To better understand how SDN works, it helps to define the basic components that create the network ecosystem. SDN architecture is made up of three layers that communicate using northbound APIs (interfaces that enable lower-level components to communicate with higher-level components) and southbound APIs that facilitate communication in the opposite direction. The three layers are:

Application layer

The application layer includes network applications and programs. The application layer communicates with the control layer through its northbound interface, informing the control layer of application resource needs. Traditional networks might use a dedicated appliance such as a firewall or load balancer, but software-defined networks instead use an application layer to control and manage the data plane. 

Control layer

The control layer serves as the brain, or network operating system that manages the movement of traffic and data. The control layer plays a key role in resource allocation throughout the network. It is the central layer that enables communication between the application layer and the infrastructure layer. 

Infrastructure layer

This layer consists of physical switches and routers that move data packets and network traffic through the network. 

In addition to these layers, software-defined networks are built with components that may or may not be located in the same physical area.  
 
These include:

Applications 

Applications are tasked with relaying information about the network or requests for specific resource availability or allocation.

SDN controllers 

SDN controllers handle communication with the apps to determine the destination of data packets. The controllers are the load balancers within SDN.

Networking devices 

Networking devices receive instructions from the controllers regarding how to route the packets. 

Open-source technologies 

Programmable networking protocols, such as OpenFlow, direct traffic among network devices in an SDN network. The Open Networking Foundation (ONF) helped to standardize the OpenFlow protocol and other open source SDN technologies.

By combining these components, organizations get a simpler, centralized way to manage networks. SDN strips away the routing and packet forwarding functions, known as the control plane, from the data plane or underlying infrastructure. SDN then implements controllers, considered the brain of the SDN network, and layers them above the network hardware in the cloud or on-premises. This lets teams use policy-based management—a kind of automation—to manage network control directly.

SDN controllers tell switches where to send packets. In some cases, virtual switches embedded in software or hardware replace the physical switches. This consolidates their functions into a single, intelligent switch that can check data packets and their virtual machine destinations to ensure there are no issues before moving packets along.

The term “virtual network” is sometimes erroneously used to mean “SDN.” These two concepts are distinct, but they do work well together.

Network functions virtualization (NFV) segments one or many logical or virtual networks within a single physical network. NFV can also connect devices on different networks to create a single virtual network, often including virtual machines. 

SDN works well with NFV; it assists NFV by refining the process of controlling data packet routing through a centralized server, improving visibility and control.

There are four primary types of software-defined networking:

Open SDN

Open protocols are used to control the virtual and physical devices responsible for routing the data packets. Open SDN enables various teams of network operators, developers and vendors to work together on optimization.

API SDN

Through programming interfaces, often called southbound APIs, organizations control the flow of data to and from each device. API SDN enables orchestration platforms, cloud management tools and network management systems to integrate with SDN infrastructure.     

Overlay Model SDN

Virtual networks run above existing hardware, creating tunnels with channels to both remote and on-premises data centers. This model then allocates bandwidth and assigns devices to each channel. 

Hybrid Model SDN

By combining SDN and traditional networking, the hybrid model assigns the optimal protocol for each type of traffic. Hybrid SDN is often used as an incremental approach to SDN, enabling enterprises to integrate SDN into legacy environments.

SDN architecture comes with many advantages, largely due to the centralization of network control and management. These benefits include:

Ease of network control

Separating the packet forwarding functions from the data plane enables direct programming and simpler network control. This includes configuring network services in real time, such as ethernet and firewalls, or quickly allocating virtual network resources to change the network infrastructure through one centralized location.

Agility

Because SDN enables dynamic load balancing to manage the traffic flow as need and usage fluctuates, it reduces latency and increases the efficiency of the network.

Flexibility

With a software-based control layer, network operators have more flexibility to control the network, change configuration settings, provision resources and increase network capacity. 

Greater control over network security

SDN lets network administrators set policies from one central location to determine access control and security policies across the network by workload type or by network segments. You can also use micro-segmentation to reduce complexity and establish consistency across any cloud network architecture—whether it’s public cloud, private cloud, hybrid cloud or multicloud.

Simplified network design and operation  

Administrators can use a single protocol to communicate with a wide range of hardware devices through a central controller. It also offers more flexibility in choosing networking equipment, since organizations often prefer to use open controllers rather than vendor-specific devices and protocols.

Modernizing telecommunications

SDN technology combined with virtual machines and network virtualization lets service providers offer distinct network separation and control to customers. This helps service providers improve their scalability and provide bandwidth on demand to customers who need greater flexibility and have variable bandwidth usage. 

SDN solutions come with significant benefits but can pose a risk if not implemented correctly. The controller is critical in maintaining a secure network. It is centralized and, therefore, a potential single point of failure. This potential vulnerability can be mitigated by implementing controller redundancy on the network with automatic failover. This may be costly but is no different from creating redundancy in other areas of the network to ensure business continuity.

Service providers and organizations alike can benefit from a software-defined wide area network or SD-WAN. A traditional WAN (wide-area network) is used to connect users to applications hosted on an organization’s servers in a data center. Typically, multiprotocol label switching (MPLS) circuits are used to route traffic along the shortest path to help ensure reliability. 

As an alternative, an SD-WAN is programmatically configured and provides a centralized management function for any cloud, on-premises or hybrid network topology in a wide area network. An SD-WAN can handle massive amounts of traffic and multiple types of connectivity, including SDN, virtual private networks, MPLS and others.