Most networks today have not changed despite the fact that SDN has being around for the last few years. One major reason for this slow change is the issue of migration faced by SDN especially Openflow. The incumbent vendors are very slow to embrace open technologies and the non-traditional vendors do not seem to have the clout to drive changes. The good news is that service providers and larger data center operators are taking Openflow seriously.
Here are a few strategies that could be explored to overcome the limitations with migration;
1. Software Upgrade - Does the incumbent vendor already have an Openflow enabled firmware image for the devices in question? If the answer is yes, do we know what level of support available?
2. Strategic Replacement - While it maybe impractical to replace all devices in the network immediately, there maybe an opportunity to replace specific devices that could enable easy migration to SDN. This could work especially for edge switches.
3. Tunneling - Another option to consider would be using tunnels in the SDN switches between each other while using the existing network as a basic transport. This is also one of the reasons why overlay technologies such as VxLAN has gotten more traction than Openflow, due to the ability to leverage the existing network and still get the benefits of SDN.
4. Hybrid Controllers - Most SDN controllers are focused only on SDN and therefore not geared towards managing existing networks. It would be useful for these controllers to speak both languages so that customers can see some unified benefit today as they slowly migrate forward.
There are many strategies to consider and employ but the bottom line is to show customers the benefits of migrating to a proper SDN Network. Building better greenfield networks is one way to make that case.
Check out this paper from the ONF on migration - Openflow Migration Paper.