The default MTU should be increased in the Cisco SD-Access fabric underlay. SD-Access data-plane traffic uses VXLAN encapsulation, and VXLAN adds overhead beyond the original endpoint frame. If the underlay remains at a standard 1500-byte MTU, encapsulated packets can fragment or be dropped, especially when applications set the DF bit. Cisco SD-Access design guidance therefore calls for an increased underlay MTU so the fabric can carry overlay-encapsulated traffic cleanly. Reducing subnets is an overlay simplification topic, not an underlay MTU consideration. The number of supported control-plane nodes is a scale and availability design consideration, not the key underlay requirement in this question. Unified policy is part of fabric policy and segmentation, not underlay transport readiness. A professional SD-Access underlay design validates routed reachability, loopback advertisements, IGP operation, redundant paths, and MTU end to end before enabling the overlay. The MTU setting is critical because a stable underlay must transport LISP control-plane traffic and VXLAN data-plane traffic without fragmentation. Reference topics: SD-Access underlay, VXLAN overhead, MTU design, fabric transport readiness.