Underlay and overlay networks

After all the talk about NFV and SDN, it behooves us to briefly describe underlay and overlay networks. This concept seems to follow the software engineering principle – solve any problem by introducing an extra level of indirection.

Very simply put, an underlay network is the underlying physical infrastructure of the network. An overlay network is a virtual logical network constructed on top of an underlay network using virtualization (see Figure 4.3):

Figure 4.3 – Overlay and underlay networks

Routers, switches, firewalls, and servers are devices found in an underlay network, which are interconnected via routing protocols. In an overlay network, which is software-based, data is transmitted via virtual links. Chapter 9 compares the security aspects of underlay and overlay networks. Suffice it to say that overlay networks provide segmentation and isolation, which helps with security. They also simplify the...