Controlling the fabric

It is very important that you understand the main technologies involved in SDN. These methods are used by SDN controllers to manage and control the SDN network.

In general, there are two methods available for controlling the fabric:

• Direct fabric programming: In this method, the SDN controller directly communicates with SDN-enabled switches via southbound protocols such as OpenFlow, NETCONF, and OVSDB. The SDN controller programs each switch member with related information about fabric, and how to forward the traffic. Direct fabric programming is the method used by OpenDaylight.
• Overlay: In the overlay method, the SDN controller doesn't rely on programming the network switches and routers. Instead it builds a virtual overlay network on top of the existing underlay network. The underlay network can be an L2 or L3 network with traditional network switches and router, just providing IP connectivity. The SDN controller uses this platform to build the overlay using encapsulation protocols such as VXLAN and NVGRE. VMware NSX uses overlay technology to build and control the virtual fabric.