The underlay network in Cisco SD-Access provides the foundational connectivity and must support physical connectivity for endpoints, which is facilitated by the underlay switches. This is essential for the operation of the overlay network where the actual data plane traffic flows. References: Implementing and Operating Cisco Service Provider Network Core Technologies source book https://www.cisco.com/c/en/us/td/docs/solutions/CVD/Campus/cisco-sda-design-guide.html#Underlay