Chapter 4
QoS
Network traffic is usually unpredictable, and the only
basic assurance that can be offered is best effort traffic
delivery. To overcome this challenge, Quality of Service
(QoS) is applied throughout the network. This ensures that
network traffic is prioritized according to specified criteria,
and that specific traffic receives preferential treatment.
QoS in the network optimizes network performance and
entails two basic facilities:
Classifying incoming traffic into handling classes, based
on an attribute, including:
The ingress interface
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Packet content
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A combination of these attributes
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Providing various mechanisms for determining the
allocation of network resources to different handling
classes, including:
The assignment of network traffic to a particular
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hardware queue
The assignment of internal resources
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Traffic shaping
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The terms Class of Service (CoS) and QoS are used in the
following context:
CoS provides varying Layer 2 traffic services. CoS
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refers to classifying traffic into traffic classes, where
each class is handled as an aggregate whole, with no
per-flow settings. CoS is usually related to the 802.1p
service that classifies flows according to their Layer 2
priority, as set in the VLAN header.
QoS refers to Layer 2 traffic and above. QoS handles
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per-flow settings, even within a single traffic class.
The QoS facility involves the following elements:
Traffic Classification—Classifies each incoming packet
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as belonging to a given traffic class, based on the
packet contents and/or the context.
Assignment to Hardware Queues—Assigns incoming
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packets to forwarding queues. Packets are sent to
a particular queue for handling as a function of the
traffic class to which they belong, as defined by the
classification mechanism.
Traffic Class-Handling Attributes—Applies QoS/CoS
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mechanisms to different classes, including Bandwidth
Management.
The QoS configuration options are as follows:
CoS Settings
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Queue Settings
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Busness Seres Smart Ggabt Ethernet Swtch
Configuration Using the Web-based Utility
DSCP Setting
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Basic Mode
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QoS > CoS Settings
The CoS Settings screen contains fields for enabling or
disabling CoS. This screen contains two sections, CoS
Settings and CoS Default.
QoS > CoS Settings
CoS Settings
QoS Mode
Indicates if QoS is enabled. The possible
values are:
Dsable
Disables QoS.
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Basc
Enables QoS. This is the default value.
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Class of Servce
Specifies the CoS priority tag values,
where 0 is the lowest and 7 is the highest.
Queue
Defines the traffic forwarding queue to which the
CoS priority is mapped. Four traffic priority queues are
supported.
Click Restore Defaults to restore the device factory
defaults for mapping CoS values to a forwarding queue.
CoS Default
Interface
The interface to which the CoS configuration
applies.
Default CoS
Determines the default CoS value for
incoming packets for which a VLAN tag is not defined. The
possible field values are 0-7. The default CoS is 0.
LAG
The LAG to which the port belongs, if relevant. If the
port is a member of a LAG, the LAG settings override the
port settings.
Click Save Settngs to save the changes, or Cancel
Changes to cancel the changes.