This application provides a method and an apparatus for determining a port orientation. The method includes: A first network device receives, through a first port, a protocol data unit PDU sent by a second network device. The first network device determines, based on the PDU, that the first port is an upstream port or a downstream port. In this embodiment of this application, the first network device may determine, based on the PDU sent by the second network device, whether the first port is the upstream port or the downstream port, to provide a basis for subsequently helping an SDN controller reduce load.