In part because of the limitations of the TCP/IP protocols used by the current Internet, the Future Internet has become an attractive network research topic. Recently, several projects focused on the Future Internet have been launched around the globe based on the OpenFlow platform, such as those that are part of the GENI project in the U.S., the OFELIA project of FP7 in the E.U., and the FIRST project in Korea. Because OpenFlow allows researchers to design and develop innovative protocols, it could be applied in different situations for constructing desired network environments. OpenFlow is not limited to single controller environment within a single network domain. It allows network engineers to construct a multi-controller environment across various network domains. In this paper, we design and implement an automatic network topology discovery mechanism based on a multi-controller OpenFlow network. The result has been applied to a large-scale OpenFlow testbed implemented across research institutes at different sites for path-finding and management.