What Your SD-WAN Isn't Telling You

Your SD-WAN is constantly making decisions. It assesses path quality based on metrics like packet loss, latency, and jitter, and steers traffic for your most critical applications accordingly. For this, it is an indispensable technology. But have you ever paused to ask a fundamental question: Is the path it chooses truly the best one available, or just the best one it can see from its limited vantage point?

This distinction is significant. Placing unconditional faith in the automated logic of an SD-WAN, without the means to independently verify its decisions, can expose your organization to performance degradation that is difficult to diagnose and even harder to solve. The question is not whether the automation is valuable, but whether it is infallible.

The logic of the software-defined path

SD-WAN was a necessary evolution from traditional routing, which was largely path-aware but not application-aware. Protocols like BGP were designed to establish and maintain connectivity, but they lacked the native intelligence to understand that a VoIP call has different performance requirements than a file transfer. SD-WAN introduced application-aware routing, creating a virtual overlay network that can steer traffic based on customized policies and real-time performance measurements. This dynamic path selection, often leveraging multiple transport types from MPLS to broadband internet, promises to optimize user experience and reduce costs. For any application sensitive to network conditions, the SD-WAN controller can programmatically switch traffic to a better-performing link, a process that is miles ahead of static, manual routing.

When the intelligent route is sub-optimal

An SD-WAN's routing decision is a programmatic response to a set of conditions and rules. Its effectiveness is therefore entirely dependent on the quality of its inputs and the accuracy of its configuration. Misconfigurations are a frequent source of problems, from incorrectly defined application policies leading to traffic misclassification, to errors in setting up the underlying IPsec tunnels or BGP route maps that govern how the SD-WAN interacts with the wider campus or data center network. These mistakes can lead to the very performance degradation the system was meant to prevent.

The bigger challenge, however, lies in the visibility gap between the SD-WAN's logical overlay and the physical underlay networks it runs on. The overlay is a virtual construct of logical tunnels; the underlay is the array of physical circuits from different ISPs responsible for actually forwarding the packets. Your SD-WAN controller might see three potential paths to a SaaS application and choose the one with the lowest latency based on its own probe measurements. What it cannot see is why the other two paths have high latency. The issue could be BGP route flapping deep within a provider's network, a congested peering exchange between two autonomous systems, or a physical last-mile fiber issue.

The SD-WAN only sees the symptom—high latency—not the root cause. This can lead to a situation where the controller diligently flips traffic between two or three equally compromised links, unable to route around the actual problem because it lacks the necessary underlay visibility. All it knows is that its pre-defined SLA threshold has been breached, triggering a path change that might not actually resolve the user's issue.

Furthermore, the SD-WAN's authority and visibility typically end at its own edge devices. When a user reports that a critical cloud application is slow, the problem could be inside the cloud provider’s network, a DNS resolution failure, or an issue with the application server itself—all segments of the end-to-end path that are invisible to your SD-WAN. To trust its decisions in this context is to operate with a significant blind spot, one that can directly impact application performance.

From blind automation to informed confidence

This does not imply a return to manual routing. The goal is not to disable the automation but to equip it with an impartial co-pilot: comprehensive, end-to-end network intelligence. You must evolve from a position of blind trust in automation to one of informed confidence.

This requires augmenting your SD-WAN vendor's native tools with an independent tool that offers true end-to-end observability. It means correlating the performance of the SD-WAN overlay with the BGP routing and hop-by-hop path performance of the ISP underlays. When your SD-WAN controller decides to move traffic from ISP A to ISP B, you should be able to validate that decision with external data. Was it a good move? You need to see the entire path to know. Perhaps the switch avoided a local fiber problem, which is a win. Or, perhaps it moved traffic from a link with a transient BGP issue to one suffering from chronic congestion at a major peering point, solving nothing.

Without external validation, you are simply managing your own device configurations. With it, you can have data-driven conversations with your service providers, holding them accountable for the performance of the underlay networks you pay for.
SD-WAN is an essential component of the modern enterprise network, but automation without comprehensive, independent visibility is a recipe for frustration. Don't just program policies and trust your network to execute them flawlessly. Give yourself the power to see the entire service delivery path, validate that the automated decisions are the correct ones, and rapidly diagnose the root cause when they are not. That is how you move beyond simply managing a technology to truly engineering a resilient digital experience.

Moving from managing SD-WAN to engineering resilience requires the right tools. To learn how you can achieve this level of informed confidence, explore how to enhance your SD-WAN Observability.