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Configuring priority mapping
This feature is supported only on SAP modules operating in bridge mode.
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 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 only have local significance. They are assigned by the device for
scheduling only. These priorities include the following types:
•
Local
precedence
—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
—Drop precedence is used for making packet drop decisions. Packets with
the highest drop precedence are dropped preferentially.
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
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.
Priority mapping configuration tasks
You can configure priority mapping in any of the following approaches:
•
Configuring priority trust mode.
In this approach, you can configure a port to look up a certain priority, 802.1p for example, in
incoming packets, in the priority mapping tables. If no packet priority is trusted, the port priority
of the incoming port is used.
•
Changing port priority.
By default, all ports are assigned the port priority of zero. By changing the port priority of a port,
you change the priority of the incoming packets on the port.
