What I Discovered About Software Defined Networking (SDN)

We’ve been hearing a LOT about Software Defined Networking (SDN) and how it’s the latest, greatest thing for computer networks and computer networking.

But what is it?

Read through this short post to see exactly what it is, how it works and what you need to know about it.

Software Defined Networking (SDN) – Different From What’s Been Used Until Now…

So most of the history of how computer networks have worked until the present-day (today) has been with hardware switches and routers. Each one (each physical router or switch) physically has 2 closely related components that function on the router or switch itself: There is a control plane and a data plane.

To understand how Software Defined Networking (SDN) works, we first need to understand what the control plane and the data plane have traditionally done ON each of these physical networking components (routers and switches).

Control Plane and Data Plane

To keep this really simple, let’s talk about the control plane first. The control plane on a router or switch is what makes actual decisions about HOW to MOVE traffic. These decisions are made from the configuration that’s been installed or programmed into the router or switch in its running configuration.

Now the control plane makes the actual decisions. The data plane is responsible for carrying OUT those decisions and actually doing what the control plane wants done.

So if we look at the control plane on a router, for instance, we see that the control plane transmits routing protocols like OSPF and BGP. It also builds routing tables that are USED by the data plane.

A router’s data plane will read incoming packets and reference that routing table to send the packets to their correct destination.

So really all you need to understand on this video is that each physical router or switch has its own control plane and data plane and these are what are used to make and carry out those decisions on where network traffic goes, if it has any priority over other traffic, what VLAN(s) it belongs in, etc.

Software Defined Networking vs. Network Management Software…

This leads us to Software Defined Networking (SDN)…With software defined networking, the control plane on each physical router or switch is no longer needed or used.

Instead, the router or switch will communicate with and rely on what’s called a Network Controller to specify how physical network components and virtual network components move traffic through a network.

So essentially for SDN to work, you need network components (routers and switches) that have data planes designed or built to take instructions FROM that network controller instead of their own in-house (so-to-speak) control plane.

An Old Concept to Help Understand SDN

To understand this concept a little better, let’s first look at (and there are plenty of instances where semi-older components have to use this) but let’s first look at network management software.

With network management software until the present day, you install and use a program like SNMP on the component itself (the router or switch) and that SNMP installation is what’s referred to as a “client” and it would report to the main management software (running on a server or desktop) allowing semi-real-time reporting of whether there are any errors reported on that switch or router as well as any log messages registered, bottlenecks in traffic, temperature thresholds, functional malfunctions, etc. and etc.

The network administrator that installs and configures the network management software and the SNMP client on each switch and router can login to the network management software running on that centralized server or desktop (or even laptop) and monitor all the components on a network that are set to report in as “clients” and that administrator/technician can even upload or change router or switch configurations remotely USING that network management software on that server or desktop. Kind of a more centrally managed network monitoring process.

SDN On A Network Controller

If you understand the concept of network management software, you can thoroughly grasp how SDN works.

Now SDN is NOT network management software.

Rather, SDN is used essentially in real-time to monitor traffic on all components (all the routers and switches set to report in to the network controller) and it can actually order them to change how they handle traffic on the network, because the network controller serves as kind of like the head boss control plane for each of those devices, instead of each device’s own control plane running the show and making decisions.

So for a while, we’ll probably be seeing Network Management Software used alongside Software Defined Networking (which again is used on the Network Controller) on larger networks.

The REAL Benefit…

But I want to leave you with this: The real benefit to software defined networking (SDN) is not so much that it’s a centrally managed control plane (of sorts, because that’s essentially what you’re doing), but the real benefit is that the network controller in Software Defined Networking is programmable. You can even write code that controls how the entire network behaves and works and that can all be done through the software defined networking setup using the Network Controller.

If we stayed with Network Management Software, administrators/technicians still have to change the configurations on each router or switch themselves to alter how traffic is handled. With software defined networking (SDN), it’s done on the fly BY the network controller acting as the centralized control plane for each of those routers and switches.

So that’s the basics of Software Defined Networking (SDN) and what you’ll need to know to correctly answer any questions you may get about it on the Network+ Exam.