What is SDN?

Software Defined Networking (SDN) means different things to different people. Some of the key words often associated with SDN are Automation, OpenFlow, Network virtualization and Overlay networking.

Lets look at each of these in turn and see how they fit with SDN.


Managing large networks has always been painful, applying changes on a system by system basis was very error prone and laborious. Over the years devices to manage and maintatin network equipment have been developed, but almost always on a proprietary basis. You would usually end up with one system to manage all of your firewalls another to manage your IPS devices etc. Today many systems will feature a central orchestration platform to push policies and effectively automate the configuration of remote devices, but is this SDN?

Programmable Networking

OpenFlow was without a doubt one of the first steps towards what has been coined SDN. OpenFlow was developed to solve a slightly different issue than simply central orchestration. It was however based on centralised control, which is why the orchestration line gets a bit blurry. The difference is that OpenFlow is a communications standard that allows separation of the control plane from the data plane. Which allows a single control plane to effectively manage multiple devices. This means in practice you configure a central controller which in turn directly manages the forwarding table on your physical equipment by means of the OpenFlow standard.

Orchestration on the other hand is where you configure a central system to manage remote end devices. By ONF’s definition an orchestration platform alone is not SDN as each end device could have it’s own control plane. That said if the communication between the central controller and the end device has a single central control plane and uses a standard such as OpenFlow then it would be classed as an SDN solution.

OpenFlow has been adopted by several Ethernet switch, router and Wireless access point vendors and is operated by the Open Network Foundation (ONF).

Virtualized Network Equipment

In a world of Virtual routers, switches, firewalls and load balancers, surely if we are only using the software then that’s Software Defined Networking, no?, Well to try and eliminate confusion the term used for taking devices usually associated with being physical and making them virtual is Network Function Virtualization or NFV. That said if the network device is managed by a central control plane then NFV solutions could be deemed part of an SDN solution.

Overlay Networking

Primarily used to provide secure multi-tenancy. Overlay networking is something provided by nearly all SDN solutions and is heavily associated with SDN in the same way that Data Center Fabrics are. However neither the underlay nor an overlay in itself can be classed as an SDN, unless…

you guessed it, a central control plane is used to implement the network in which case they can both be classified as SDN.

What is SDN?

SDN is a little bit of everything mentioned, but at its heart we have to stick to the definition provided by the ONF and  say that SDN can only exist with a centralized control plane. If that isn’t present, technically we should be calling it something else.

About Stephen Ransome

Stephen Ransome is an IT consultant and network nerd with experience ranging from SMBs to Service Providers, he has a passion for learning new technologies and delivering solutions that count. He has some alphabet soup, including CCIE#41102 and is far more cynical than he should be.