Northbound and southbound
A controller sits in the middle and talks two directions:
| API | Direction | Talks to | Typically |
|---|---|---|---|
| Northbound (NBI) | Up, from the controller to apps and people | Your scripts, a web dashboard | A REST API (JSON over HTTPS) |
| Southbound (SBI) | Down, from the controller to the devices | The switches and routers | NETCONF, RESTCONF, OpenFlow, SSH |
You (or an app) ask the controller for what you want through the northbound API; the controller works out the details and programs each device through the southbound API.
Underlay, overlay and fabric
| Term | Meaning |
|---|---|
| Underlay | The physical network: the real switches, routers and cables, and the IP routing between them |
| Overlay | A virtual network built on top with tunnels (such as VXLAN), carrying the user traffic |
| Fabric | The underlay and overlay together, managed as one system by the controller |
Cisco's controllers
| Product | What it manages |
|---|---|
| Catalyst Center (was DNA Center) | The campus LAN: design, provision, assurance from one place |
| SD-Access | A campus fabric with policy by user/role, not by subnet |
| SD-WAN (vManage) | WAN links between sites, choosing the best path by policy |
Tip: Catalyst Center also does assurance: it watches the network and tells you what's wrong (and increasingly, why), instead of you hunting through logs.
Remember: Northbound = to apps/people (REST). Southbound = to devices (NETCONF/RESTCONF/OpenFlow). Underlay (physical) + overlay (tunnels) = fabric.