Cisco Cisco IOS Software Release 12.2(27)SBC
QoS: Percentage-Based Shaping
Information About QoS: Percentage-Based Shaping
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Cisco IOS Release: Multiple releases (see the Feature History table)
Defining Class and Policy Maps for QoS: Percentage-Based Shaping
To configure the QoS: Percentage-Based Shaping feature, you must define a traffic class, configure a
policy map, and then attach that policy map to the appropriate interface. These three tasks can be
accomplished by using the Modular Quality of Service (QoS) Command-Line Interface (CLI) (MQC).
policy map, and then attach that policy map to the appropriate interface. These three tasks can be
accomplished by using the Modular Quality of Service (QoS) Command-Line Interface (CLI) (MQC).
The MQC is a command-line interface that allows you to define traffic classes, create and configure
traffic policies (policy maps), and then attach these traffic policies to interfaces.
traffic policies (policy maps), and then attach these traffic policies to interfaces.
In the MQC, the class-map command is used to define a traffic class (which is then associated with a
traffic policy). The purpose of a traffic class is to classify traffic.
traffic policy). The purpose of a traffic class is to classify traffic.
The MQC consists of the following three processes:
•
Defining a traffic class with the class-map command.
•
Creating a traffic policy by associating the traffic class with one or more QoS features (using the
policy-map command).
policy-map command).
•
Attaching the traffic policy to the interface with the service-policy command.
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
(that is, match-all or match-any). The traffic class is named in the class-map command line; for example,
if you enter the class-map cisco command while configuring the traffic class in the CLI, the traffic class
would be named “cisco”.
match command exists in the traffic class, an instruction on how to evaluate these match commands
(that is, match-all or match-any). The traffic class is named in the class-map command line; for example,
if you enter the class-map cisco command while configuring the traffic class in the CLI, 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.
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.
Traffic Regulation Mechanisms and Bandwidth Percentages
Cisco IOS quality of service (QoS) offers two kinds of traffic regulation mechanisms—traffic policing
and traffic shaping. A traffic policer typically drops traffic that violates a specific rate. A traffic shaper
typically delays excess traffic using a buffer to hold packets and shapes the flow when the data rate to a
queue is higher than expected.
and traffic shaping. A traffic policer typically drops traffic that violates a specific rate. A traffic shaper
typically delays excess traffic using a buffer to hold packets and shapes the flow when the data rate to a
queue is higher than expected.
Traffic shaping and traffic policing can work in tandem and can be configured in a class map. Class maps
organize data packets into specific categories (“classes”) that can, in turn, receive a user-defined QoS
treatment when used in policy maps (sometimes referred to as “service policies”).
organize data packets into specific categories (“classes”) that can, in turn, receive a user-defined QoS
treatment when used in policy maps (sometimes referred to as “service policies”).
Before this feature, traffic policing and traffic shaping were configured on the basis of a user-specified
amount of bandwidth available on the interface. Policy maps were then configured on the basis of that
specific amount of bandwidth, meaning that separate policy maps were required for each interface.
amount of bandwidth available on the interface. Policy maps were then configured on the basis of that
specific amount of bandwidth, meaning that separate policy maps were required for each interface.
This feature provides the ability to configure traffic policing and traffic shaping on the basis of a
percentage of bandwidth available on the interface. Configuring traffic policing and traffic shaping in
this manner enables customers to use the same policy map for multiple interfaces with differing amounts
of bandwidth.
percentage of bandwidth available on the interface. Configuring traffic policing and traffic shaping in
this manner enables customers to use the same policy map for multiple interfaces with differing amounts
of bandwidth.