H3C Technologies H3C S12500 Series Switches - Manual (Page 40)

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Configuring priority mapping
Overview
Introduction to priority mapping
When a packet arrives, a device assigns a set of QoS priority parameters to the packet based on a
certain priority field carried in the packet or the port priority of the incoming port, depending on your
configuration. This process is called "priority mapping". During this process, the device can modify the
priority of the packet depending on device status. The set of QoS priority parameters decides the
scheduling priority and forwarding priority of the packet.
Priority mapping is implemented with priority mapping tables and involves priorities such as 802.11e
priority, 802.1p priority, DSCP, EXP, IP precedence, local precedence, and drop precedence.
Introduction to priorities
Priorities include the following types: priorities carried in packets, and priorities locally assigned for
scheduling only.
The packet carried priorities include 802.1p priority, DSCP precedence, IP precedence, EXP, and so on.
These priorities have global significance and affect the forwarding priority of packets across the network.
For more information about these priorities, see "Appendix."
The locally assigned priorities have only local significance. They are assigned by the device for
scheduling only. These priorities include the local precedence, drop precedence, and user precedence,
as follows.
• Local precedence is 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.
• Drop precedence is used for making packet drop decisions. Packets with the highest drop
precedence are dropped preferentially.
• User precedence is the precedence that the device automatically extracts from a certain priority
field of the packet according to its forwarding path. The user precedence represents the 802.1p
precedence for Layer-2 packets, the IP precedence for Layer-3 packets, and the EXP precedence for
MPLS packets.
Priority mapping tables
The device provides various types of priority mapping tables, or rather, priority mappings. By looking up
a priority mapping table, the device decides which priority value is to assign to a packet for subsequent
packet processing.
The default priority mapping tables (as shown in
Appendix B Default priority mapping tables ) are
available for priority mapping. They are adequate in most cases. If a default priority mapping table
cannot meet your requirements, you can modify the priority mapping table as required.