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24-2
Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 24 Configuring QoS
Understanding QoS
Understanding QoS
Typically, networks operate on a best-effort delivery basis, which means that all traffic has equal priority
and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an
equal chance of being dropped.
When you configure the QoS feature, you can select specific network traffic, prioritize it according to
its relative importance, and use traffic-management techniques to provide preferential treatment.
Implementing QoS in your network makes network performance more predictable and bandwidth
utilization more effective.
Figure 24-1 shows the MQC model.
Figure 24-1 Modular QoS CLI Model
Basic QoS includes these actions.
• Packet classification organizes traffic on the basis of whether or not the traffic matches a specific
criteria. When a packet is received, the router identifies all key packet fields: class of service (CoS),
Differentiated Services Code Point (DSCP), or IP precedence. The router classifies the packet based
on this content or based on an access-control list lookup. For more information, see the
“Classification” section on page 24-7.
• 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. You can configure a committed information rate (CIR) and peak
information rate (PIR) and set actions to perform on packets that conform to the CIR and PIR
(conform-action), packets that conform to the PIR, but not the CIR (exceed-action), and packets that
exceed the PIR value (violate-action). For more information, see the “Policing” section on
page 24-14.
• Packet prioritization or marking evaluates the classification and policer information to determine the
action to take. All packets that belong to a classification can be remarked. When you configure a
policer, packets that meet or exceed the permitted bandwidth requirements (bits per second) can be
conditionally passed through, dropped, or reclassified. For more information, see the “Marking”
section on page 24-18.
• Congestion management uses queuing and scheduling algorithms to queue and sort traffic that is
leaving a port. The router supports these scheduling and traffic-limiting features: class-based
weighted fair queuing (CBWFQ), class-based traffic shaping, port shaping, and class-based priority
queuing. You can provide guaranteed bandwidth to a particular class of traffic while still servicing
other traffic queues. For more information, see the “Congestion Management and Scheduling”
section on page 24-19.
Classification Policing Marking Congestion
Avoidance
Queuing
Scheduling
Congestion
Drops
Policer
Drops
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