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Chapter 14 - Quality of Service
The Quality of Service feature is applied throughout the network to ensure that network traffic is
prioritized according to required criteria and the desired traffic receives preferential treatment.
The QoS feature is used to optimize network performance. It provides classification of incoming
traffic to traffic classes, based on attributes, including:
•
Device configuration
•
Ingress interface
•
Packet content
•
Combination of these attributes
QoS includes the following:
•
Traffic Classification—Classifies each incoming packet as belonging to a specific traffic
flow, based on the packet contents and/or the port.
•
Assignment to Hardware Queues—Assigns incoming packets to forwarding queues.
Packets are sent to a particular queue for handling as a function of the traffic class to
which they belong. See Queue Scheduling.
•
Other Traffic Class-Handling Attribute—Applies QoS mechanisms to various classes,
including bandwidth management.
QoS Operation
When using the QoS feature, all traffic of the same class receives the same treatment, which
consists of a single QoS action of determining the egress queue on the egress port, based on the
indicated QoS value in the incoming frame. This is the VLAN Priority Tag (VPT) 802.1p value in
Layer 2 and the Differentiated Service Code Point (DSCP) value for IPv4 or Traffic Class (TC)
value for IPv6 in Layer 3. When operating in Basic Mode, the device trusts this external assigned
QoS value. The external assigned QoS value of a packet determines its traffic class and QoS.
The type of header field to be trusted is entered in the Basic QoS page. For every value of that
field, an egress queue is assigned, indicating through which queue the frame is sent, in the
CoS/802.1p to Queue page or the DSCP to Queue page (depending on whether the trust mode is
CoS/802.1p or DSCP, respectively).
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