📘 CCNA 200-301 v1.1
6.3 Describe controller-based, software defined architecture (overlay, underlay, and fabric)
1. Controller-Based Networking
What it is:
A controller-based network separates the decision-making part of the network from the actual devices (switches or routers).
- Controller: Think of it as the “brain” of the network.
- Devices (Switches/Routers): Think of them as “workers” that follow instructions from the brain.
Why use it?
It makes networks easier to manage, automate, and scale. Instead of configuring each device individually, you configure the controller, and the devices automatically get the instructions.
Key points for CCNA:
- Centralized control = easier management
- Devices can be simpler because the intelligence is in the controller
- Often used in software-defined networks (SDN)
2. Software-Defined Networking (SDN)
What it is:
SDN is a networking approach where the control plane is separated from the data plane:
- Control Plane: Makes decisions about where traffic goes (like routing decisions).
- Data Plane: Forwards traffic according to the control plane’s rules.
In simple terms:
The controller decides where data should go, and the network devices just send it there.
Benefits for IT environments:
- Easier network automation
- Centralized policy enforcement
- Faster response to changes (like adding new users or applications)
3. Overlay and Underlay Networks
In SDN and modern networks, we often see two layers: underlay and overlay.
a) Underlay
- The physical network (switches, routers, cables, and IP addresses).
- It’s responsible for basic connectivity and getting packets from point A to point B.
Example in IT networks:
- Your core routers connecting different data centers.
- The underlay must be reliable, fast, and predictable.
b) Overlay
- A virtual network on top of the underlay.
- Uses technologies like VXLAN to create logical networks independent of physical connections.
- The overlay network can move workloads or devices without changing physical connections.
Example in IT networks:
- Virtual machines in a data center can communicate across different physical switches as if they were on the same network.
- Policies like security or QoS can be applied on the overlay without touching the underlay.
Key Exam Points:
- Overlay = virtual/logical network
- Underlay = physical network
- Overlay depends on underlay, but it abstracts complexity for users or applications
4. Fabric Networks
A fabric is a network design approach that combines controller-based management and overlay/underlay concepts.
Definition:
- A fabric is a simplified, scalable, and automated network where devices work together under a controller to make deployment and management easier.
Features of fabric networks:
- Automation: The controller automatically configures devices.
- Scalability: You can add new switches without reconfiguring the entire network.
- Consistent policies: Security, QoS, and routing rules are applied consistently.
- Overlay support: Logical networks can be created on top of the physical fabric.
Common Cisco Technologies for Fabrics:
- Cisco SD-Access: Enterprise fabric for campus networks
- VXLAN: For overlay virtualization
- ISE (Identity Services Engine): For applying policies based on user/device identity
5. Putting It All Together
| Concept | Role in Network | Example for IT Environment |
|---|---|---|
| Controller-Based | Central “brain” controls switches and routers | Cisco DNA Center controlling campus switches |
| SDN | Separates control plane (decision) from data plane (forwarding) | Controller decides paths, switches just forward traffic |
| Underlay | Physical network | Core routers, physical switches, IP routing |
| Overlay | Virtual/logical network on top of underlay | Virtual networks connecting VMs across multiple switches |
| Fabric | Simplified, automated network design | Campus fabric with automated policies, overlay, and underlay |
6. Key CCNA Exam Takeaways
- Know the difference between controller-based and traditional networking.
- Understand SDN concepts: separation of control and data plane.
- Understand underlay vs overlay: physical vs virtual networks.
- Know fabric benefits: automation, scalability, consistent policies.
- Examples like VXLAN, SD-Access, DNA Center may appear in questions.
