The chief problem with centralized control is that there is a single point of failure. So when ARPA sponsored the design of a network that needed to survive network element failures (no, despite popular belief, there was no design goal to survive nuclear attack), it was decided to rely on distributed routing protocols. Using these protocols routers speak with other routers, and learn enough about the underlying topology to properly forward packets. Local forwarding decisions miraculously lead to globally optimal paths.
Yet, the optimality just mentioned is that of finding the shortest path from source to destination, not that of optimally utilizing network resources. Routing protocols can support traffic engineering, but this means reserving local resources for a flow, not locating under-utilized resources elsewhere and pressing them into service.„