A switch fabric for a modular router may be tested without connecting the switch fabric portion of the router to the other modular portions of the router. The switch fabric may generate test data units and insert the test data units into one or more elements of the switch fabric. The switch fabric may operate with the inserted test data units. A control component may receive data units from the switch fabric after operation of the switch fabric and analyze the received data units to determine whether the received data units correspond to the inserted test data units.