SD-WAN Interview Questions and Answers (With Explanations)

Interview panels love SD-WAN questions because they reveal whether a candidate actually understands why SD-WAN exists, not just the buzzwords. This guide walks through the questions that come up most often, from the basics to more scenario-based ones, with answers written the way you’d actually explain them in an interview room.

Basic SD-WAN Interview Questions

1. What is SD-WAN and why was it introduced?

SD-WAN stands for Software-Defined Wide Area Network. It separates the control plane (decision-making) from the data plane (traffic forwarding), and lets a central controller manage routing policy across all branch sites.

It was introduced because traditional WANs, built mostly on MPLS, were expensive, slow to provision, and inefficient for cloud-heavy traffic. A new branch circuit could take weeks to activate, and all traffic — including simple internet browsing — had to backhaul through a central data center.

2. How is SD-WAN different from a traditional WAN?

Point	Traditional WAN	SD-WAN
Transport	Mostly MPLS, expensive	Any transport: MPLS, broadband, LTE/5G
Management	Box-by-box configuration	Centralized controller/orchestrator
Path selection	Static routing	Application-aware, dynamic path selection
Cloud traffic	Backhauled to data center	Can break out locally to the internet
Provisioning	Slow, manual	Fast, often zero-touch

3. What is Zero Touch Provisioning (ZTP) in SD-WAN?

ZTP lets a new SD-WAN device at a branch site connect to the internet, authenticate with the central controller, and pull its configuration automatically — without an engineer manually configuring it on site. This is one of SD-WAN’s biggest operational advantages over traditional WAN rollouts.

4. What are the typical SD-WAN architecture components?

• Edge device (CPE): sits at the branch, handles traffic forwarding
• Controller/Orchestrator: central brain that pushes policy to all edges
• Management console: where admins configure and monitor the fabric
• Overlay tunnels: encrypted tunnels between edge devices, usually over IPsec

Vendors name these differently — Cisco calls them vManage, vSmart and vBond; Fortinet, VMware and others use their own terms — but the concept is the same.

Intermediate SD-WAN Interview Questions

5. What is application-aware routing in SD-WAN?

Instead of routing every packet based only on destination IP, SD-WAN identifies the actual application (such as Zoom, Salesforce, or a VoIP call) and applies a routing policy specific to that application. For example, VoIP traffic can be sent over the lowest-latency link, while bulk file transfers go over a cheaper broadband link.

6. What is local internet breakout, and why does it matter?

Local breakout means sending internet-bound traffic (like SaaS or general browsing) directly out from the branch, instead of routing it back through a central data center first. This reduces latency and saves WAN bandwidth, which matters a lot since so much enterprise traffic now goes to cloud applications rather than internal servers.

7. What is the difference between underlay and overlay in SD-WAN?

The underlay is the actual physical transport — MPLS, broadband, LTE — that carries traffic between sites. The overlay is the virtual network SD-WAN builds on top of it, typically encrypted IPsec tunnels, so applications see one logical network regardless of which underlying transport is used.

8. How does SD-WAN handle link failover?

SD-WAN continuously monitors link quality using metrics like latency, jitter and packet loss. If the active link degrades beyond a set threshold, traffic is automatically steered to a healthier link, often within milliseconds for latency-sensitive traffic like voice and video. This is more granular than traditional routing failover, which typically reacts only to a link going fully down.

9. Can SD-WAN work over MPLS?

Yes. SD-WAN doesn’t replace MPLS by default, it works alongside it. A common design keeps MPLS for critical, latency-sensitive traffic while adding broadband or LTE links for less critical traffic, with SD-WAN deciding dynamically which link each type of traffic should use.

10. What routing protocols are commonly used with SD-WAN?

SD-WAN controllers often use BGP to exchange routes between sites and with the SD-WAN fabric itself, while individual sites may still run OSPF internally for LAN routing. Understanding both matters here — our OSPF vs BGP blog covers how each protocol works if you want a refresher before the interview.

Scenario-Based SD-WAN Interview Questions

11. A branch office has two internet links: a 100 Mbps broadband and a 20 Mbps LTE backup. How would you design the SD-WAN policy?

A reasonable approach: set broadband as the primary path for all traffic, with LTE reserved as a backup for internet breakout. Business-critical, real-time applications (VoIP, video conferencing) get priority queuing and the lowest-latency path available at any given time. Bulk or non-critical traffic (software updates, backups) can be scheduled or restricted to off-peak hours over the backup link to control LTE data costs.

12. How would you troubleshoot a branch site where voice calls keep dropping but the internet otherwise works fine?

Start by checking real-time path metrics (latency, jitter, packet loss) on the active link for that site, since voice is sensitive to even small degradation that wouldn’t be noticeable in general browsing. Confirm the SD-WAN policy is actually classifying the voice traffic correctly and steering it onto the best-performing link. If metrics look fine but calls still drop, check for interface errors, QoS misconfiguration, or an issue with the voice application itself rather than the network path.

13. Why might a company choose to keep some MPLS circuits even after deploying SD-WAN?

Some applications are genuinely latency-sensitive and benefit from a predictable, dedicated circuit rather than best-effort internet transport. Certain compliance or contractual requirements may also call for a private circuit. A hybrid design — MPLS for critical traffic, broadband/LTE for everything else — is common precisely because it balances cost savings with reliability where it matters most.

SD-WAN Vendor and Tooling Questions

14. Name a few SD-WAN vendors you’re aware of.

Cisco (Viptela-based SD-WAN), Fortinet Secure SD-WAN, VMware (VeloCloud), Palo Alto Prisma SD-WAN, and Versa Networks are commonly seen in the market. Each has its own controller architecture and terminology, but the underlying concepts — centralized control, application-aware routing, overlay tunnels — stay consistent across vendors.

15. Is SD-WAN the same as SASE?

No, though they’re related. SD-WAN focuses on WAN connectivity and path optimization. SASE (Secure Access Service Edge) is a broader framework that combines SD-WAN with cloud-delivered security functions like a firewall, secure web gateway and zero-trust access, all managed from the cloud. SD-WAN is often described as one building block within a SASE architecture, not a replacement for it.

Frequently Asked Questions

1. Is SD-WAN knowledge required for CCNA or CCNP?
SD-WAN isn’t a core CCNA topic, but it’s increasingly relevant at the CCNP Enterprise level and in real enterprise network roles. A solid grounding in routing and WAN concepts makes SD-WAN much easier to pick up.

2. Do I need to know a specific vendor’s SD-WAN product for interviews?
It depends on the job posting. Concepts matter more than vendor-specific commands at most interview stages, but if the job explicitly mentions a vendor (Cisco, Fortinet, VMware), it’s worth getting familiar with that platform’s terminology and console.

3. What’s the most common mistake candidates make in SD-WAN interviews?
Describing SD-WAN purely as “cheaper than MPLS” without explaining the architecture — the overlay/underlay separation, centralized control, and application-aware routing are what interviewers are actually listening for.

4. How much hands-on practice is needed before an SD-WAN interview?
Reading concepts helps with theory questions, but scenario-based questions are much easier to answer confidently after lab practice — configuring policies, simulating link failures, and observing how traffic gets steered.

Conclusion

SD-WAN interview questions tend to reward candidates who understand the “why” behind the technology, not just definitions. If you want structured, hands-on SD-WAN training that covers architecture, policy design and real troubleshooting scenarios, check out the SD-WAN course at PM Networking.