traffic
class, you can create two different flow groups with different priorities,
to give web traffic a higher priority than FTP. Web traffic would then be
given preferential access to bandwidth, but would be limited to the
bandwidth limit of the traffic class.
Traffic Classes
Traffic classes are the central component of the QoS solution. They
provide most of the QoS controls that allow a QoS solution to be
deployed. A traffic class can be assigned to only one policy. Once
assigned, it cannot be used by any other policies. Traffic classes consist
of a set of QoS parameters and a group of QoS Flow Groups. Traffic can
be prioritized, marked (IP TOS or DSCP field set), and bandwidth limited.
Traffic is matched in the order of traffic class. For example, if a policy has
traffic classes 1, 3, 2 and 5, this is the order in which the packets are
matched.
Policies
QoS policies consist of a collection of user defined traffic classes. A policy
can be assigned to more than one port, but a port may only have one
policy.
QoS controls are applied to ingress traffic on ports. Therefore, to control
a particular type of traffic, an appropriate QoS policy must be attached
to each port that type of traffic ingresses. In most situations, the same
policy can be applied to all ports, and to classify according to an egress
port.