The classic core, aggregation, and access (3-tier) topology has been commonplace in most enterprises and service providers for many years and provides a scalable modular architecture for networking services.

 

Applications which required a Layer 2 adjacency had to be connected within the same Pod, because each Pod was separated by a Layer 3 wide-area network (WAN) connection. This was one of the primary reasons for the introduction for a leaf-spine fabric design.