Cisco ASR 9000 Series Configuration Manual page 13

Modular Quality of Service Overview on Cisco ASR 9000 Series Routers
Packet Classification
Packet classification techniques identify the traffic flow, and provide the capability to partition network
traffic into multiple priority levels or classes of service. After traffic flow is identified, it can be marked
as a traffic class.
Identification of a traffic flow can be performed by using several methods within a single router: access
control lists (ACLs), protocol match, IP precedence, IP differentiated service code point (DSCP), and so
on.
Marking of a traffic flow is performed by
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Marking can be carried out:
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For detailed conceptual and configuration information about packet marking, see the "Configuring
Modular Quality of Service Packet Classification on Cisco ASR 9000 Series Routers" module in this
guide for unconditional marking, and the "Configuring Modular Quality of Service Congestion
Management on Cisco ASR 9000 Series Routers" module in this guide for conditional marking.
Congestion Management
Congestion management techniques control congestion after it has occurred. One way that network
elements handle an overflow of arriving traffic is to use a queueing algorithm to sort the traffic, then
determine some servicing method of prioritizing it onto an output link.
Cisco IOS XR software implements the Low Latency Queueing (LLQ) feature, which brings strict
priority queueing (PQ) to the Modified Deficit Round Robin (MDRR) scheduling mechanism. LLQ with
strict PQ allows delay-sensitive data, such as voice, to be dequeued and sent before packets in other
queues are dequeued.
Cisco IOS XR software includes traffic policing capabilities available on a per-class basis as well as
class-based shaping.
The traffic policing feature limits the input or output transmission rate of a class of traffic based on
user-defined criteria, and can mark packets by setting values such as IP Precedence, QoS group, or DSCP
value.
Traffic shaping allows control over the traffic that leaves an interface to match its flow to the speed of
the remote target interface and ensure that the traffic conforms to the policies contracted for it. Thus,
traffic adhering to a particular profile can be shaped to meet downstream requirements, thereby
eliminating bottlenecks in topologies with data-rate mismatches.
Cisco IOS XR software supports a class-based traffic shaping method through a CLI mechanism in
which parameters are applied per class.
For detailed conceptual and configuration information about congestion management, see the
"Configuring Modular Quality of Service Congestion Management on Cisco ASR 9000 Series Routers"
module.
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D r a f t — C i s c o C o n f i d e n t i a l
Setting IP Precedence or DSCP bits in the IP Type of Service (ToS) byte.
Setting Class of Service (CoS) bits in the Layer 2 headers.
Setting EXP bits within the imposed or the topmost Multiprotocol Label Switching (MPLS) label.
Setting qos-group and discard-class bits.
Unconditionally—As part of the class-action.
Conditionally—As part of a policer-action.
Combination of conditionally and unconditionally.
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
Information About Modular Quality of Service on Cisco ASR 9000 Series Routers
QC-3