What is a network of overlays? Does it include routers? What are the edges in the network of overlays?

What is routing, and how does it work? Defintion of two popular routing methods is necessary.

Destination IP-based forwarding is used by the per-router control plane (sometimes called the classical network) to connect with other routers. If messages are forwarded depending on their destinations' IP addresses, what are the boundaries of this approach? Include a description of at least three limitations, preferably with examples. To what extent does Software-Defined Networking (SDN) contribute to solving these problems? The examples will serve as the explanation.

Communication between routers on the per-router control plane (or classical network) is accomplished using IP destination forwarding. To what extent does this message forwarding mechanism based on a recipient's IP address have restrictions? Provide at least three limitations, illuminating them via specific examples. The question is, what part does Software-Defined Networking (SDN) play in solving these problems? This will be explained with certain instances.