H3C S5120-EI series Configuration Manual

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H3C S5120-EI series Configuration Manual Manual pdf 37 page image

27Configuring priority mappingOverviewWhen a packet enters a device, depending on your configuration, the device assigns a set of QoSpriority parameters to the packet based on either a certain priority field carried in the packet or the portpriority of the incoming port. This process is called "priority mapping". During this process, the devicecan modify the priority of the packet depending on device status. The set of QoS priority parametersdecides the scheduling priority and forwarding priority of the packet.Priority mapping is implemented with priority mapping tables and involves priorities such as 802.1ppriority, DSCP, IP precedence, local precedence, and drop precedence.Types of prioritiesPriorities fall into the following types: priorities carried in packets, and priorities locally assigned forscheduling only.The packet-carried priorities include 802.1p priority, DSCP precedence, IP precedence, and so on. Thesepriorities have global significance and affect the forwarding priority of packets across the network. Formore information about these priorities, see "Appendix B Packet precedences."The locally assigned priorities only have local significance. They are assigned by the device forscheduling only. These priorities include the local precedence and drop precedence, as follows:• Local precedence—Local precedence is used for queuing. A local precedence value corresponds toan output queue. A packet with higher local precedence is assigned to a higher priority outputqueue to be preferentially scheduled.• Drop precedence—Drop precedence is used for making packet drop decisions. Packets with thehighest drop precedence are dropped preferentially.Priority mapping tablesPriority mapping is implemented with priority mapping tables. By looking up a priority mapping table,the device decides which priority value to assign to a packet for subsequent packet processing. Theswitch provides the following priority mapping tables:• dot1p-dp—802.1p-to-drop priority mapping table.• dot1p-lp—802.1p-to-local priority mapping table.• dscp-dot1p—DSCP-to-802.1p priority mapping table, which is applicable to only IP packets.• dscp-dp—DSCP-to-drop priority mapping table, which is applicable to only IP packets.• dscp-dscp—DSCP-to-DSCP priority mapping table, which is applicable to only IP packets.The default priority mapping tables (see "Appendix A Default priority mapping tables") are available forpriority mapping. In most cases, they are adequate for priority mapping. If a default priority mappingtable cannot meet your requirements, you can modify the priority mapping table as required.