Automation & Programmability // Lesson 03

Software-Defined Architecture.

Describe underlay, overlay and fabric, control/data-plane separation, and northbound versus southbound APIs.

UnderlayOverlayFabricNorthbound APISouthbound API
01 // Mental model

Start with the big picture.

The physical IP transport that provides reachability between infrastructure nodes.

CCNA focus: Troubleshoot from the bottom up: physical and underlay reachability first, then control relationships and overlay state, then policy.
02 // Building blocks

Know what each part does.

01

Underlay

The physical IP transport that provides reachability between infrastructure nodes.

02

Overlay

A logical topology built over the underlay, often using encapsulation to carry endpoint traffic.

03

Fabric

The coordinated system of edge, control and border functions that delivers policy and connectivity.

04

API direction

Northbound interfaces expose controller services to applications; southbound interfaces program or collect from infrastructure.

03 // Compare and recognise

Read the clues.

ItemWhat to remember
Control-plane separationA controller can centralise or logically centralise decisions.
Data planeInfrastructure nodes continue to forward traffic at line rate.
EncapsulationCarries overlay identifiers across underlay IP reachability.
PolicyCan follow users, devices or groups instead of relying only on topology.
04 // Layered view

Layered view.

Applications and automation
        | northbound APIs
Controller / policy / assurance
        | southbound APIs
Fabric control and edge nodes
        | overlay tunnels
Routed IP underlay
        | physical links
Endpoints and services

Read the example from top to bottom, then verify the resulting state. Configuration is only complete when the output matches the intended design.

05 // Exam and troubleshooting

Turn facts into a method.

  • A controller can centralise or logically centralise decisions.
  • Infrastructure nodes continue to forward traffic at line rate.
  • Carries overlay identifiers across underlay IP reachability.
  • Can follow users, devices or groups instead of relying only on topology.
Exam checkpoint: Troubleshoot from the bottom up: physical and underlay reachability first, then control relationships and overlay state, then policy.
06 // Check yourself

Software-Defined Architecture quiz.

1. What provides basic IP transport between fabric nodes?

2. Which API direction connects applications to a controller?

3. Which API direction connects a controller to infrastructure?

4. What is an overlay?

5. Where are packets normally forwarded?

Score: 0 / 5