4-1
v1.0, July 2009
Chapter
4
Configuring Quality
of
Service
This section gives an overview of Quality of Service (QoS) and explains the QoS features
available from the Quality of Service navigation tree menu. This section contains the following
subsections:
•
“Configuring Class of Service” on page
4-1
In a typical switch, each physical port consists of one or more queues for transmitting packets on
the attached network. Multiple queues per port are often provided to give preference to certain
packets over others based on user-defined criteria. When a packet is queued for transmission in a
port, the rate at which it is serviced depends on how the queue is configured and possibly the
amount of traffic present in the other queues of the port. If a delay is necessary, packets get held in
the queue until the scheduler authorizes the queue for transmission. As queues become full,
packets have no place to be held for transmission and get dropped by the switch.
QoS is a means of providing consistent, predictable data delivery by distinguishing between
packets that have strict timing requirements from those that are more tolerant of delay. Packets
with strict timing requirements are given “special treatment” in a QoS-capable network. With this
in mind, all elements of the network must be QoS-capable. The presence of at least one node
which is not QoS-capable creates a deficiency in the network path and the performance of the
entire packet flow is compromised.
Configuring Class of Service
The Class of Service (CoS) queueing feature lets you directly configure certain aspects of switch
queueing. The priority of a packet arriving at an interface can be used to steer the packet to the
appropriate outbound CoS queue through a mapping table. CoS queue characteristics that affect
queue mapping, such as minimum guaranteed bandwidth, or transmission rate shaping are user-
configurable at the queue (or port) level.
Four queues per port are supported.
The Class of Service folder contains links to the following features:
•
“Basic CoS Configuration” on page
4-2
•
“CoS Interface Configuration” on page
4-3