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Contents
Overview
This chapter contains the following sections:
- Information About Quality of Service
- Traffic Classification and Marking
- QoS Commands
- Default QoS Behavior
- Supported RFCs
- High Availability Requirements for QoS Features
- Commonly Used DSCP Values
- IP Precedence Values
- QoS Configuration Limits
Information About Quality of Service
You can use QoS to provide the most desirable flow of traffic through a network. QoS allows you to classify your network traffic, police and prioritize the traffic flow, and provide congestion avoidance. Traffic is processed based on how you classify it and the QoS policies that you put in place.
You can implement a QoS policy using the following steps:
- Define a traffic class by using the class-map command. For more information, see Configuring QoS Classification.
- Create a traffic class by using the policy-map command. A traffic policy defines how specific traffic is to be acted upon to improve the quality of service. For more information, see Configuring QoS Marking Policies.
- Attach the traffic policy to an interface or port profile by using the service-policy command. For more information, see Creating Ingress and Egress Policies.
- Police the traffic. For more information, see Configuring QoS Policing.
Traffic Classification and Marking
QoS Commands
QoS configuration commands are shown in the following table:
Command
Configuration
Description
class-map
Global configuration
Defines a class map that represents a class of traffic.
table-map
Global configuration
Defines a table map that represents a mapping from one set of field values to another set of field values. You can reference a table map from a policy map.
policy-map
Global configuration
Defines a policy map that represents a set of policies to be applied to a set of class maps. Policy maps can reference table maps.
match
Class map QoS configuration
Applies a specified policy map to input or output packets on interfaces configured as follows:
set
Policy map QoS configuration
Defines the action to take regarding packet data rates.
service-policy
Interface or port profile configuration
Defines the criteria for a class map.
police
Policy map class QoS configuration
Defines the packet header values for a policy map.
Default QoS Behavior
QoS has no default behavior. Policing and prioritization of traffic are only implemented when you apply a policy map to an interface. The only exception is that, by default, the CoS value for control and packet VLAN traffic is set to 6. This value can be overridden with an explicit QoS policy that is configured on the interface that carries the control and packet VLAN traffic. However, when designing your QoS and ACL policies, note that access control lists (ACLs) that are referenced within a QoS policy are processed as follows as part of the QoS policy:
High Availability Requirements for QoS Features
QoS recovers its previous state after a software restart, and it is able to switch over from the active supervisor to the standby supervisor without a loss of state.
Commonly Used DSCP Values
You can mark both incoming and outgoing packets. The following commonly used DSCP values are described in RFC 2475.
DSCP Value
Decimal Value
Meaning
Drop Probability
Equivalent IP Precedence Values
101 110
46
High Priority Expedited Forwarding (EF)
N/A
101 - Critical
000 000
0
Best effort
N/A
000 - Routine
001 010
10
AF11
Low
001 - Priority
001 100
12
AF12
Medium
001 - Priority
001 110
14
AF13
High
001 - Priority
010 010
18
AF21
Low
001 - immediate
010 100
20
AF22
Medium
001 - immediate
010 110
22
AF23
High
001 - immediate
011 010
26
AF31
Low
011 - Flash
011 100
28
AF32
Medium
011 - Flash
011 119
30
AF33
High
011 - Flash
100 010
34
AF41
Low
100 - Flash Override
100 100
36
AF42
Medium
100 - Flash Override
100 110
38
AF43
High
100 - Flash Override
001 000
8
CS1
1
010 000
16
CS2
2
011 000
24
CS3
3
100 000
32
CS4
4
101 000
40
CS5
5
110 000
48
CS6
6
111 000
56
CS7
7
000 000
0
Default
101 110
46
EF