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What is Software-Defined Networking?

Jan. 15, 2019
Chris Olsen

As organizations migrate to all-digital solutions in areas like cloud, mobility, and Internet of Things (IoT), the strength of the network hosting these technologies will be more critical than ever. The traditional network engineering model of configuring one device at a time simply doesn’t scale to the growing needs of today’s network.

Software-defined networking (SDN) is one solution that will enable your organization to meet the demands of network programmability and automation. The result will be greater network agility to support new applications while complying with increasing security needs.

SDN is a new approach to the network infrastructure and protocols that make networks more agile, add simplicity to the networking protocols and allow bandwidth to scale exceptionally high.

The defining characteristics of SDN are two types of protocols—underlay and overlay—that work together as a powerful team. Simply put, an underlay is a protocol that runs lower on the OSI model, typically right above Ethernet. An overlay is a protocol that runs higher in the OSI model, analogous to an application running on UDP.



The most popular overlay is Virtual Extensible Local Area Network (VXLAN). While a VXLAN extends a layer 2 network like a VLAN always has, VXLAN improves on a VLAN in several ways. First, the total number of VLANs is a 12 bit field or 4,096. The VXLAN standard allows for 24 bits, or 16,777,216 total VNIs. A VNI is called the VXLAN network identifier.

While VLANs stop at a router, VXLAN is a tunneling protocol that can go over routers, allowing VXLAN to extend a layer 2 subnet within or to another data center. VXLANs have no distance limitations and VXLANs never block a port like spanning tree does.

Another example of an overlay is Network Virtualization using Generic Routing Encapsulation (NVGRE). Like VXLAN, NVGRE is also a tunneling protocol.



Two common underlays are Intermediate System to Intermediate System (IS-IS) and BGP Ethernet VPN (EVPN), both of which are standards.

The combination of the underlay and overlay completely replace the need for any of these protocols within the SDN fabric:

  1. Spanning Tree
  2. Routing Protocols
  3. Static routes
  4. Port Channels
  5. Virtual Port Channels
  6. FabricPath
  7. Trill

The overlay and underlay team creates equal-cost multipathing (ECMP), inherently allowing the infrastructure to scale to exceptionally high bandwidth.

SDN solutions usually contain a centralized server for networking intelligence called an SDN controller. When a user connects to the controller over the network, they are using the northbound interface. The controller does all of the work for all of the network devices with the southbound interface. There typically is nothing to configure or install in the southbound interface.


Declarative programming

A powerful approach to adding power to network programming is declarative programming, which is an alternate to imperative programming. Imperative programming uses direct statements that change a state—basically, what you do is what gets done. As an example, the Cisco IOS command-line interface (CLI) is based on imperative programming. When you configure an interface, it reflects that exact configuration, no more and no less. Imperative programming is simple, but not powerful.

Declarative programming is a way to express logic or intent without describing the details of what is being done. While imperative programming is direct, it does not scale efficiently to many devices. To illustrate a point, who could give an argument that the CLI is the most efficient mechanism to make changes to many different network devices? Declarative programming solves this scale issue by allowing a network engineer to declare the intent of a network configuration change to a centralized device called a controller, which is the northbound protocol. Then the controller does the configuration to all of the devices on its own. This is the southbound SDN protocol.

The networking industry has spoken, and the ability to intend changes to multiple devices at the same time is far more valuable than the traditional manual configuration paradigm of one network device at a time. Software-defined networking solutions are based on declarative programming.


About the author

Chris Olsen has been an IT trainer since 1993 and an independent consultant and technical writer since 1996. He has taught over 60 different IT, data center and telephony classes to over 15,000 students. He is a technical editor for Global Knowledge’s lab manuals and has published three books with Cisco Press, CIPT part 2 version 6 and 8 and CCNA Voice Flash Cards. He is an author and technical editor for both Microsoft OCS 2007 and 2007 R2 certification exams. He is a technical author for Cisco-certified courses. He has also authored technical exams for Cisco’s certification program. Mr. Olsen can be reached at

Learn more

Learn more about software-defined networking and Cisco programming.

Cisco Software-Defined Networking
Cisco Application Centric Infrastructure (ACI)
Cisco Digital Network Architecture (DNA)
Cisco Software-Defined Access (SD-Access)
Cisco Software-Defined WAN (SD-WAN)
Cisco Network Programmability

Visit or call 1-800-COURSES (1-800-268-7737) to speak with a Global Knowledge training advisor.

Related resource

White paper: Cisco’s Intent-Based Networking and the Journey to Software-Defined Networks