Networks - Underlay Network

An underlay network in the context of networking refers to the physical infrastructure that forms the foundation over which overlay networks are built. This underlying network consists of the actual routing, switches, cabling, and other hardware that facilitate the transport of data.

The underlay network delivers the physical infrastructure, which is comprised of the physical components like routers, switches, cables, and wireless access points upon which the overlay network is built. The underlay network is responsible for the encapsulation and the actual transport of packets across the network. It handles the basic networking tasks such as packet forwarding based on IPv4 or IPv6 addresses and routing protocols.

Overlay networks depend on the underlay network to function. While overlay networks can implement advanced networking features and virtualization, they still rely on the physical infrastructure of the underlay network to transmit data. The underlay network requires traditional network management and maintenance, including physical repairs, upgrades, and monitoring for performance and faults. The underlay network uses standard routing protocols (like BGP, OSPF) and technologies inherent to physical networking.

The underlay network is the foundational physical network that supports all network activities. Its performance and capabilities are critical, as they set the baseline on which overlay networks and virtualized services operate.

Examples of technologies that rely on an underlay network to operate include VPNs, VXLAN, OTV, MPLS, and SD-WAN among others. Underlay networks are also a necessary foundational component of network designs that require the spanning of Layer 2 networks across remote sites.

The network that is built on top of the underlay network is called the overlay network.