Cisco ASR 9000 Series User manual

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Cisco ASR 9000 Series Aggregation
Services Router Modular Quality of
Service Configuration Guide
Cisco IOS XR Software Release 3.7 FCI
Text Part Number: OL-17239-01

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Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
© 2009 Cisco Systems, Inc. All rights reserved.

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CONTENTS
Preface QC-ix
Changes to This Document QC-ix
Obtaining Documentation and Submitting a Service Request QC-ix
Modular Quality of Service Overview on Cisco ASR 9000 Series Routers QC-1
Contents QC-1
Information About Modular Quality of Service on Cisco ASR 9000 Series Routers QC-2
Benefits of Cisco ASR 9000 Series Router QoS Features QC-2
QoS Techniques on Cisco ASR 9000 Series Routers QC-2
Packet Classification QC-3
Congestion Management QC-3
Congestion Avoidance QC-4
Differentiated Service Model for Cisco IOS XR Software QC-4
Access Node Control Protocol QC-4
Modular QoS Command Line Interface QC-4
Where to Go Next QC-5
Additional References QC-5
Related Documents QC-5
Standards QC-5
MIBs QC-6
RFCs QC-6
Technical Assistance QC-6
Configuring Modular Quality of Service Packet Classification on
Cisco ASR 9000 Series Routers QC-7
Contents QC-7
Prerequisites for Configuring Modular QoS Packet Classification on
Cisco ASR 9000 Series Routers QC-8
Information About Configuring Modular QoS Packet Classification on
Cisco ASR 9000 Series Routers QC-8
Packet Classification Overview QC-8
Traffic Class Elements QC-9
Traffic Policy Elements QC-9
Default Traffic Class QC-10

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Class-based Unconditional Packet Marking Feature and Benefits QC-10
Specification of the CoS for a Packet with IP Precedence QC-11
IP Precedence Bits Used to Classify Packets QC-11
IP Precedence Value Settings QC-12
IP Precedence Compared to IP DSCP Marking QC-12
Hierarchical Ingress Policing QC-13
How to Configure Modular QoS Packet Classification on Cisco ASR 9000 Series Routers QC-13
Creating a Traffic Class QC-13
Restrictions QC-13
Creating a Traffic Policy QC-16
Restrictions QC-17
Attaching a Traffic Policy to an Interface QC-19
Prerequisites QC-19
Restrictions QC-19
Attaching a Traffic Policy to Multiple Subinterfaces QC-20
Prerequisites QC-20
Restrictions QC-20
Configuring Class-based Unconditional Packet Marking QC-22
Configuring Hierarchical Ingress Policing QC-25
Restrictions QC-26
Configuration Examples for Configuring Modular QoS Packet Classification on Cisco IOS XR
Software QC-28
Traffic Classes Defined: Example QC-28
Traffic Policy Created: Example QC-29
Traffic Policy Attached to an Interface: Example QC-29
Traffic Policy Attached to Multiple Subinterfaces: Example QC-29
Default Traffic Class Configuration: Example QC-29
class-map match-any Command Configuration: Example QC-30
Traffic Policy as a QoS Policy (Hierarchical Traffic Policies) Configuration: Example QC-30
Class-based, Unconditional Packet Marking Examples QC-31
IP Precedence Marking Configuration: Example QC-31
IP DSCP Marking Configuration: Example QC-31
QoS Group Marking Configuration: Example QC-32
QoS Group and Discard Class Marking Configuration: Example QC-32
CoS Marking Configuration: Example QC-32
MPLS Experimental Bit Imposition Marking Configuration: Example QC-33
MPLS Experimental Topmost Marking Configuration: Example QC-33
Hierarchical Ingress Policing: Example QC-33
Where to Go Next QC-34

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Additional References QC-35
Related Documents QC-35
Standards QC-35
MIBs QC-35
RFCs QC-35
Technical Assistance QC-36
Configuring Modular Quality of Service Congestion Management
on Cisco ASR 9000 Series Routers QC-37
Contents QC-38
Prerequisites for Configuring QoS Congestion Management on Cisco ASR 9000 Series Routers QC-38
Information About Configuring QoS Congestion Management on Cisco ASR 9000 Series Routers QC-38
Congestion Management Overview QC-39
Modified Deficit Round Robin QC-39
Low-Latency Queueing with Strict Priority Queueing QC-40
Traffic Shaping QC-40
Traffic Shaping Mechanism Regulates Traffic QC-41
Traffic Policing QC-42
Packet Marking Through the IP Precedence Value, IP DSCP Value, and the MPLS Experimental
Value Setting QC-42
Traffic Policing Mechanism Regulates Traffic QC-43
Traffic Shaping Versus Traffic Policing QC-44
How to Configure QoS Congestion Management on Cisco ASR 9000 Series Routers QC-45
Configuring Guaranteed and Remaining Bandwidths QC-45
Restrictions QC-45
Configuring Low-Latency Queueing with Strict Priority Queueing QC-48
Restrictions QC-48
Configuring Traffic Shaping QC-51
Restrictions QC-51
Configuring Traffic Policing QC-54
Restrictions QC-54
Configuration Examples for Configuring QoS Congestion Management on Cisco ASR 9000 Series
Routers QC-57
Traffic Shaping for an Input Interface: Example QC-57
Traffic Policing for a Bundled Interface: Example QC-58
Traffic Policing for an IPSec Interface: Example QC-59
Multiaction Set/Policer: Example QC-59
Where to Go Next QC-60
Additional References QC-60
Related Documents QC-60

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Standards QC-60
MIBs QC-60
RFCs QC-61
Technical Assistance QC-61
Configuring Modular QoS Congestion Avoidance on Cisco ASR 9000 Series Routers QC-63
Contents QC-63
Prerequisites for Configuring Modular QoS Congestion Avoidance on
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Information About Configuring Modular QoS Congestion Avoidance on
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Random Early Detection and TCP QC-64
Weighted Random Early Detection for Preferential Traffic Handling QC-64
Average Queue Size for WRED QC-65
WRED Support for IPv4 Multicast Egress QoS Traffic QC-66
Restrictions QC-67
Tail Drop and the FIFO Queue QC-67
How to Configure Modular QoS Congestion Avoidance on Cisco IOS XR Software QC-67
Configuring Non-weighted Random Early Detection QC-67
Restrictions QC-67
Configuring Weighted Random Early Detection QC-70
Restrictions QC-70
Configuring Tail Drop QC-73
Restrictions QC-73
Configuration Examples for Configuring Policy Maps QC-76
Enabling WRED for IPv4 Egress Multicast Traffic QC-76
Where to Go Next QC-77
Additional References QC-77
Related Documents QC-77
Standards QC-77
MIBs QC-77
RFCs QC-78
Technical Assistance QC-78
Configuring Access Node Control Protocol
on Cisco ASR 9000 Series Routers QC-79
Contents QC-79
Prerequisites for Configuring ANCP on Cisco ASR 9000 Series Routers QC-80
Restrictions for Configuring ANCP on Cisco ASR 9000 Series Routers QC-80
Information About Configuring ANCP on Cisco ASR 9000 Series Routers QC-80

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ANCP Adjacencies QC-80
Neighbor Adjacency Timing QC-81
ANCP Messages QC-81
Port Mapping QC-81
Rate Adjustment QC-82
Prioritization of ANCP Traffic QC-82
Process Restart QC-82
How to Configure ANCP on Cisco ASR 9000 Series Routers QC-82
Enabling ANCP QC-83
Prerequisites QC-83
Configuring ANCP Server Sender Name QC-84
Configuring ANCP Neighbors QC-85
Mapping AN Ports to VLAN Subinterfaces QC-88
SUMMARY STEPS QC-88
Configuring ANCP Rate Adjustment QC-90
Configuration Examples for Configuring ANCP on Cisco ASR 9000 Series Routers QC-92
Configuring ANCP Server Sender Name: Example QC-92
Configuring ANCP Neighbors: Example QC-93
Mapping AN ports to VLAN Subinterfaces: Example QC-96
Configuring ANCP Rate Adjustment: Example QC-97
Additional References QC-99
Related Documents QC-99
Standards QC-99
MIBs QC-99
RFCs QC-99
Technical Assistance QC-100
Index

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Preface
This document describes the Cisco ASR 9000 Series Aggregation Services Router Quality of Service
(QoS) feature for prioritizing traffic flow and providing preferential forwarding for higher-priority
packets. The QoS techniques described include allocating bandwidth, improving loss characteristics,
avoiding and managing network congestion, metering network traffic, and setting traffic-flow priorities
across the network.
For QoS command descriptions, usage guidelines, and examples, refer to the Cisco ASR 9000 Series
Aggregation Services Router Modular Quality of Service Command Reference.
The preface contains the following sections:
• Changes to This Document, page ix
• Obtaining Documentation and Submitting a Service Request, page ix
Changes to This Document
Table 1 lists the technical changes made to this document since it was first printed.
Obtaining Documentation and Submitting a Service Request
For information on obtaining documentation, submitting a service request, and gathering additional
information, see the monthly What’s New in Cisco Product Documentation, which also lists all new and
revised Cisco technical documentation, at:
http://www.cisco.com/en/US/docs/general/whatsnew/whatsnew.html
Subscribe to the What’s New in Cisco Product Documentation as a Really Simple Syndication (RSS) feed
and set content to be delivered directly to your desktop using a reader application. The RSS feeds are a free
service and Cisco currently supports RSS version 2.0.
Ta b l e 1 Changes to This Document
Revision Date Change Summary
OL-17239-01 March 2009 Initial release of this document.

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Modular Quality of Service Overview on
Cisco ASR 9000 Series Routers
Quality of Service (QoS) is the technique of prioritizing traffic flows and providing preferential
forwarding for higher-priority packets. The fundamental reason for implementing QoS in your network
is to provide better service for certain traffic flows. A traffic flow can be defined as a combination of
source and destination addresses, source and destination socket numbers, and the session identifier. A
traffic flow can more broadly be described as a packet moving from an incoming interface that is
destined for transmission to an outgoing interface. The traffic flow must be identified, classified, and
prioritized on all routers and passed along the data forwarding path throughout the network to achieve
end-to-end QoS delivery. The terms traffic flow and packet are used interchangeably throughout this
module.
To implement QoS on a network requires the configuration of QoS features that provide better and more
predictable network service by supporting bandwidth allocation, improving loss characteristics,
avoiding and managing network congestion, metering network traffic, or setting traffic flow priorities
across the network.
This module contains overview information about how to implement modular QoS features on
Cisco ASR 9000 Series Aggregation Services Routers within a service provider network.
Note For additional conceptual information about QoS in general and complete descriptions of the QoS
commands listed in this module, see the “Related Documents” section of this module. To locate
documentation for other commands that might appear in the course of executing a configuration task,
search online in the Cisco ASR 9000 Series Router software master command index.
Contents
• Information About Modular Quality of Service on Cisco ASR 9000 Series Routers, page 2
• Where to Go Next, page 5
• Additional References, page 5

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Information About Modular Quality of Service on
Cisco ASR 9000 Series Routers
Before configuring modular QoS on your network, you should understand the following concepts:
• Benefits of Cisco ASR 9000 Series Router QoS Features, page 2
• QoS Techniques on Cisco ASR 9000 Series Routers, page 2
• Differentiated Service Model for Cisco IOS XR Software, page 4
• Access Node Control Protocol, page 4
Benefits of Cisco ASR 9000 Series Router QoS Features
Cisco IOS XR QoS features enable networks to control and predictably service a variety of networked
applications and traffic types. Implementing Cisco IOS XR QoS in your network promotes the following
benefits:
• Control over resources. You have control over which resources (bandwidth, equipment, wide-area
facilities, and so on) are being used. For example, you can limit bandwidth consumed over a
backbone link by FTP transfers or give priority to an important database access.
• Tailored services. If you are an Internet Service Provider (ISP), the control and visibility provided
by QoS enables you to offer carefully tailored grades of service differentiation to your customers.
• Coexistence of mission-critical applications. Cisco IOS XR QoS features make certain of the
following conditions:
– That your WAN is used efficiently by mission-critical applications that are most important to
your business.
– That bandwidth and minimum delays required by time-sensitive multimedia and voice
applications are available.
– That other applications using the link get their fair service without interfering with
mission-critical traffic.
QoS Techniques on Cisco ASR 9000 Series Routers
QoS on Cisco IOS XR software relies on the following techniques to provide for end-to-end QoS
delivery across a heterogeneous network:
• Packet classification
• Congestion management
• Congestion avoidance
Before implementing the QoS features for these techniques, you should identify and evaluate the traffic
characteristics of your network because not all techniques are appropriate for your network environment.

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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
• 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.
Marking can be carried out:
• Unconditionally—As part of the class-action.
• Conditionally—As part of a policer-action.
• Combination of conditionally and unconditionally.
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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Congestion Avoidance
Congestion avoidance techniques monitor network traffic flows in an effort to anticipate and avoid
congestion at common network and internetwork bottlenecks before problems occur. These techniques
are designed to provide preferential treatment for traffic (such as a video stream) that has been classified
as real-time critical under congestion situations while concurrently maximizing network throughput and
capacity utilization and minimizing packet loss and delay. Cisco IOS XR software supports the Random
Early Detection (RED), Weighted RED (WRED), and tail drop QoS congestion avoidance features.
For detailed conceptual and configuration information about congestion avoidance techniques, see the
“Configuring Modular Quality of Service Congestion Management on Cisco ASR 9000 Series Routers”
module in this guide.
Differentiated Service Model for Cisco IOS XR Software
Cisco IOS XR software supports a differentiated service that is a multiple-service model that can satisfy
different QoS requirements. However, unlike in the integrated service model, an application using
differentiated service does not explicitly signal the router before sending data.
For differentiated service, the network tries to deliver a particular kind of service based on the QoS
specified by each packet. This specification can occur in different ways, for example, using the IP
Precedence bit settings in IP packets or source and destination addresses. The network uses the QoS
specification to classify, mark, shape, and police traffic, and to perform intelligent queueing.
The differentiated service model is used for several mission-critical applications and for providing
end-to-end QoS. Typically, this service model is appropriate for aggregate flows because it performs a
relatively coarse level of traffic classification.
Access Node Control Protocol
Access Node Control Protocol (ANCP) creates a control plane between a service-oriented aggregation
device and an access node (AN) (for example, a DSLAM) in order to perform QoS-related,
service-related, and subscriber-related operations. An ANCP Network Access Server (NAS) accepts and
maintains ANCP adjacencies (sessions with an ANCP neighbor), and sending and receiving ANCP
messages.
ANCP allows static mapping between AN ports and VLAN subinterfaces so that DSL rate updates for a
specific subscriber received by the ANCP server are applied to the QoS configuration corresponding to
that subscriber. DSL train rates received via ANCP are used to alter shaping rates on subscriber-facing
interfaces and subinterfaces on the Cisco ASR 9000 Series Router.
Modular QoS Command Line Interface
In Cisco IOS XR software, QoS features are enabled through the Modular QoS command line interface
(MQC) feature. The MQC is a command line interface (CLI) structure that allows users to create policies
and attach these policies to interfaces. A traffic policy contains a traffic class and one or more QoS
features. A traffic class is used to classify traffic, while the QoS features in the traffic policy determine
how to treat the classified traffic. One of the main goals of MQC is to provide a platform-independent
interface for configuring QoS across Cisco platforms.

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For detailed conceptual and configuration information about the MQC feature, see the “Configuring
Modular Quality of Service Packet Classification on Cisco ASR 9000 Series Routers” module in this
guide.
Where to Go Next
To configure the packet classification features that involve identification and marking of traffic flows,
see the “Configuring Modular Quality of Service Packet Classification on
Cisco ASR 9000 Series Routers” module in this guide.
To configure the queueing, scheduling, policing, and shaping features, see the “Configuring Modular
Quality of Service Congestion Management on Cisco ASR 9000 Series Routers” module in this guide.
To configure the WRED and RED features, see the “Configuring Modular QoS Congestion Avoidance
on Cisco ASR 9000 Series Routers module in this guide.
To configure Access Node Control Protocol (ANCP) features, see the “Configuring Access Node
Control Protocol on Cisco ASR 9000 Series Routers” module in this guide.
Additional References
The following sections provide references related to implementing QoS on Cisco ASR 9000 Series
Routers.
Related Documents
Standards
Related Topic Document Title
Cisco IOS XR QoS commands: complete command
syntax, command modes, command history, defaults,
usage guidelines, and examples
Cisco ASR 9000 Series Aggregation Services Router Modular
Quality of Service Command Reference
Cisco IOS XR getting started material Cisco ASR 9000 Series Aggregation Services Router Getting
Started Guide
Standards Title
No new or modified standards are supported by this
feature, and support for existing standards has not been
modified by this feature.
—

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MIBs
RFCs
Technical Assistance
MIBs MIBs Link
—To locate and download MIBs using Cisco IOS XR software, use the
Cisco MIB Locator found at the following URL and choose a
platform under the Cisco Access Products menu:
http://cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml
RFCs Title
No new or modified RFCs are supported by this
feature, and support for existing RFCs has not been
modified by this feature.
—
Description Link
The Cisco Technical Support website contains
thousands of pages of searchable technical content,
including links to products, technologies, solutions,
technical tips, and tools. Registered Cisco.com users
can log in from this page to access even more content.
http://www.cisco.com/techsupport

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Configuring Modular Quality of Service Packet
Classification on Cisco ASR 9000 Series Routers
Packet classification identifies and marks traffic flows that require congestion management or
congestion avoidance on a data path. The Modular Quality of Service (QoS) command-line interface
(MQC) is used to define the traffic flows that should be classified, where each traffic flow is called a
class of service, or class. Subsequently, a traffic policy is created and applied to a class. All traffic not
identified by defined classes falls into the category of a default class.
This module provides the conceptual and configuration information for QoS packet classification on
Cisco ASR 9000 Series Aggregation Services Routers.
Note For additional conceptual information about QoS in general and complete descriptions of the QoS
commands listed in this module, see the “Related Documents” section of this module. To locate
documentation for other commands that might appear in the course of executing a configuration task,
search online in the Cisco IOS XR software master command index.
Feature History for Configuring Modular QoS Packet Classification on Cisco ASR 9000 Series Routers
Contents
• Prerequisites for Configuring Modular QoS Packet Classification on
Cisco ASR 9000 Series Routers, page 8
• Information About Configuring Modular QoS Packet Classification on
Cisco ASR 9000 Series Routers, page 8
• How to Configure Modular QoS Packet Classification on Cisco ASR 9000 Series Routers, page 13
• Configuration Examples for Configuring Modular QoS Packet Classification on Cisco IOS XR
Software, page 28
• Where to Go Next, page 34
• Additional References, page 35
Release Modification
Release 3.7 FCI This feature was introduced on Cisco ASR 9000 Series Routers.

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Prerequisites for Configuring Modular QoS Packet
Classification on Cisco ASR 9000 Series Routers
The following prerequisites are required for configuring modular QoS packet classification on your
network:
• You must be in a user group associated with a task group that includes the proper task IDs. The
command reference guides include the task IDs required for each command. If you suspect user
group assignment is preventing you from using a command, contact your AAA administrator for
assistance.
• You must be familiar with Cisco IOS XR QoS configuration tasks and concepts.
Information About Configuring Modular QoS Packet
Classification on Cisco ASR 9000 Series Routers
To implement QoS packet classification features in this document, you must understand the following
concepts:
• Packet Classification Overview, page 8
• Traffic Class Elements, page 9
• Traffic Policy Elements, page 9
• Default Traffic Class, page 10
• Class-based Unconditional Packet Marking Feature and Benefits, page 10
• Specification of the CoS for a Packet with IP Precedence, page 11
• IP Precedence Compared to IP DSCP Marking, page 12
• Hierarchical Ingress Policing, page 13
Packet Classification Overview
Packet classification involves categorizing a packet within a specific group (or class) and assigning it a
traffic descriptor to make it accessible for QoS handling on the network. The traffic descriptor contains
information about the forwarding treatment (quality of service) that the packet should receive. Using
packet classification, you can partition network traffic into multiple priority levels or classes of service.
The source agrees to adhere to the contracted terms and the network promises a quality of service. Traffic
policers and traffic shapers use the traffic descriptor of a packet to ensure adherence to the contract.
Traffic policers and traffic shapers rely on packet classification features, such as IP precedence, to select
packets (or traffic flows) traversing a router or interface for different types of QoS service. For example,
by using the three precedence bits in the type of service (ToS) field of the IP packet header, you can
categorize packets into a limited set of up to eight traffic classes. After you classify packets, you can use
other QoS features to assign the appropriate traffic handling policies including congestion management,
bandwidth allocation, and delay bounds for each traffic class.
On Cisco IOS XR software, methods of classification may consist of the logical combination of any
fields in the packet header, where a packet header may be a Layer 2, a Layer 3, or a Layer 4 header; or
classification based on the incoming or outgoing physical or virtual interface.

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Note IPv6-based classification is supported only on Layer 3 interfaces.
Traffic Class Elements
The purpose of a traffic class is to classify traffic on your router. Use the class-map command to define
a traffic class.
A traffic class contains three major elements: a name, a series of match commands, and, if more than
one match command exists in the traffic class, an instruction on how to evaluate these match commands.
The traffic class is named in the class-map command. For example, if you use the word cisco with the
class-map command, the traffic class would be named cisco.
The match commands are used to specify various criteria for classifying packets. Packets are checked
to determine whether they match the criteria specified in the match commands. If a packet matches the
specified criteria, that packet is considered a member of the class and is forwarded according to the QoS
specifications set in the traffic policy. Packets that fail to meet any of the matching criteria are classified
as members of the default traffic class. See the “Default Traffic Class” section on page 10.
The instruction on how to evaluate these match commands needs to be specified if more than one match
criterion exists in the traffic class. The evaluation instruction is specified with the class-map match-any
command. If the match-any option is specified as the evaluation instruction, the traffic being evaluated
by the traffic class must match at least one of the specified criteria. The match-any keyword is the only
evaluation option supported. If the match-all option is specified, the traffic must match all of the match
criteria.
The function of these commands is described more thoroughly in the Cisco ASR 9000 Series
Aggregation Services Router Modular Quality of Service Command Reference.
The traffic class configuration task is described in the “Creating a Traffic Class” section on page 13.
Traffic Policy Elements
The purpose of a traffic policy is to configure the QoS features that should be associated with the traffic
that has been classified in a user-specified traffic class or classes. The policy-map command is used to
create a traffic policy. A traffic policy contains three elements: a name, a traffic class (specified with the
class command), and the QoS policies. The name of a traffic policy is specified in the policy map
Modular Quality of Service (MQC) (for example, the policy-map policy1 command creates a traffic
policy named policy1). The traffic class that is used to classify traffic to the specified traffic policy is
defined in class map configuration mode. After choosing the traffic class that is used to classify traffic
to the traffic policy, the user can enter the QoS features to apply to the classified traffic.
The MQC does not necessarily require that users associate only one traffic class to one traffic policy.
When packets match to more than one match criterion, as many as 512 traffic classes can be associated
to a single traffic policy. The 512 class maps include the default class and the classes of the child policies,
if any.
The order in which classes are configured in a policy map is important. The match rules of the classes
are programmed into the TCAM in the order in which the classes are specified in a policy map.
Therefore, if a packet can possibly match multiple classes, only the first matching class is returned and
the corresponding policy is applied.
The function of these commands is described more thoroughly in the Cisco ASR 9000 Series
Aggregation Services Router Modular Quality of Service Command Reference.

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The traffic policy configuration task is described in the “Creating a Traffic Policy” section on page 16.
Default Traffic Class
Unclassified traffic (traffic that does not meet the match criteria specified in the traffic classes) is treated
as belonging to the default traffic class.
If the user does not configure a default class, packets are still treated as members of the default class.
However, by default, the default class has no enabled features. Therefore, packets belonging to a default
class with no configured features have no QoS functionality. These packets are then placed into a first
in, first out (FIFO) queue and forwarded at a rate determined by the available underlying link bandwidth.
This FIFO queue is managed by a congestion avoidance technique called tail drop. For further
information about congestion avoidance techniques, such as tail drop, see the “Configuring Modular
QoS Congestion Avoidance on Cisco ASR 9000 Series Routers” module in this guide.
Class-based Unconditional Packet Marking Feature and Benefits
The Class-based, Unconditional Packet Marking feature provides users with a means for efficient packet
marking by which the users can differentiate packets based on the designated markings.
The Class-based, Unconditional Packet Marking feature allows users to perform the following tasks:
• Mark packets by setting the IP precedence bits or the IP differentiated services code point (DSCP)
in the IP ToS byte.
• Mark Multiprotocol Label Switching (MPLS) packets by setting the EXP bits within the imposed or
topmost label.
• Mark packets by setting the Layer 2 class-of-service (CoS) value. The priority value within an SRP
header can also be set.
• Mark packets by setting the value of the qos-group argument.
• Mark packets by setting the value of the discard-class argument.
Unconditional packet marking allows you to partition your network into multiple priority levels or
classes of service, as follows:
• Use QoS unconditional packet marking to set the IP precedence or IP DSCP values for packets
entering the network. Routers within your network can then use the newly marked IP precedence
values to determine how the traffic should be treated.
For example, weighted random early detection (WRED), a congestion avoidance technique, uses IP
precedence values to determine the probability that a packet is dropped. In addition, low-latency
queueing (LLQ) can then be configured to put all packets of that mark into the priority queue.
• Use QoS unconditional packet marking to assign MPLS packets to a QoS group. The router uses the
QoS group to determine how to prioritize packets for transmission. To set the QoS group identifier
on MPLS packets, use the set qos-group command in policy map class configuration mode.
• Use CoS unconditional packet marking to assign packets to set the priority value of IEEE 802.1p/
Inter-Switch Link (ISL) packets. The router uses the CoS value to determine how to prioritize
packets for transmission and can use this marking to perform Layer 2-to-Layer 3 mapping. To set
the Layer 2 CoS value of an outgoing packet, use the set cos command in policy map configuration
mode.
The configuration task is described in the “Configuring Class-based Unconditional Packet Marking”
section on page 22.