Priority mapping

Packets transmitted over different networks carry different QoS precedence fields, for example, EXP field on an MPLS network, 802.1p field on a virtual local area network (VLAN), and DSCP field on an IP network. The mapping between priority fields must be configured on the gateway to retain priorities of packets when the packets traverse different networks.
The priority mapping mechanism provides the mapping from precedence fields of packets to internal priorities (local precedences) or the mapping from internal priorities to precedence fields of packets. This mechanism uses the DiffServ domain to manage and record the mapping of precedence fields and internal priorities. When packets reach the AR, the AR maps priorities in packets or the default 802.1p priorities of inbound interfaces to local priorities. The AR then determines the queues that packets enter based on the mapping between internal priorities and queues and performs traffic shaping, congestion avoidance, and queue scheduling. In addition, the AR can re-mark priorities in outgoing packets so that the downstream device can provide different QoS levels based on packet priorities.

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