SD-WAN: Why this satnav for your data traffic is only as good as the network

Aerial View of Beijing Traffic Jam


  • SD-WAN adoption is growing as organisations seek better application performance and the ability to respond to changing demands on their WAN.
  • The excitement about SD-WAN tends to focus on the overlay because of what it enables network managers to do.
  • However, the overall performance depends on the underlay network to do its job. Correct design and connectivity choices will influence the effectiveness of SD-WAN.

Article content

This article is based on content from the Telstra Ultimate SD-WAN Guide – a comprehensive guide containing 40+ pages of handy tips, pitfalls to avoid, security risks to be aware of, lessons learned and more.

SD-WAN is surging in popularity among businesses and enterprises as they seek to boost productivity and optimise application performance across their wide area networks (WANs). SD-WAN also offers organisations the ability to respond faster to changing demands on their networks while making it easier to manage hybrid connectivity.

If SD-WAN delivers all these outcomes, why should we worry about the underlay (the network itself)?

One way of viewing the relationship between the SD-WAN overlay and the underlay is to think of another form of traffic.

SD-WAN guides data packets along transport paths in the same way satellite navigation (satnav) in your device would direct you to your destination when you’re driving. If there’s congestion on one road, the satnav will provide an alternative route.

However, the satnav can’t change a road from a single carriageway urban street complete with speedbumps into a six-lane freeway. Nor can it fix potholes or create new roads.

The same could be said for the SD-WAN overlay. How you design the network, including the underlying carriage, will significantly affect your outcomes.

There are four main factors to keep in mind when considering the underlay for your SD-WAN solution.

1. Right-size the bandwidth

Many companies have shifted their applications and data away from on-premises data centres to private or public clouds. Business-critical apps such as Microsoft O365 or Google Suite are destined for the cloud, while adoption of high-bandwidth video collaboration tools has increased exponentially. This shift has consequences for your network.

Your SD-WAN underlay network must be able to accommodate these data flows. WAN selection and bandwidth sizing must be reworked to get the most out of SD-WAN. This means existing WAN links will need to be right-sized for bandwidth, latency and resiliency.

2. Create redundant, resilient options

The resilience and performance of the underlying network is critical. A single link network with SD-WAN overlay will remain a precarious proposition for businesses because outages on the underlay network will result in outages on the overlay network. Similarly, deploying SD-WAN on a consumer-grade internet link will not deliver the business benefits. To extend our transport metaphor above, traffic that can only move along a single road will always be at risk of congestion.

A well-designed underlay network will leverage multiple redundancies to achieve high availability. The choice of network routes will also influence performance, making it vital to select vendors with network capabilities to suit your organisation’s needs.

3. Think mobile

Your underlay networks can use more than just fixed networks. Network architects can supplement fixed MPLS, Layer 2 and internet links with high speed mobile connectivity via 4G LTE (and 5G as vendors release CPEs to support the next-generation mobile standard).

Mobile networks provide useful back up options that can help achieve last mile diversity (something that is challenging in a multi-provider WAN solution) to increase site availability. They also enable organisations to serve sites where fixed networks may not be present or sufficiently performant.

Here’s the Telstra plug – if you need mobile as a backup, what better option than to have Australia’s Best Mobile Network on your side?

4. Remember: not all internet services are equal

Some internet services, particularly consumer-grade and some lower-end business-grade services, are shared by other customers.

Typical residential internet services can have a contention ratio – the number of users sharing the same network capacity – of anywhere from 30:1 and up [1]. Even with a contention ratio of 4:1 offered by many business grade plans, sharing capacity with other businesses means a spike in their traffic may affect your application performance. Again, we can draw a parallel with our roads: if you had the entire road to work to yourselves, your commute would be much shorter than when you share it with hundreds of others.

If the performance of your internet connectivity is important to you, consider using uncontended services such as Telstra Internet Direct. Uncontended services provide dedicated capacity for your needs, helping to achieve optimal performance at all times.


Keep traffic moving

Just as satnav is a useful tool for drivers to get from A to B, so SD-WAN is becoming an essential solution for network managers seeking to get the most from their wide area networks. As long as we take account of their limitations, they both help keep traffic moving in the right direction as efficiently as possible.

For more in-depth tips and insights about SD-WAN, download the Telstra Ultimate SD-WAN Guide or find out more about Telstra SD-WAN here.