4000 Router Series ACL and QoS Configuration Guide

Configuring priority mapping

Overview

When a packet arrives, depending on your configuration, a device assigns a set of QoS priority

parameters to the packet based on either a certain priority field carried in the packet or the port priority

of the incoming port. This process is called "priority mapping." During this process, the device can

modify the priority of the packet according to the priority mapping rules. The set of QoS priority

parameters decides the scheduling priority and forwarding priority of the packet.

Priority mapping is implemented with priority maps and involves priorities such as 802.1p priority, DSCP,

EXP, and IP precedence.

Introduction to priorities

Priorities include the following types: priorities carried in packets, and priorities locally assigned for

scheduling only.

Packet-carried priorities include 802.1p priority, DSCP precedence, IP precedence, and EXP. These

priorities have global significance and affect the forwarding priority of packets across the network. For

more information about these priorities, see "Appendix."

Locally assigned priorities only have local significance. They are assigned by the device only for

scheduling. These priorities include the local precedence, drop priority, and user priority, as follows:

• Local precedence—Used for queuing. A local precedence value corresponds to an output queue. A

packet with higher local precedence is assigned to a higher priority output queue to be

preferentially scheduled.

• User priority—Precedence that the device automatically extracts from a certain priority field of the

packet according to its forwarding path, and is a parameter for determining the scheduling priority

and forwarding priority of the packet. The user priority represents the 802.1p priority for Layer-2

packets, the IP precedence for Layer-3 packets, and the EXP for MPLS packets.

Priority maps

The device provides various types of priority maps. By looking through a priority map, the device decides

which priority value to assign to a packet for subsequent packet processing.

The default priority maps (as shown in Appendix B Default priority maps) ar

e available for priority

mapping. They are adequate in most cases. If a default priority map cannot meet your requirements, you

can modify the priority map as required.

Priority mapping configuration tasks

You can configure priority mapping by using one of the following approaches: