HQoS and LFI are performed in two stages that involve double traversal (dual-pass) of traffic through the carrier IOM and the BB-ISA. The following are the functions performed in each pass:
In the first pass through the carrier IOM, traffic is marked (dot1p bits) as high or low priority. This plays a crucial role in the execution of LFI in the BB-ISA.
In the first pass through the BB-ISA this prioritization from the first step, is an indication (along with the internally calculated fragment size) of whether the traffic is interleaved (non MLPPP encapsulated) or not (MLPPP encapsulated). Consequently, the BB-ISA adds the necessary padding related to last mile wire overhead to each packet. This padding is in the second pass on the carrier IOM and performs last mile wire based QoS functions.
In the second pass through the carrier IOM, the last-mile wire-based HQoS is performed based on the padding added in the first pass through the BB-ISA.
In the second pass through the BB-ISA, the previously added overhead is stripped off and LFI/MLPPP encapsulation functions are performed.