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Quality of Service
Cisco Small Business SG200 Series 8-port Smart Switches Administration Guide
This chapter provides an overview of Quality of Service (QoS) and explains the
QoS features available from the Quality of Service menu.
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QoS Properties
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Defining Queues
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Mapping CoS/802.1p Priorities to Queues
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Mapping IP Precedence to Queues
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Mapping DSCP Values to Queues
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Defining Rate Limit Profiles
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Applying Rate Limit Profiles to Interfaces
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Traffic Shaping
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
configured 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 might be dropped by the switch.
QoS is a means of providing consistent, predictable data delivery by
distinguishing 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.
In networks where QoS operation is enabled, 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.
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