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How to compare SD WAN underlay service providers

Written by Robert Sturt

Robert Sturt is Managing Director of Netify, an SD-WAN, SASE security & connectivity market network where you can login free to compare and shortlist vendors.

icon-cloud@2xWhat is the difference between SD WAN overlay vs underlay?

SD WAN overlay brings resources together using intelligent routing and path selection. The underlay is the connectivity transport upon which the overlay aspect delivers seamless WAN connectivity. In many cases, network underlay connectivity often consists of a hybrid of Internet, MPLS and dedicated fibre connectivity across Ethernet, Broadband, 4G and 5G.

Selecting an SD WAN vendor is now one of the key priorities for IT teams, making the transition from MPLS VPN or re-evaluating their working practices. The dizzying amount of solution features and benefits associated with Software WAN market often removes the focus on evaluating the underlay connectivity service. With the need to access resources and SaaS applications which reside within public cloud, the underlying connectivity should form a major component of your WAN architecture deployment.

There are two paths IT teams move through when evaluating SD WAN overlay technology and the underlay network.

  • Follow the true SD WAN path of using the best-fit Internet connectivity vs your location, i.e. lowest cost, proximity to the providers PoP and associated individual support levels.
  • Single backbone, similar to legacy MPLS where traffic follows one network path end to end creating the best possible performance.

In point 1, I mention the 'true SD WAN path' is based on using the Internet as an actual concept when evaluating suitability. The reasons make sense in many ways as your traffic should be free to transit your WAN across the best possible path per destination address.

One of the main features of SD WAN overlay technology is Cloud Path Selection which will evaluate the best possible application flows to Cloud resources wherever the user is located. In addition to location, multiple connections are constantly analysed to ensure the best possible underlay route is used based on latency, bandwidth and throughput figures.

In the days of MPLS, the WAN link topology was typically between head office and branch-office locations. With this in mind, creating a single backbone architecture makes sense to achieve consistent levels of application performance. When dealing with straight-forward HQ to branch requirements, there is no need to leverage the routing benefits of the Internet.

One of the reasons for the demise of MPLS is because the technology cannot meet the inherent challenges associated with delivering hybrid network connectivity today vs only a few years back which is largely due to the home working revolution across public Cloud adoption. In many ways, the Internet is responsible for the new working model as 4G and Superfast Broadband offer flexible user access some connections performing better vs the user's static office Ethernet environment. With resources typically located within the public cloud (rather than a specific location) the Internet is the obvious and clear optimal access method, And, this very reason is one of the main leading contributors for SD WAN adoption vs the reduction of MPLS VPN deployments.

Certain vendors are offering local VPN access into private backbone networks and data centres. In short, this architecture delivers something comparable to MPLS because your traffic is sent over a single, well scaled and engineered backbone. The industry views on SD WAN with private backbone access are somewhat polarising; SD WAN leaders offering the 'pure Internet access model' believe using a private backbone removes the benefit of SD WAN path selection and optimisation.

Software-WAN is bringing service assurance across WAN services because the complexity of the underlay is all but hidden to the IT team. Where issues and problems occurred in the past, the fault was usually related to connectivity which may have caused a loss of revenue due to protracted fix times.

We no longer need to rely on a single service provider because SD WAN overlay brings the intelligence to dynamically select from multiple connectivity types from Ethernet to Broadband with the capability to prioritise mission-critical applications. And while the fault may take time to fix, policies are deployed to maintain underlay connectivity while the fault is being addressed.

In certain business sectors, private cloud infrastructure/data centre is adopted to add an additional layer of privacy and security but the majority of businesses are adopting public cloud as their main strategy.

Single or multi-ISP strategy for your SD WAN connectivity?

By following the true SD WAN path, one of the benefits is cost savings because your IT team is positioned to evaluate the lowest cost provider vs your address and SLA requirements. However, thought must be given to whether or not a single ISP backbone will deliver better overall network performance, service capabilities and support.

Again, it is important to evaluate the benefits of both options for your business. While SD WAN is the enabler to connecting any ISP connection, multinational Enterprise business should consider and weigh up multiple and single provider underlay.

Multiple-ISP SD WAN

  1. Underlay latency and jitter - difficult to ascertain using a multi-provider strategy since IP traffic will transit multiple ISP backbones.
  2. SLA - the Enterprise cannot leverage a single SLA across multiple providers.
  3. Service - support can be difficult when involving numerous points of contact, there are options here such as single point of contact ISP management.
  4. Diversity - low-cost second circuit can be added, but there are pro guarantee of diversity.

Single-ISP SD WAN Underlay

  • Underlay latency and jitter - the performance of your network is typically more predictable because your traffic is not transiting multiple AS backbones.
  • SLA - the provider will deliver a single SLA covering network performance and uptime.
  • Service - support is made simpler since your team is interfacing with one contact.
  • Diversity - adding a second circuit with no single point of failure is only possible when using a single underlay provider

How to identify providers which fit your post/ZIP code?

The Netify pricing tool is available to research up to 20 sites online (more via Spreadsheet) via the following link. Our research team is positioned to ascertain which service providers are within reach of your office address including availability for Ethernet, 4G, 5G and Broadband.

What types of underlay connectivity are available for SD WAN?

  1. 100Mbps/1Gbps/10Gbps Ethernet
  2. FTTC (Fibre to the Cabinet) Broadband
  3. FTTP (Fibre to the Premises) Broadband
  4. 4G 5G


Robert Sturt Last Updated: 25.08.2020

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