Defining SDN in the Open Networking Era

Software-defined networking (SDN) represents the future of networking. A “software-defined” network enables an organization to virtualize their network, automate operations to enable efficient network configuration, and integrate network functions across dozens of switches creating a unified network architecture that is programmable and dynamically definable. The goal of SDN is to create a network that is easier to manage and is automated to accelerate the roll out of services as IT stands up new applications and adds new users. SDN should also help IT simplify the operational model and the movement and sharing of applications, resources and users to various locations across the organization.

When initially created, the spirit of SDN was to separate the control plane and the data plane functions. The separation enables programmability and also enables the use of low-cost, less intelligent network switches. For years SDN was commonly associated with the OpenFlow protocol and controllers with the purpose of determining the path for network packets across network switches and centralizing management of the network.

While SDN protocols and architectures have evolved, the notion of the SDN controller has remained. Even today most SDN architectures still use controllers to support the separation of the control plane functions from the physical switches to enable programmability, allow centralized and automated control over the network.

What’s Not SDN?

Vendors now use the term “software-defined” to describe everything, including networking. Unfortunately, some networking vendors use the SDN term more liberally than they should. Stated simply, today’s SDN is creating a “fabric” of network switches that operate together as a single logical switch that through the SDN software is programmable and automated.

A common term used along with SDN and Open Networking is disaggregation. Disaggregation decouples switch hardware from the networking operating system and enables IT to mix and match the hardware and software components. SDN though is not always about disaggregation and disaggregation is not always an SDN solution. There are SDN solutions that are available from traditional networking vendors. These are the closed hardware and software offerings that use a controller to enable their ability for programming and automation. There are also disaggregated SDN solutions from Open Networking OS vendors as well. These use open networking hardware switches and a Network OS that enables the SDN functionality. With Open Networking SDN, some SDN architectures use a controller, while others do not. But not all Open Networking OSes are capable of implementing SDN.

Our last blog established the value of Open Networking and disaggregation, but the benefits of Open Networking translate through to SDN deployments as well. To see those benefits, one needs to understand the evolution of SDN.

SDN – The First Generation

The first generation SDN architectures have two common elements – they are dependent on a controller for operation, and they use an SDN related communication protocol such as Open Flow between the controller and switches.

The SDN controller is the “brains” of the network. It holds the configurations, tells the switches what to do and is the point of programming and control for the switches that are in the SDN fabric. The controller enables centralized control, automation and programmability. The use of a controller has a significant downside though, the network will not function without the controller.

SDN controllers also add complexity and impede resiliency. Since the network is dependent on them for operation, they are a single-point-of-failure and can become a performance bottleneck. In the event of controller failures, fail-over to a redundant controller can cause network reconvergence impacting network operations. A reconverging network can interrupt applications and create user experience issues as the network resets itself through the controller.

In addition to resiliency considerations, SDN controllers inhibit the ability to extend the SDN fabric across physical locations. Some first-generation SDN vendors can interconnect SDN islands, but each island still has its group of controllers that are overseeing the local fabric of switches.

SDN can significantly improve network functionality, agility and business value, but the complexity of controllers and dependency on new protocols like Open Flow, has been cited as a common inhibitor to widespread SDN adoption.

Next Generation SDN

The next generation of SDN addresses the fundamental challenges created by first-generation controller-bound SDN architectures. It is still software-defined, and programmable, but it eliminates the complexities that are common with controllers and SDN protocols. Next-Generation SDN is a fully distributed fabric architecture that operates without a controller. Many IT professionals find it easier to manage and easier to automate. It is also considered more resilient since it isn’t dependent on a controller architecture. IT can deploy next generation SDN more easily into an existing networking infrastructure allowing IT organizations to orchestrate a more graceful migration to SDN.

The next generation of SDN removes the complexities that have inhibited SDN adoption by building the controller function into the SDN operating system. The controller functionality then operates within the distributed network devices. The peer-to-peer approach shares the controller functionality across all of the network’s switches. The distribution of the controller function across switches is similar to how computing and storage cluster technologies work.

With next-generation SDN there is still the separation of control plane functions that was the intent of the initial SDN definitions. The distribution of the control plane instead of isolating on a few servers makes the network switches more intelligent and more integrated with the rest of the network.

Next-generation SDN simplifies the operational model and should lead to increasing adoption for several reasons. First, automation is now integrated into the network and is not dependent on a controller. This means that you can manage and program the network from any device in the network. The switches in a next-generation SDN network are intelligent and understand the state of the network – not only for the local switch, but for all switches across the network. The network awareness of each switch increases resiliency, eliminates reconvergance and makes the network inherently smarter.

Second, next-generation SDN virtualizes the network, like how a hypervisor virtualizes a server, which means you can segment the network for security and multi-tenant operations, but you can also virtualize network services. Through network virtualization IT can segment networks for better security control to reduce the attack surface and prevent attack movements.

Third, next-generation SDN enables IT to build a geographically distributed SDN fabric to simplify Data Center Interconnection. Data Center Interconnection via SDN means organizations can have a single logical switch that can be distributed to many physically separate places. Even in the geographically disperse configuration IT still has the control and programmability over the environment as if all of the distributed switches were one logical switch.

Finally, the next-generation SDN is also is important for virtualized applications and hyper-converged infrastructure environments because it allows distributed locations to operate as if they are located in a single data center. Next-generation SDN improves application resiliency, optimizes resource sharing, speeds application mobility and supports business continuity requirements.


In our last blog we discussed the importance of the Network Operating Systems when implementing White Box Switches. Open Networking changes the economics of networking in the data center and beyond. However, not all White Box OS options are SDN, and not all SDN systems are built on white boxes. There are two steps to consider. The first is taking the journey to white box architectures, and then considering adoption of SDN for your network.

An Open Networking enabled SDN architecture can lower costs by leveraging the cost efficiencies and flexibility of white box switches. The operational value of SDN is of equal importance. It will improve network efficiency and organizational agility through automation and programmability. The next-generation of SDN can remove the obstacles associated with first generation SDN implementations and enable the organization to undertake a staged migration to SDN and all its benefits.

Sponsored by Pluribus Networks

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Twelve years ago George Crump founded Storage Switzerland with one simple goal; to educate IT professionals about all aspects of data center storage. He is the primary contributor to Storage Switzerland and is a heavily sought after public speaker. With over 25 years of experience designing storage solutions for data centers across the US, he has seen the birth of such technologies as RAID, NAS and SAN, Virtualization, Cloud and Enterprise Flash. Prior to founding Storage Switzerland he was CTO at one of the nation's largest storage integrators where he was in charge of technology testing, integration and product selection.

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