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How is network virtualization implemented in a software-defined network?
The goal of SDN-based network virtualization is to decouple a virtual network from a physical network. You want to create these virtual networks quickly with diverse requirements -- low latency, security separation, etc. -- and want to be able to have a programmatic interface. Considering, more and more network functions are being automated.
There are a variety of ways network virtualization can be implemented in a software-defined network. One approach is when a controller implements Layer 2 and Layer 3 constructs -- bridging and routing, if you will. I can use Layer 2 constructs to provide bridging between virtual machines. Sticking with that, I can use Layer 3 constructs to provide routing between my Layer 2 networks.
Overlay networks and SDN is another way of doing network virtualization. With overlay networks, there are competing approaches. The market is still spending time thinking about them. In SDN, OpenFlow is a standard, but how the controller implements network virtualization is not standardized.
When thinking about overlay networks, it's really a traditional approach. You're saying, "I have a problem. I'm going to encapsulate it." One protocol runs on top of another protocol. A cynic would say, "That's building a tower of complexity," but with SDN, it's really a new approach, and you can strip away some of that complexity. It's exciting. It's a new way to think about things differently. With network overlays, there is the potential for added complexity, but SDN can significantly reduce that complexity.
There is a significant need for network virtualization, and it fits in with major trends and things we are trying to do in IT: be faster, cheaper and have more agility while being cost-effective. Network overlays are the traditional approach, but the fact that there are many vendor options may slow deployment. SDN is a fundamental change -- it can do network virtualization and add value, but the price you pay is that it's a fundamental change in how you do things.
The difference between an overlay network and software-defined networking
Virtual overlay network use tunneling protocols to allow multi-tenant networks
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