Allied Telesis AT-S63, Features Manual

Page: 162 / 538

Chapter 14: Quality of Service
162 Section II: Advanced Operations
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.
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 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.
Although a policy can be applied to an egress port, the classifiers and the
QoS controls are actually applied by the switch on the ingress ports of the
traffic. This means the parameters used to classify the traffic and the
actions specified by the policy are checked and applied on the ingress
traffic of every port, before the traffic reaches an egress queue. As a
consequence, a policy is never applied to the whole aggregated traffic of a
designated egress port, but rather to the individual ingress flows destined
to the port.
The effects of this behavior become evident when using the maximum
bandwidth feature of QoS. Here is an example. Suppose you have a policy
that assigns 5 Mbps of maximum bandwidth to an egress port. Now
assume there are 10 ports on the switch where ingress traffic matches the
criteria specified in the classifier assigned to the policy of the egress port.
Since the policy considers each ingress flow separately, the result would
be a maximum bandwidth of 50 Mbps (10 x 5 Mbps) on the egress port,
because there are 10 flows, one from each ingress port, directed to the
egress port.
An additional factor to consider when specifying an egress port in a policy
is that if the destination MAC address of the traffic flow has not been
learned by the egress port or, alternatively, added as a static address to
the port, the policy remains inactive. This is because the ingress ports
consider the traffic as unknown traffic and flood the traffic to all the ports.
This applies equally to unknown unicast and unknown multicast traffic, as
well as broadcast traffic.