Ip Precedence Value Settings; Ip Precedence Compared To Ip Dscp Marking - Cisco ASR 9000 Series Configuration Manual

Information About Configuring Modular QoS Packet Classification on Cisco ASR 9000 Series Routers
Table 1
Number
5
6
7
The IP precedence feature allows you considerable flexibility for precedence assignment. That is, you
can define your own classification mechanism. For example, you might want to assign precedence based
on application or access router.
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.
You can use policy-based routing to set the IP precedence in packets. See
Unconditional Packet Marking" section on page

IP Precedence Compared to IP DSCP Marking

IP precedence and Differentiated Services Code Point (DSCP) markings are used to decide how packets
should be treated in WRED.
The IP DSCP value is the first six bits in the ToS byte, and the IP precedence value is the first three bits
in the ToS byte. The IP precedence value is actually part of the IP DSCP value. Therefore, both values
cannot be set simultaneously. If both values are set simultaneously, the packet is marked with the IP
DSCP value.
If you need to mark packets in your network and all your devices support IP DSCP marking, use the IP
DSCP marking to mark your packets because the IP DSCP markings provide more unconditional packet
marking options. If marking by IP DSCP is undesirable, however, or if you are unsure if the devices in
your network support IP DSCP values, use the IP precedence value to mark your packets. The IP
precedence value is likely to be supported by all devices in the network.
You can set up to 8 different IP precedence markings and 64 different IP DSCP markings.
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide
QC-12
Configuring Modular Quality of Service Packet Classification on Cisco ASR 9000 Series Routers
D r a f t — C i s c o C o n f i d e n t i a l
IP Precedence Values (continued)
Name
critical
internet
network
"Configuring Class-based
22.
OL-17239-01