Various network programmability options exist today. Network management applications and monitoring tools traditionally accessed devices using CLI, SNMP, and NetFlow methods.
As SDN emerged, vendors introduced vendor-specific APIs to control existing devices, often maintaining the control and data planes within the same device (Option 1). Open APIs, such as NETCONF and RESTCONF, were later added.
Pure SDN (Option 2) separated the control plane to a controller, initially using OpenFlow, which required hardware upgrades.
Hybrid SDN (Option 3) retains a control plane on network devices and employs an abstraction layer between applications and devices, enabling programmable communication and automation. This approach is most commonly used today by most vendors, including Cisco.
The overlay approach (Option 4), often utilizing protocols like Virtual Extensible LAN (VXLAN), maintains existing devices while creating virtual overlay networks for automation and programmability.