This application discloses a flow entry generating method and apparatus. The method includes: receiving a service path establishment request, where the service path establishment request includes a constraint, a source device, and a target device; finding, in a data transmission network according to a cross-layer information model, a service path that meets the constraint and is from the source device to the target device, where the cross-layer information model is a model that describes, at a same layer, an overall topology relationship between an IP layer and an optical layer; and generating a respective corresponding flow entry for each forwarding device in the service path, and sending the flow entry to a corresponding forwarding device. This application can greatly reduce time complexity of computing the service path, and improve efficiency of computing the service path, thereby improving efficiency of generating a flow entry.