Hack the Route Start free Open app Switches and VLANs
Underlay and overlay
The physical routed network (underlay) and the virtual networks built on top of it with tunnels (overlay).
The underlay moves packets between devices' own addresses: links, routing (OSPF or BGP), ECMP. The overlay is virtual networks made by tunnelling between endpoints over it: VXLAN segments, GRE or IPsec tunnels, an SD-WAN. Overlay endpoints only need to reach each other; the underlay only needs to route their outer packets, and neither knows the other's addresses. A fabric is the whole system run as one: the underlay, the overlays, and usually a controller or EVPN that sets them up.