An Underlay Network is the physical IP-routed infrastructure of switches, links, and routing protocols that provides basic reachability across a data center fabric. It transports the encapsulated packets of any overlay running on top of it. In modern fabrics the underlay is typically a simple, stable Layer 3 leaf-spine network using ECMP and a routing protocol such as BGP or OSPF.
Category: Network & Connectivity · Subcategory: Data Center Network Architecture · Part of the AIDC Technical Terminology reference.