Recently Cisco launched their ACI product and started a IETF draft for their OPFLEX protocol. They have taken on the issue of Centralize control versus distributed. At the end of the day what they are trying to say is not totally accurate. First of all ACI is Centralizes as you have to define policies from the APIC controller, that is the begin and end of it. Yes, once the policy is installed, you do not need the controller for traffic forwarding but so is the case with most of the other SDN protocols.
What about the other SDN protocols? Notably Openflow and the various Overlays. On the surface Openflow can be seen as a Centralize Control Plane approach while Overlays may be seen as distributed. The reality is that yes, in Openflow you need to define flow rules centrally but you can make these rules proactive to reduce reliance on the controller. In the case of overlays, you need some centralize entity to manage end point addresses across the fabric. If this overlay address management entity goes offline, your network could break significantly.
In summary, the centralized model does give us the advantage of agility and flexible network management but you do need schemes that would reduce the dependency on controller reliability. For example in Openflow, you can implement proactive flows to ensure less dependence on the controller.
Openflow as emerged as the pioneering protocol relating to Software Defined Networking and the hopes to transform networking as we know it. One of the biggest complaints with Openflow for real deployment is the lack of scale.
Recently, Broadcom an ASIC manufacturer published a new API scheme that shows the implementation of Multiple table pipe-lining which started in Openflow 1.1 but mostly talked about with 1.3.1. This pipe-lining approach allows switch vendors to exploit the many tables already in the current hardware. This API is significant as its implementation could turn the corner for real Openflow deployments in the year or so.
The other side to the scaling challenge is that Broadcom and other chip vendors are developing ASICs with much larger ACL tables closer to 100K to provide more flexibility in designing Openflow networks.
The idea of application centric networking is getting more attention nowadays. This has been Cisco's response to the SDN world through its Insieme Networks spin out and spin-in. There are some merits to the concept of declarative network management which was started by the likes of Puppet and Chef to declare policies which are enforced by the end points.
Cisco is also trying to push their ACI based technology as open by submitting a draft standard to the IETF for OPFLEX along with other companies. This is also going to be part of the Open Day Light project as well.
You can read more about Cisco's ACI Here..