This feature allows the user to perform hierarchical scheduling of subscriber host packets in a way that the packet encapsulation overhead and ATM bandwidth expansion (when applicable) because of the last mile for each type of broadband session, that is, PPPoEoA LLC/SNAP and VC-Mux, IPoE, IPoEoA LLC/SNAP and VC-Mux, and so on, is accounted for by the 7450 ESS and 7750 SR acting as the Broadband Network Gateway (BNG).
The intent is that the BNG distributes bandwidth among the subscriber host sessions fairly by accounting for the encapsulation overhead and bandwidth expansion of the last mile so the packets are less likely to be dropped downstream in the DSLAM DSL port.
The last mile encapsulation type can be configured by the user or signaled using the Access-loop-encapsulation sub-TLV in the Vendor-Specific PPPoE Tags or DHCP Relay Options as per RFC 4679.
Furthermore, this feature allows the BNG to shape the aggregate rate of each subscriber and the aggregate rate of all subscribers destined for a specific DSLAM to prevent congestion of the DSLAM. The subscriber aggregate rate is adjusted for the last mile overhead. The shaping to the aggregate rate of all subscribers of a specific destination DSLAM is achieved by a new scheduling object, referred to as Virtual Port or Vport in CLI, which represents the DSLAM aggregation node in the BNG scheduling hierarchy