Allied Telesis AT-S63 - Manual (Page 300)

Chapter 16: Quality of Service
300 Section II: Advanced Operations
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. 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.
Note that the switch can only perform error checking of parameters and
parameter values for the policy and its traffic classes and flow groups
when the policy is set on a port.
QoS Policy
Guidelines
Following is a list of QoS policy guidelines:
ˆ A classifier may be assigned to many flow groups. However, assigning
a classifier more than once within the same policy may lead to
undesirable results. A classifier may be used successfully in many
different policies.
ˆ A flow group must be assigned at least one classifier but may have
many classifiers.
ˆ A flow group may only be assigned to one traffic class.
ˆ A traffic class may have many flow groups.
ˆ A traffic class may only be assigned to one policy.