# What is a Software-Defined Wide Area Network (SD-WAN)?

> A software-defined wide area network (SD-WAN) uses centrally defined policy and software-controlled edge functions to connect sites, data centers, cloud environments, and remote locations across one or more WAN transports.

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## Content

It typically creates an overlay across underlays such as broadband, cellular, Carrier Ethernet, or Multiprotocol Label Switching (MPLS), then selects paths according to application and network conditions.

SD-WAN edges classify traffic, establish overlay connectivity, measure paths, and apply forwarding policies. A controller or orchestrator distributes configuration and policy while forwarding devices move traffic locally. Implementations differ in topology, interoperability, encryption, security inspection, and operating responsibility.

### Key points

- **Path control:** Edges can classify applications, measure paths, select routes by policy, fail over, and perform traffic engineering across available underlays.
- **Operations:** Central policy distribution and zero-touch provisioning can make geographically distributed sites easier to configure consistently and observe.
- **Service scope:** Some offerings add encryption or security inspection, but buyers must verify those functions and where they are enforced; they are not inherent to every SD-WAN.
- **Important limitation:** Multiple links do not guarantee independent failure paths or predictable end-to-end performance. Shared carriers and physical routes can create common points of failure, and centralized management or edge compromise can propagate incorrect policy across many sites.

### Related terms

[Wide area network (WAN)](<https://yellowcube.eu/glossary/wide-area-network/>) · [Multiprotocol Label Switching (MPLS)](<https://yellowcube.eu/glossary/multiprotocol-label-switching/>) · [Quality of service (QoS)](<https://yellowcube.eu/glossary/quality-of-service/>) · [Secure access service edge (SASE)](<https://yellowcube.eu/glossary/secure-access-service-edge/>) · [Software-defined networking (SDN)](<https://yellowcube.eu/glossary/software-defined-networking/>) · [Branch networking](<https://yellowcube.eu/glossary/branch-networking/>) · [WAN aggregation](<https://yellowcube.eu/glossary/wan-aggregation/>)

### Sources

[Mplify Alliance (formerly MEF), MEF 70.2: SD-WAN Service Attributes and Service Framework](https://www.mplify.net/resources/mef-70-2-sd-wan-service-attributes-and-service-framework/) · [NIST SP 800-215: Guide to a Secure Enterprise Network Landscape](https://csrc.nist.gov/pubs/sp/800/215/final)

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