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Sampling the Many Flavors of SD-WAN

Not everyone is fully aware that software-defined WAN (SD-WAN) solutions can be designed differently. Enterprises considering SD-WAN should understand how the design impacts total costs, network reliability, application performance, and service-level promises. The good news is that your choices are no longer limited to expensive MPLS networks or unreliable Internet-based VLANs.
Appliance-Centric Design
The first type of SD-WAN design is the appliance-based pure overlay, which uses an on-premises router. This unit often replaces the standard site router at smaller offices or is a software feature activated in an edge router. In this design, the SD-WAN router leverages whatever WAN connections are provisioned – it’s agnostic about the uplinks. The unit will provide traffic shaping, prioritizing real-time protocols and latency sensitive data, and will seek the best performance pathway among the WAN links.
In many cases, the SD-WAN appliance is sold as a way to move from expensive MPLS circuits to a combination of standard ISP circuits. However, the SD-WAN router allows a customer to choose the WAN connectivity, including leaving one link as MPLS while providing an alternate to improve reliability (failover) or to increase total bandwidth. Using a non-telco provider such as a cable provider or a 4G LTE wireless link can provide local loop protection.
It’s important to note that with each of these WAN link options there can be disadvantages. A design that leaves in the deterministic MPLS circuit is unlikely to save money, although the SD-WAN performance and failover advantages may be more important than simply costs. Using a combination of ISP links, however, may save money, but it’s reliant upon the open “best-effort” Internet, which cannot provide any assurance that the best available ISP link chosen by the SD-WAN router is going to always perform acceptably. The wireless 4G link may incur usage charges, even when it’s only from test packets, and the upstream handoff is usually the open Internet.
Cloud Gateway Design
To combat the performance concerns raised by relying upon standard Internet connections, some SD-WAN providers have established a cloud-based gateway for the on-premises router to use as the uplink. This design allows the provider to enhance overall performance by using peering arrangements and direct backbone connections to key cloud services such as AWS, Microsoft Azure,, etc.
Many of these providers also offer or link to real-time communications services, including voice and video, as part of the directly connected services. All of the previously described SD-WAN benefits and options remain in place, but now using standard Internet links (at a lower cost) does not carry as much risk of negative impact from uncontrolled Internet events.
A detailed example of how one vendor, Juniper Networks, has combined the on-site SD-WAN router with gateway services is explained in this No Jitter post by UC analyst Zeus Kerravala.
Carrier-Based SD-WAN
The third SD-WAN design is carrier-centric. The on-site SD-WAN appliance is not the key here, although some router functions still help control the client-side access and uplink choices. The router will connect the local loops to the nearest carrier point-of-presence, where the carrier can separate out the standard Internet traffic. All corporate and critical traffic is delivered across the carrier-backbone, which can provide low levels of latency, packet loss, and jitter. Like the gateway design above, the carrier backbone is also directly connected with major cloud application providers to increase the performance and reliability of those applications.
Although some enterprises may feel carrier diversity has its benefits, it’s also common for some customers to stay with their existing carrier to help with the technology migration or to enable a smooth contract transition. Costs may be higher, but better service-level agreements are often available to support the performance promises.
Which Design Is Best?
Of course, no single design implementation is right for everyone. The best approach is to understand the different options and to engage with different supplier choices before making a commitment. Also keep in mind that even with similar designs, there will be differences in service offerings from the various providers.
Lessons-learned from peers sharing their experiences can be helpful as well. Join me at Enterprise Connect Orlando, for my session, “SD-WAN Case Studies,” taking place on Monday, March 18 at 2:00 p.m. You’ll hear from five enterprise IT executives who have overseen SD-WAN implementations at their organizations and have advice for you to take back to your own enterprise.
Enterprise Connect is coming to Orlando, Fla., March 18 to 21. If you don’t have your pass yet, there’s still time to get in on the action! Register now with code NJPOSTS to save an extra $200 off your conference pass or get a free Expo Plus pass!
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"SCTC Perspectives" is written by members of the Society of Communications Technology Consultants, an international organization of independent information and communications technology professionals serving clients in all business sectors and government worldwide.