Is SDN a threat to large network equipment manufacturers?
I think the short answer is yes. Cisco and others have developed strategies to deal with this threat, but fundamentally, the main idea under SDN and the promises it holds are incompatible with the existing platforms of these large network equipment providers, and their existing business models.
By decoupling the control plane and the data plane in the complex networking equipment, SDN (Software Defined Networking) makes it possible to simplify the design and development of computer networking hardware. It moves the complex control plane decisions to a remote server, and leaves only the physical layer and a limited layer 2 functionality in the networking hardware itself. It embodies all the recent trends in the computer industry such as the open source software development and cloud computing. Moving the control plane to a remote server is similar to the idea behind cloud computing, and the OpenFlow and OpenStack software that are widely used in the SDN networking equipment are open source software, which means anyone can use them to develop their own flavor of a network switch.