MPLS in the SDN Era

Chapter 10. Underlay and Overlay Architectures

Software-Defined Networking (SDN) is an umbrella concept that means many things—probably, too many things. Depending on whom you ask, you will hear a completely different definition of what SDN is. Every new solution is wrapped with the SDN-ready mention and every new project is immediately overvalued when one says we will do it with SDN.

To separate hype from reality, it is very important to know precisely what we are talking about. Two key ideas, overlay and underlay, are at the center of the discussion and are often treated very lightly. They happen to be very complex concepts, with many derivatives, gray areas, and nuances. And, most important, they have a long history. Paraphrasing Martin Luther King Jr.: history is a great teacher. Ignoring it is a sure recipe for reinventing the wheel or, even worse, resurrecting architectures that have already been seen to fail repeatedly in the past.

The centralized and distributed control-plane dilemma is an old one. Phrases such as “centralize what you can, distribute what you must” have been told so many times and belong to the culture of good-old network engineers.

Here is the structure of this chapter:

• Introduction to the overlay and underlay concepts

• Architecture of multiforwarder network devices

• Discussion of the challenges of legacy data center networking and the need for an overlay

• Architecture of IP fabrics, with a distributed or centralized control plane

• Architecture ...