One embodiment provides a system that performs in-order traffic aggregation from a number of low-speed ports to a high-speed port. During operation, the system receives at a low-speed port a packet, stores it in a store-and-forward FIFO associated with the low-speed port, extracts a sequence number associated with the stored packet, and stores the extracted sequence number in a sequence-number FIFO associated with the low-speed port. The system further generates an expected sequence number, which maintains a linear order with respect to sequence numbers associated with previously forwarded packets, and determines whether a front end of the sequence-number FIFO matches the expected sequence number. If so, the system removes the front end of the sequence-number FIFO buffer, retrieves a packet associated with it, forwards the retrieved packet on the high-speed port, and updates the expected sequence number by adding 1 to the packet number of the retrieved packet.