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Configuring priority mapping
Overview
When a packet arrives, a device assigns a set of QoS priority parameters to the packet based on
either a 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 the following priorities:
• 802.1p priority.
• DSCP.
• EXP.
• IP precedence.
• Local precedence.
• Drop priority.
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 "Appendixes."
Locally assig
ned priorities only have local significance. They are assigned by the switch only for
scheduling. These priorities include the local precedence and drop 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.
• Drop priority—Used for making packet drop decisions. Packets with the highest drop priority
are dropped preferentially.
Priority maps
The switch provides various types of priority maps. By looking through a priority map, the switch
decides which priority value to assign to a packet for subsequent packet processing. The switch
provides the following priority mapping tables:
• dot1p-dp—802.1p-to-drop priority mapping table.
• dot1p-exp—802.1p-to-EXP priority mapping table.
• dot1p-lp—802.1p-to-local priority mapping table.
• dscp-dot1p—DSCP-to-802.1p priority mapping table, which is applicable only to IP packets.
• dscp-dp—DSCP-to-drop priority mapping table, which is applicable only to IP packets.
• dscp-dscp—DSCP-to-DSCP priority mapping table, which is applicable only to IP packets.
• exp-dot1p—EXP-to-802.1p priority mapping table.
• exp-dp—EXP-to-drop priority mapping table.
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