Cisco ASR 9000 Series Service Configuration Guide

Configuring Modular QoS Service Packet Classification and Marking on Cisco ASR 9000 Series Routers
Information About Configuring Modular QoS Packet Classification and Marking on Cisco ASR 9000 Series Routers
QC-16
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
OL-23108-02
So that each subsequent network element can provide service based on the determined policy, IP
precedence is usually deployed as close to the edge of the network or administrative domain as possible.
This allows the rest of the core or backbone to implement QoS based on precedence.
The configuration task is described in the “Configuring Class-based Unconditional Packet Marking”
section on page 29.
IP Precedence Bits Used to Classify Packets
Use the three IP precedence bits in the ToS field of the IP header to specify the CoS assignment for each
packet. As mentioned earlier, you can partition traffic into a maximum of eight classes and then use
policy maps to define network policies in terms of congestion handling and bandwidth allocation for
each class.
For historical reasons, each precedence corresponds to a name. These names are defined in RFC 791.
Table 1 lists the numbers and their corresponding names, from least to most important.
Note IP precedence bit settings 6 and 7 are reserved for network control information, such as routing updates.
IP Precedence Value Settings
By default, Cisco IOS XR software leaves the IP precedence value untouched. This preserves the
precedence value set in the header and allows all internal network devices to provide service based on
the IP precedence setting. This policy follows the standard approach stipulating that network traffic
should be sorted into various types of service at the edge of the network and that those types of service
should be implemented in the core of the network. Routers in the core of the network can then use the
precedence bits to determine the order of transmission, the likelihood of packet drop, and so on.
Because traffic coming into your network can have the precedence set by outside devices, we recommend
that you reset the precedence for all traffic entering your network. By controlling IP precedence settings,
you prohibit users that have already set the IP precedence from acquiring better service for their traffic
simply by setting a high precedence for all of their packets.
The class-based unconditional packet marking, LLQ, and WRED features can use the IP precedence bits.
Ta b l e 1 IP Precedence Values
Number Name
0 routine
1 priority
2 immediate
3 flash
4 flash-override
5 critical
6 internet
7 network

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English