Methods and apparatus for transmit buffers for network devices. One apparatus includes a packing unit, a buffer manager and a plurality of aggregated port buffers, each coupled to receive output from the packing unit. The packing unit is configured to receive packet data as input segments of a first size; generate storage units of a second size; and write each storage unit to a particular aggregated port buffer identified by the buffer manager. The buffer manager is configured to: select a particular aggregated port buffer for each storage unit, and send information to the buffer manager about the selected particular aggregated port buffer; monitor availability of storage space in the aggregated port buffers; control reception of input segments based on storage space availability; and manage transmission of the storage units from the aggregated port buffers to one or more external destinations as output segments of a third size.