Ip Precedence Bits Used To Classify Packets - Cisco ASR 9000 Series Configuration Manual

Configuring Modular QoS Service Packet Classification
Using the ToS bits, you can define up to eight classes of service. Other features configured throughout the
network can then use these bits to determine how to treat the packet in regard to the ToS to grant it. These
other QoS features can assign appropriate traffic-handling policies, including congestion management strategy
and bandwidth allocation. For example, queuing features such as LLQ can use the IP precedence setting of
the packet to prioritize traffic.
By setting precedence levels on incoming traffic and using them in combination with the Cisco IOS XR QoS
queuing features, you can create differentiated service.
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

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. This
table lists the numbers and their corresponding names, from least to most important.
Table 2: IP Precedence Values
Number
0
1
2
3
4
5
6
7
Note IP precedence bit settings 6 and 7 are reserved for network control information, such as routing updates.
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide, Release 6.1.x
Specification of the CoS for a Packet with IP Precedence
Configuring Class-based Unconditional Packet
Name
routine
priority
immediate
flash
flash-override
critical
internet
network
Marking.
107