A unified control plane (UCP) for IP/Dense Wavelength Division Multiplexing (DWDM) multi-layer optical networks is a very important issue for network carriers. Generalized Multi-Protocol Label Switching (GMPLS), with a decade of development and standardization work, is a mature control plane technique for optical transport networks. But GMPLS-based UCP for IP/DWDM multi-layer networks is overly complex for deployment in a real operational scenario. On the other hand, OpenFlow has been verified as a promising solution for a UCP, but it is not mature enough to control optical switching nodes so far. Therefore, as an intermediate step towards a fully OpenFlow-based UCP, a logical choice at present is to introduce an OpenFlow/GMPLS interworking control plane which is able to use GMPLS to control the optical layer and use OpenFlow to dynamically coordinate between IP and optical layers. In light of this, in this paper, we experimentally present three interworking solutions (parallel, overlay and integrated) between OpenFlow and GMPLS for intelligent wavelength path control in IP/DWDM multi-layer optical networks. The overall feasibility of these solutions is assessed and their performance is quantitatively evaluated and compared on an actual network testbed.