An underlay network is the underlying physical infrastructure of a network typically built from a number of routers and switches with a layer 3 routing protocol. When building an underlay foundation for a SDA network the recommendation is to implement a routed access model. This prevents the need for protocols such as STP and HSRP. The network is made simpler while optimising reconvergence and all available links via non-blocking ECMP (equal cost multipathing).
Routed access = No layer 2 links = no STP = No reconvergence and use of all available links.
DNA-C is able to automatically discover, provision, and deploy your network underlay from scratch using best practices from Cisco's Validated Design. It uses IS-IS or OSPF for routing. This is known as 'LAN Automation'.