Underlay vs Overlay need to understand

Forum|Forum|11 months ago
July 1, 2025
6 replies
3686 views

Dear All,

Anybody can explain in laymon term what is under lay and over lay in SDWAN concept and how does it work.

Why under lay and over lay need.

Thank you in advanced for sharing the knowledge.

FortiGate

sjoshi

Staff

Hi @Umesh

In SD-WAN, the underlay refers to the actual physical network connections like MPLS, broadband, or 4G/5G that carry the data between locations, while the overlay is the virtual, software-defined network built on top of the underlay that creates secure tunnels and intelligently manages how traffic is routed across those physical links.

Thanks, Salon

kaman

Staff

Hi Umesh,

The underlay is the physical network infrastructure itself — the actual cables, routers, internet links, and circuits that carry your data.

It includes all your WAN connections like MPLS, broadband internet, LTE, etc. The underlay just moves packets from point A to point B, without any intelligence about the applications or services running on top.

The overlay is a virtual network layer built on top of the underlay.

It creates logical tunnels or connections (like VPN tunnels) between sites over the physical underlay.

FortiGate’s SD-WAN feature monitors each WAN link (underlay) for performance (latency, jitter, packet loss).

If you have found a solution, please like and accept it to make it easily accessible to others.

Regards,
Aman

Toshi_Esumi

SuperUser

Tricky thing is if you add site-to-site IPsec VPNs to SD-WAN members, those are also considered as underlay. They're equal, from SD-WAN's view, to other physical interfaces in the members.

Toshi

F

finmark2

New Member

Hi @Umeshmyhtspace
Underlay vs Overlay need to understand
Dear All,
Anybody can explain in laymon term what is under lay and over lay in SDWAN concept and how does it work.
Why under lay and over lay need.
Thank you in advanced for sharing the knowledge

Underlay is the physical network (internet/MPLS) that carries traffic, while overlay is the virtual SD-WAN layer that securely routes and controls how data travels over it.We need both because underlay provides connectivity, and overlay adds intelligence like routing, encryption, and performance optimization on top.