Written by Jim Metzler
Jim Metzler is an expert at researching and analyzing emerging technology trends within the networking and service provider space.
The overlay and underlay models present specific advantages and disadvantages to the implementation of SDN. Overlay does not require touching the underlying infrastructure, which deprives the network of some of the promises of SDN. While Underlay requires attention to the underlying switches and routers, it allows for the dynamic movement of VMs, centralized configuration management, and the ability for applications to signal for resources they need.
There are two fundamental models that people are implementing relative to SDN. One is the overlay model. The overlay model is from companies such as VMware and Nuage Networks. It focuses on the virtual machine on the vSwitch inside of that. It's entirely focused on a virtual environment. Initially, the main use case for that was to support the dynamic movement of VMs around a data center or in theory, between data centers. More recently, companies like VMware have introduced other use cases such as microsegmentation to increase the security within a data center. I'm talking about rolling out yet additional functionality.
The advantage of that approach is it is an overlay. You don't have to touch the underlying network. You don't worry about new protocols, say OpenFlow. It doesn't have to be there. You just lay this on top. That makes it much easier to implement. That's also the bad news. It doesn't touch the underlying infrastructure. Some of the promises of SDN in terms of centralizing configuration aren't there. Now, if we look at the other approach, the underlay, it's the opposite. It does get down to the individual switches and routers, which means you need some kind of a protocol to communicate between the controller and those switches.
That may mean rolling out new technology. That can be an impediment. On the other hand, you've got all of the advantages of the overlay in terms of supporting dynamic movement of VMs, but you also get other advantages such as I mentioned, centralizing configuration management and at least in theory, the ability of applications to dynamically signal the network for the resources it needs. Now, I've also seen in the last year or so, is the attempt to bring these two solutions together.
Just one example, VMware is working with HP. HP, NEC, OpenDaylight are good examples of vendors or sources doing the underlay model. VMware is working with HP and others to integrate ... Or, really federate is the proper word, their two solutions. We in theory, now get the best of both worlds, but obviously, introduce the complexity of federating controllers from different companies.