SDN: Software Defined Networks

Chapter 2. Centralized and Distributed Control and Data Planes

One of the tenets expressed early in the introduction of SDN is the potential advantage in the separation of a network device’s control and data planes. This separation affords a network operator certain advantages in terms of centralized or semi-centralized programmatic control. It also has a potential economic advantage based on the ability to consolidate in one or a few places what is often a considerably complex piece of software to configure and control onto less expensive, so-called commodity hardware.

Introduction

The separation of the control and data planes is indeed one of the fundamental tenets of SDN—and one of its more controversial, too. Although it’s not a new concept, the contemporary way of thinking has some interesting twists on an old idea: how far away the control plane can be located from the data plane, how many instances are needed to exist to satisfy resiliency and high-availability requirements, and whether or not 100% of the control plane can be, in fact, relocated further away than a few inches are all intensely debated. The way we like to approach these ideas is to think of them as a continuum of possibilities stretching between the simplest, being the canonical fully distributed control plane, to the semi- or logically centralized control plane, to finally the strictly centralized control plane. Figure 2-1 illustrates the spectrum of options available to the network operator, as well as some ...