June 16, 2015. Technical Mobility Author: James Hankins

Software Defined Networking

Software Defined Networking (SDN) is currently a hot topic in the IT industry with vendors like VMware heavily promoting their NSX product. I am getting asked more and more from our customers about SDN, so I thought I would explore some of the basics and some of the differences between the various SDN solutions out there.

SDN is an approach to networking in which control is decoupled from the physical infrastructure, allowing network administrators to support a network fabric across multi-vendor equipment.

Social media, mobile devices, and cloud computing are pushing traditional networks to their limits. Compute and storage have benefited from incredible innovations in virtualisation and automation, but those benefits are constrained by limitations in the network. Administrators may spin up new compute and storage instances in minutes, only to be held up for weeks by rigid and often manual network operations.


Overlay and Underlay

SDN technologies are broadly split into two fundamentally different paradigms: “overlay” SDN and “underlay” SDN.

With overlay SDN, the SDN is implemented on top of an existing physical network. With underlay SDN, the fabric of the underlying network is reconfigured to provide the paths required to provide the inter-endpoint SDN connectivity. Overlay SDN (e.g., VMware NSX and Juniper Contrail) use tunnelling technologies such as VXLANSTT and GRE to create endpoints within the hypervisor’s virtual switches and rely on the existing network fabric to transport the encapsulated packets to the relevant endpoints using existing routing and switching protocols. One advantage of using encapsulation is that only the tunnelling protocol endpoint IP addresses (TPEP IPs) are visible in the core network. The IP addresses of the inter-communicating VMs are not exposed (of course the downside of this is that without specific VXLAN awareness, traffic sniffers, flow analysers etc. can only report on TPEP IP-IP conversations and not inter-VM flows). Another advantage of encapsulated overlay networks is that there is no need for tenant segregation within the core (e.g., using MPLS VPNs, 802.1q VLANs, VRFs, etc.) as segregation is implicitly enforced by the tunnelling protocol and the TPEPs.

Underlay SDN (OpenFlowCisco ACI, Juniper QFabricFabricPath, etc.) directly manipulate network component forwarding tables to create specific paths through the network. In other words, they intrinsically embed the end-to-end network paths within the network fabric. The SDN controller is responsible for directly manipulating network element configuration to ensure that the requirements presented at the controller’s northbound API are correctly orchestrated. With intimate knowledge of network topology, configured paths through the fabric and link-level metrics (e.g. bandwidth, latency and cost), much more efficient utilisation of network infrastructure can be achieved using more complex route packing algorithms, for example, sub-optimal routing. Another advantage of underlay SDN is that the controller dictates exactly where in the network each traffic flow traverses, which is invaluable for troubleshooting, impact analysis and security.


Benefits of SDN

  • Reduce CapEx: SDN potentially limits the need to purchase purpose-built ASIC-based networking hardware and instead supports pay-as-you-grow models.
  • Reduce OpEX: SDN enables algorithmic control of the network elements (such as hardware or software switches/routers) that are increasingly programmable, making it easier to design, deploy, manage and scale networks. The ability to automate provisioning and orchestration optimizes service availability and reliability by reducing overall management time and the chance for human error.
  • Deliver Agility and Flexibility: SDN helps organisations rapidly deploy new applications, services, and infrastructure to quickly meet changing business goals and objectives.
  • Enable Innovation: SDN enables organisations to create new types of applications, services and business models that can offer new revenue streams and more value from the network.


In Summary, SDN has the potential to revolutionise legacy data centres by providing a flexible way to control the network so it can function more like the virtualised versions of compute and storage today.

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