SDN Fabric, Underlay, Overlay Networks and Architectures

This tutorial explains the Software-Defined Networking and its components, such as the underlay network, the overlay network, and the SDN fabric.

There are two types of networking: traditional networking and controller-based networking . In traditional networking, we individually configure and manage each network device. For example, if we have ten routers in our network, we must configure and manage them separately.

In Controller-based networking, we configure and manage devices from a central location. We use a controller software or an SDN device that automates configuration changes on the network and manages the deployment of configuration changes to multiple devices simultaneously.

Since we use software for applying and managing configuration changes to devices on the network, it is also known as Software-Defined Networking (SDN).

An SDN controller centralizes the control of the networking devices. Based on the network's requirements, administrators can configure the level of the control. For example, they can configure the controller to perform all control plane functions, replacing the devices’ distributed control plane. Alternatively, they can configure it to monitor the ongoing work of the distributed data, control, and management planes on the network devices, without changing how those operate.

There are three ways to implement an SDN.

  1. You can install it as software on a server available on the network.
  2. You can use an SDN device. An SDN device contains the controller in it.
  3. You can install it on the cloud as a remote controller.

No matter how we install or implement the controller device or software, it uses various tools such as Puppet, Chef, and Ansible to manage and automate changes on the network.

SDN provides a graphical overview of the network. It allows us to manage a network device directly from the GUI screen. To manage a device, we select it on the SDN screen and change its settings. The SDN then applies those changes to that device. With SDN, we don't need to physically visit devices to manage them. We can manage them directly from the SDN screen.

SDN Architecture

Unlike traditional networking which uses a flat architecture, SDN uses layered architecture. It divides the network into two layers; underlay and overlay. Let us understand both layers in detail.

Underlay Network

It is the bottom layer. It contains all regular network components, such as routers and switches. To build an SDN environment, first, we have to configure appropriate protocols and features on all network devices to enable access and network communication. It means we have to configure IP addresses on all interfaces and enable routing between all routers. We must use an open standard protocol such as OSPF, BGP, or IS-IS as the routing protocol.

Overlay network

It is the upper layer. It builds on the top of the underlay network. It includes the virtual networks created on top of the underlay network that uses virtual technologies such as VRF (virtual routing and forwarding), MPLS VPN(multiprotocol label switching virtual private network), and VXLAN (Virtual Extensible LAN).

Differences between underlay and overlay networks

The following table compares the underlay and overlay networks and lists their differences.

parameter Underlay Network Overlay network
Network type Physical network Virtual network
Protocol BGP, IS-IS, OSPF, Ethernet Switching, VLAN VXLAN , OTV , VPLS
Data transmission between network devices such as routers and switches between virtual nodes through virtual links
Deployment time Require more time as each device needs to be set up individually Require less time as multiple devices can be configured simultaneously
Scalability Less scalable More scalable
Packet encapsulation Packets are encapsulated in Layer 2 and Layer 3. Packets are encapsulated based on the source and destination.
Packet control Hardware Software

SDN Fabric

The SDN fabric includes all the components that make up the SDN network. It is made up of the physical network devices that make the underlay network and are controlled by the SDN controller. In other words, it is a combination of both underlay and overlay networks.

ComputerNetworkingNotes CCNA Study Guide SDN Fabric, Underlay, Overlay Networks and Architectures