The difference between SDN and SD-WAN – and why we care

The best way to think of SDN is that SDN is to data routing what a sat nav is to an A-Z roadmap; they both get you where you need to go, but one can react to real-time traffic conditions and the other cannot. Similarly, SDN can offer virtualised orchestration and deployment – a true cloud-based router or firewall – for all network functions.

Where SDN covers the broad spectrum of networking, applications, routing, virtualisation orchestration and implementation, SD-WAN is emerging as a more focused sibling. SD-WAN is becoming shorthand for those network elements outside of the core network, while SDN is the core. The core here could be the carrier or service provider core network, or it could refer to a local core network such as in a data centre or a LAN.

This means that as software defined networks emerge and are implemented we are likely to see hybrid models being employed, where carrier SDNs are used to present a network segmented at the branch to offer WAN and Internet over the same connection.

Part of the challenge when implementing SD-WANs is to utilise the benefits of these carrier-based SDNs whilst retaining the benefits and flexibility that comes from using multiple carriers, such as:

  • Additional resilience
  • Best value
  • Wider technology and access connection choice

This means that in practice, the difference between SDN and SD-WAN becomes starker as SDNs occupy silos within the core or locally in the premise, whilst service providers overlay SD-WAN to offer benefits such as:

  • Routing over the fastest available route
  • Reactively re-routing to avoid disruptions, even if only temporary or intermittent
  • Optimising applications by only sending core data elements over the WAN

Using a carrier’s SDN means that you can't provide the wide range of technology and access connectivity offered by using multiple providers, or the additional resilience and reliability of using two distinct and separate network providers. The type of SD-network you use will be dictated by a number of factors, including:

  • Legacy decisions and infrastructure
  • Contract expirations and upgrade options
  • Key drivers such as traffic management, security, ease of deployment and change
  • Desire for resilience and flexibility

As requirements are reviewed the network should evolve and adapt to meet those needs, as increasingly we will see that one-size definitely does not fit all.

In conclusion – do we care about SDN versus SD-WAN?

On the one hand the use of a single carrier SDN will meet the needs of many customers. In contrast SD-WANs using multiple carriers’ infrastructure will become more attractive, where resilience, flexibility and value are key. There is not, therefore a “right” or “wrong” choice – rather it should be driven by what requirements are driving the network design. The key, as ever, is to understand the pros and cons of the core-SDN versus multi-carrier SD-WAN approaches and make the best choice for your needs.