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Introduction to Software Defined Networking (SDN)

Written by Ken Corkins, Director of Network Solutions, Datatrend Technologies, Inc.

As a new and emerging technology, Software Defined Networking (SDN) is somewhat difficult to define. Ask 10 different people what SDN means, and you are likely to receive 10 different answers. (Does this remind you of the term “Cloud” in its early days?)

The Open Network Foundation (ONF) is the organization that manages the OpenFlow standards. The ONF defines SDN as “The physical separation of the network control plane from the forwarding plane, and where a control plane controls several devices”. 1

To illustrate this concept, an Ethernet switch (the IBM G8264 is pictured here) can be described as three functional parts;

First, processes within the switch decide what to do with each individual packet as it arrives on a port. This function is called the “Control Plane” (or the “brains” of the device).

Second, the device contains the electronic circuitry to actually perform the sending and receiving of packets. This function is called the “Data Plane” (or the “muscles” of the device).

Third, There are also management processes in the device used to configure the switch. Although not always depicted, this is referred to as the “Management Plane” (or the “secretary”.)

With a software defined networking model, the data plane (packet forwarding) and management plane stay with the device. However, the control plane (forwarding decisions) is consolidated in a central location, which can be an OpenFlow controller, or a hypervisor manager. The controller takes the information about packets arriving at the ingress port and makes a decision as to how to process the packet. This decision is made based on the first packet, then the controller programs all of the SDN devices in the path with the forwarding information for subsequent packets, which is called a “flow”.


Generally, there are two recognized models for Software Defined Networking:
The OpenFlow Model, and Network Virtualization.

OpenFlow is an open standards protocol maintained by the ONF. The OpenFlow protocol uses a software program called a controller to configure the data planes on the devices it controls.

The other model for SDN falls under the umbrella of Network Virtualization. This term commonly refers to hypervisor environments. In this type of deployment, the control plane is generally deployed as a virtual machine or in the hypervisor manager, and the data plane is a module loaded onto each hypervisor host.

Use Cases

There are many use cases for SDN, including the ability to seamlessly tap network traffic to a security appliance. Once a threat is identified, the security application could send information to the controller to reroute the threat traffic for further analysis, or to simply discard the packets.

With SDN, the network can quickly and programmatically respond to dynamic network events, such as workloads being deployed or de-provisioned, or network congestion issues.
Traffic for different applications or customers can be controlled separately, or sent through different network services (firewall, load-balancers, etc) based on specific requirements.


While showing great potential, SDN is not without its share of concerns. The newness of the technology is the primary concern for many who look at SDN as their network operational model. Standards are still evolving, and vendors are working to carve out a place within the market. Security is also a potential concern. As a new and emerging technology, SDN may have security exposures that are not yet fully understood. Additionally, network administrators will need to learn new and different ways of thinking about, designing and operating within this new networking paradigm.

Where to begin

SDN does not require a wholesale “rip and replace” strategy to get started. If you’re considering a private cloud for a department or a given set of development projects, this may represent an excellent opportunity to pilot some of these technologies. Many network devices can operate in “mixed” mode where some portion of the device operates in a traditional fashion and other portions of the device can be controlled by an OpenFlow controller. If you are in the process of planning a restructuring of your network, or planning for the rollout of a new application or service, it may make sense to take a look at SDN as a potential technology with an eye to integrating more SDN services as you move forward.


While still an immature technology, SDN represents the future of networking. All the major network vendors have SDN products available and roadmaps to bring forward more SDN offerings; and, a number of startups are trying to make their way into the market. While standards are still evolving, and it will also take some time for network administrators to learn new skills and processes, one thing is clear: Software Defined Networking will be the next major wave of network technology.

For information on Software Defined Networking (SDN), please contact Ken Corkins at ken.corkins@datatrend.com, or call us at 1-800-367-7472.

1 https://www.opennetworking.org/sdn-resources/sdn-definition