Lesson Overview
Software-Defined Networking (SDN) separates application intent from the device-by-device implementation of that intent. In a controller-based architecture, applications communicate with a controller through northbound interfaces, while the controller communicates with routers, switches, wireless infrastructure, and security devices through southbound interfaces.
Learning Objectives
- Explain the role of the application, controller, and infrastructure layers in an SDN architecture.
- Differentiate northbound APIs from southbound interfaces and identify where each is used.
- Describe REST API requests, HTTP methods, status codes, JSON, authentication, and CRUD operations.
- Explain how NETCONF, RESTCONF, OpenFlow, and model-driven interfaces can be used southbound.
- Explain the relationship between YANG data models, NETCONF, and RESTCONF.
- Follow an end-to-end enterprise automation workflow from application intent to device configuration.
- Recognize CCNA exam traps related to REST, JSON, YANG, NETCONF, RESTCONF, and controller-based networking.
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EXAM FOCUS |
For CCNA, the essential direction is: Application ↔ Controller = northbound. Controller ↔ network devices = southbound. |
1. Why SDN Interfaces Are Needed
Traditional networks are commonly configured device by device using the CLI. As the number of routers, switches, firewalls, wireless controllers, and access points increases, manual configuration becomes slower, harder to maintain, and more prone to inconsistency.
Software-Defined Networking (SDN) introduces a controller that provides centralized visibility, policy, automation, and management. For the controller to communicate with both applications above it and network devices below it, standardized interfaces and APIs are required.
These interfaces are generally divided into:
| Interface | Communication Direction | Purpose |
|---|---|---|
| Northbound APIs | Applications ↔ SDN Controller | Allow applications, automation tools, dashboards, and business systems to request network information or services |
| Southbound Interfaces/APIs | SDN Controller ↔ Network Devices | Allow the controller to configure, monitor, collect state, and manage routers, switches, firewalls, WLCs, and other devices |
Why Interfaces Are Important
SDN interfaces provide a consistent way for different systems to exchange information and instructions without requiring every application to understand the CLI syntax of every network platform.
A controller-based architecture introduces a programmable abstraction layer. Applications communicate with the controller, and the controller translates the requested intent into actions on the infrastructure. This creates centralized visibility, consistent policy, and repeatable automation.
Figure 1. Controller-based SDN architecture with northbound and southbound interfaces.
1.1 The Three Logical Layers
|
Layer |
Typical Components |
Primary Role |
|
Application layer |
Automation applications, monitoring, security, analytics, cloud orchestration |
Express business or operational intent and consume network information. |
|
Controller layer |
Cisco Catalyst Center, SD-WAN controllers, other SDN controllers |
Maintains topology and inventory, applies policy, exposes APIs, and translates intent. |
|
Infrastructure layer |
Routers, switches, firewalls, WLCs, APs, virtual network devices |
Forwards traffic and applies configuration received through supported management interfaces. |
2. Northbound APIs
A northbound interface sits logically above the controller. It allows applications, scripts, portals, and automation systems to request information from the controller or ask the controller to perform an action. The application does not have to log in separately to every device.
Figure 2. An automation application uses a northbound REST API to interact with the controller.
2.1 Typical Northbound Functions
- Retrieve device inventory, platform, serial number, software version, and reachability.
- Retrieve site, topology, interface, wireless client, and health information.
- Create or modify network policy and intent.
- Provision devices and sites.
- Trigger workflows such as software image distribution or configuration deployment.
- Integrate the network controller…