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218
5
5. Configure Quality of Service
In a 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 can no longer be held for transmission and are dropped by the switch.
Quality of Service (QoS) is a means of providing consistent, predictable data delivery by
distinguishing packets with 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 that is not QoS capable creates a deficiency in the network path, and the performance of
the entire packet flow is compromised.
This chapter covers the following topics:
• Manage Class of Service
• Manage Differentiated Services
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