NEW YORK — The single biggest challenge enterprise IT organizations now face as they make the transition to software-defined wide area networks (SD-WANs) is determining to what degree business-class Internet connections can replace MPLS lines.

At a WAN Summit conference here, that debate played out as IT leaders, vendors, and service providers debated the merits of hybrid approaches to SD-WANs, spanning both MPLS and Internet connections.

The consensus is that hybrid SD-WANs will soon become commonplace across the enterprise. There are many scenarios where deploying an SD-WAN to replace MPLS makes economic sense. But outside of an SD-WAN deployed by a cloud service provider, service level agreements (SLAs) based on services provided over Internet connections are rare. As such, IT organizations need to carefully weigh the performance, security, and location requirements associated with any application.

IDC predicts that 70 percent of enterprise IT organizations will adopt SD-WAN in some form over the next 18 months. Sen Chokkan, senior manager for network engineering at VWR International, a provider of supplies for laboratories, said that VWR has been able to reduce its total WAN costs by 60 percent since it first adopted SD-WANs. But the existence of SD-WANs has not obviated VWR’s need for MPLS lines or other classes of networking technologies in the branch office.

Chokkan said VWR groups the activities occurring in its branch offices by type of function. The function determines what type of WAN service is needed.

“It’s very important to know what’s happening in each branch office,” Chokkan said.

In general, interest in SD-WANs is rising because they provide a network overlay that can be employed to centrally manage both MPLS and Internet connections. That hybrid approach to SD-WANs is needed because outside of some dedicated cloud computing deployments, there are no service providers willing to commit to an SLA for an application based on Internet connections.

“There are no application SLAs,” said Steve Garson, president of consulting firm SD-WAN Experts. “The SLAs are only at the network level.”

The exception is cloud service providers that have made available an SD-WAN as a service. Steve Woo, vice president of product for Velocloud, a provider of a cloud-based SD-WAN services, said that cloud service providers essentially aggregate responsibility for all the underlying services it employs.

“The cloud service provider provides the end-to-end service agreement,” said Woo.

But Colin Spence, vice president for the eastern region for BT Global Services noted that not all application workloads will be moving into the cloud.

“We don’t believe everything will be moving into the cloud,” said Spence. “Just like there are hybrid clouds there will be hybrid networks."

Chris Alberding, senior vice president for product for Global Capacity, added that while it’s clear there’s a major shift toward SD-WANs, an Internet connection is always going to be a tradeoff when it comes to performance, price, and location. In some instances, the Internet connection might provide more bandwidth. But in other instances, the packet losses might not be acceptable.

“It’s always going to come down to finding the right service for the right location,” said Alberding.

In fact, for all the excitement over SD-WANs, Samir Desai, director of product management for GTT, said IT organizations would do well to remember the importance of the physical network.

“The network underlay is always going to be the weakest link in the chain,” said Desai.