Cisco ACI is a policy-driven CLOS or Spine/Leaf based switching fabric utilizing layer 3 ECMP routing in the underlay and VXLAN encapsulation in the overlay to transport layer two and layer three traffic East/West across the fabric and North/South in and out of the fabric. ACI consists of the Application Policy Infrastructure Controller (APIC), a centralized controller that manages all aspects of the ACI fabric. The leaf switches are ToR switches that provide connectivity between servers and external networks, and the spine switches are Layer 3 switches that provide ECMP high-bandwidth connectivity between leaf switches. An ACI fabric can be expanded East/West by adding leafs, cabling to the spines, and registering them. ACI was designed to operate as "One Big Switch" (like a chassis-based NEXUS 7K) with the controllers acting like the Supervisors, the spines as fabric modules, and leafs acting as blades. We can decouple these elements from the chassis and place them anywhere in the data center by taking this approach. The leafs (blades) can be placed anywhere in the data center, so you are not limited to a chassis.