The SD-Access underlay must run a routing protocol to establish IP connectivity between infrastructure devices. Cisco SD-Access uses a routed underlay to provide basic reachability among fabric edge nodes, border nodes, control-plane nodes, and intermediate nodes. Cisco design documentation states that all network elements in the fabric underlay must have IP connectivity through routing. Catalyst Center LAN Automation commonly deploys IS-IS for this purpose, although the underlay can be built manually with another supported routing design. The user endpoints and client subnets are not part of the underlay; they belong to the overlay, where fabric services, segmentation, and policy are applied. CEF should not be disabled, because efficient IP forwarding is necessary for the routed underlay. Two or more switches are not required merely to avoid spanning-tree loops; the SD-Access underlay is normally Layer 3 routed and avoids large Layer 2 loop domains. Reference topics: Cisco SD-Access underlay, routed access, LAN Automation, IS-IS, fabric node reachability.