Which campus design supports migrating from a traditional campus to a programmable fabric?

A routed access design is the most suitable for migrating to a programmable fabric like Cisco SD-Access. In a routed access design, Layer 3 routing extends to the access layer, meaning each access switch acts as a Layer 3 device. This aligns well with the underlay requirements of SD-Access, which typically uses an IP-based underlay for VXLAN encapsulation. This design simplifies the integration of fabric nodes and reduces the complexity of extending the fabric. A two-tier or three-tier design, while common, doesn't inherently imply a routed access layer and often relies on Layer 2 at the access, which complicates fabric integration. Layer 2 access, by definition, uses Layer 2 at the access layer, requiring additional configuration or redesign to support a Layer 3 underlay for fabric solutions.