3-2z exp-dp: EXP-to-drop priority mapping table.The default priority mapping tables (as shown in Appendix B Default Priority Mapping Tables) areavailable for priority mapping. Generally, they are sufficient for priority mapping. If a default prioritymapping table cannot meet your requirements, you can modify the priority mapping table as required.Priority Trust Mode on a PortThe priority trust mode on a port decides which priority is used for priority mapping table lookup. Thereare two priority trust modes on the S7900E series switches:z dot1p: Uses the 802.1p priority carried in packets for priority mapping.z dscp: Uses the DSCP carried in packets for priority mapping.In addition, port priority was introduced for 802.1q untagged packets. Thus, when a port configured withthe 802.1p trust mode receives an 802.1q untagged packet, the priority of the port is used as the 802.1ppriority of the packet for priority mapping table lookup.The priority mapping procedure varies with the priority modes, as described in the next section PriorityMapping Procedure.Priority Mapping ProcedureFigure 3-1 presents how the S7900E performs priority mapping for an Ethernet packet. The procedurediffers depending on whether the packet is 802.1q tagged or not.Figure 3-1 Priority mapping procedure for an Ethernet packetWhich priority istrusted on theport?Receive apacket on a portUse the portpriority as the802.1p priority ofthe packetLook up thedot1p-dp anddot1p-lp tablesMark the packetwith localprecedence anddrop precedenceIs the packet802.1q tagged?DSCP in packetsLook up thedscp-dp, dscp-dot1p, and dscp-dscp tables802.1p inpacketsMark the packetwith 802.1ppriority, dropprecedence, andnew DSCPprecedenceLook up thedot1p-lp tableMark the packetwith localprecedenceLook up thedot1p-dp anddot1p-lp tablesMark the packetwith localprecedence anddrop precedenceSchedule the packetaccording to its localprecedence and dropprecedenceYN
