SD-WAN is an overlay. You still need an underlay!
When you ask “What is SD WAN?” it’s really important to understand exactly which definition of SD WAN is being described to you. Since most SD WAN vendors are WAN edge device companies they will not be offering the connectivity within their solution. That will be left to the customer or their Managed Service Provider (who may or not be a carrier) to provide and manage, or at least to monitor.
The WAN edge devices (otherwise known as CPE), their functionality and management are known as the Overlay, since they overlay the Wide Area Network. Similarly, the connectivity is known as the Underlay. This could be public (internet) or private (MPLS, VPLS, Ethernet P2P etc.) or semiprivate network (Optimised IP).
SD WAN makes it easier to mix connectivity types within your underlay to create Hybrid Networking which is a topic in its own right, with its own hype cycle.
To complement your SD WAN CPE solution you must carefully choose your Underlay, since this is where the likely cost savings will arise. And this isn’t just true for customers with international sites. To maximise your application performance and budget and minimise your management overhead, you need an in-depth understanding of the carrier market place. You need to understand access options and associated SLA’s, private and public networks, and details of your users and applications (and where they both reside). Take a look at our Guide to Reducing WAN Costs for further information.