69 PRIORITY MAPPINGWhen configuring priority mapping, go to these sections for information you areinterested in:■ “Priority Mapping Overview” on page 883■ “Configuring a Priority Mapping Table” on page 884■ “Configuring the Port Priority” on page 885■ “Configuring Port Priority Trust Mode” on page 886■ “Displaying and Maintaining Priority Mapping” on page 887Priority MappingOverviewWhen a packet reaches a switch, the switch assigns the packet parametersaccording to it configuration, such as 802.1p precedence, DSCP precedence, IPprecedence, local precedence, and drop precedence.The local precedence and drop precedence are described as follows.■ Local precedence is the precedence that the switch assigns to a packet and it iscorresponding to the number of an outbound queue on the port. Localprecedence takes effect only on the local switch.■ Drop precedence is a parameter that is referred to when dropping packets. Thehigher the drop precedence, the more likely a packet is dropped.the Switch 4800G provide the following two priority trust modes:■ Trusting the DSCP precedence of received packets. In this mode, the switchsearches the dscp-dot1p/dp/dscp mapping table based on the DSCPprecedence of the received packet for the 802.1p precedence/dropprecedence/DSCP precedence to be used to mark the packet. Then the switchsearches the dot1p-lp mapping table based on the marked 802.1p precedencefor the corresponding local precedence and marks the received packet with thelocal precedence.■ Trusting the 802.1p precedence of received packets. In this mode, if a packet isreceived without an 802.1q tag, the switch takes the priority of the receivingport as the 802.1p precedence of the packet and then based on the prioritysearches the dot1p-dp/lp mapping table for the local/drop precedence for thepacket. If packet is received with an 802.1q tag, the switch searches thedot1p-dp/lp mapping table based on the 802.1p precedence in the tag forlocal/drop precedence for the packet.The default dot1p-lp/dp mapping and dscp-dot1p/dp/dscp mapping providedby the Switch 4800G are shown in the following two tables.
