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5. Configuring Quality of Service
Use the features in the QoS tab to configure Quality of Service (QoS) settings on the switch.
The QoS tab contains links to the following features:
• Class of Service on page 331
• Differentiated Services on page 339
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.
Class of Service
The Class of Service (CoS) queueing feature lets you directly configure certain aspects of
switch queueing. This provides the desired QoS behavior for different types of network traffic
when the complexities of DiffServ are not required. 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.
Eight queues per port are supported.