In network architectures, how are underlay and overlay related?

• A. The overlay is the physical IP network; the underlay is a virtual network carried on top.
• B. Underlay is physical IP network; overlay is a virtual network (VXLAN/GRE) carried on top.
• C. The underlay and overlay refer to data center power distribution and cooling.
• D. The overlay replaces the underlay in modern networks.

Answer: (B)

The fundamental idea is that the underlay provides the physical transport, while the overlay provides a virtual network on top of that transport. The underlay is the actual physical IP network—the cables, switches, routers, and the routing that moves packets between devices. On top of this fabric runs the overlay, which creates virtual networks that span the underlay using encapsulation such as VXLAN or GRE. This lets you build logical L2/L3 networks that can stretch across the entire data center, enabling multi‑tenancy and scalable network segmentation without needing a separate physical network for each tenant. The underlay handles the real forwarding and path reliability, while the overlay defines the virtual topology and tenant isolation, independent of the physical wiring. The other options blur or reverse these roles or bring in unrelated concepts like power and cooling, which aren’t part of how underlay and overlay relate.