Arista brings the benefits of leaf-spine to routing
About a decade ago almost all data centers were built on a traditional three- (or sometimes more) tier architectures that used the spanning tree protocol (STP). That prevented routing loops but also deactivated all the backup links, which accounted for almost half the ports in large environments. This caused organizations to significantly overspend on their networks.
Leaf-spine networks, on the other hand, have only two tiers, are much flatter and use something called ECMP (equal cost multi-pathing). So all routes are active, creating a much more efficient network that more agile and costs less.
The traditional three-tier data center was designed to scale up, which was the key requirement in the client/server era. Leaf-spine is optimized for rapid scale out, which has become critical in data centers today, as more and more traffic is moving in an East-West direction.