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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter30 Co nf iguring QoS
Understanding QoS
Figure30-1 Modular QoS CLI Model
Basic QoS includes these actions.
•Packet classification allows you to organ iz e traffic on the basis of whether or not the traff ic m atche s
a specific criteria. When a packet is received, the switch identifies all key packet fields: class of
service (CoS), Differentiated Services Code P oint (DSCP), or IP precedence. The switch class i fies
the packet based on this content or based on an access-control list l ookup. For more information, see
the “Classification” section on page 30-5.
•Packet policing determines whether a packet is in or out of profile by comparing the rate of the
incoming traffic to the configured policer. You can control the traffic flow for packets that conform
to or exceed the configured policer. For more information, see the “Policing” section on page30-12.
•Packet prioritization or marking evaluates the classification and policer information to d etermine the
action to take. All packets that belong to a classification ca n be r emar ked. W hen y ou co nfigure a
policer, packets that meet or exceed the permitted bandwidth requirements (bits per second) can be
conditionally passed through, dropped, or reclassified. For more in forma tio n, se e the “ Mar king ”
section on page 30-16.
•Congestion management uses queuing and scheduling algorithms to queue and sort traffic leaving a
port. The switch supports these scheduling and traffic-limiting features: class-based weighted fair
queuing (CBWFQ), class-based traffic shaping, port shap ing, an d class-b ased pri ority qu euing. You
can provide guaranteed bandwidth to a particular class of traffic while still servicing other traffic
queues. For more information, see the “Congestion Management a nd Sc hedul ing ” sec ti on on
page 30-18.
•Queuing on the switch is enhanced with the weighted tail-drop (WTD) algorithm, a
congestion-avoidance mechanism. WTD differentiated traffic classes and regulates the queue size
(in number of packets) based on the classification. For more informati on, se e the “Co ngesti on
Avoidance and Queuing” section on page 3 0-24.
This section includes information about these topics:
•Modular QoS CLI, page 30-3
•Input and Output Policies, page 30-4
•Classification, page 30-5
•Table Maps, page 30-11
•Policing, page 30-12
•Marking, page 30-16
•Congestion Management and Scheduling, page 30-18
•Congestion Avoidance and Queuing, page 30-24
Classification Policing Marking Congestion
Avoidance Queuing
Scheduling
Congestion
Drops
Policer
Drops
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