Final answer:
The collection of devices that connects to each fabric node in an IP network is known as the Underlay. It is the physical infrastructure of the network, as opposed to the virtual Overlay network or the network Fabric term that suggests an integrated topology of network devices.
Step-by-step explanation:
The collection of devices that comprises the IP network and connects to each fabric node is known as the Underlay. The underlay network is the physical infrastructure consisting of routers, switches, and other networking equipment that provides the foundational connectivity. In contrast, the Overlay is a virtual network built on top of the underlay, which allows for services like network virtualization and can help in segmenting the network. Fabric in networking terms usually refers to the integrated set of network switches and routers that are interconnected to form a network topology, often used to manage data flow in a data center. The northbound API, on the other hand, is an interface that allows communication from the SDN (Software-Defined Networking) controller towards applications or higher-level orchestration tools.