Explanation/Reference:
In a modern Clos topology, every lower-tier switch is connected to each of the top-tier switches in a full- mesh topology. If there isn't any oversubscription taking place between the lower-tier switches and their uplinks, then a non-blocking architecture can be achieved. The advantage of the Clos network is you can use a set of identical and inexpensive devices to create the tree and gain high performance and resilience that would otherwise cost must more to construct. To prevent any one uplink path from being chosen, the path is randomly chosen so that the traffic load is evenly distributed between the top-tier switches. If one of the top tier switches were to fail, it only slightly degrades performance through the data center.
Reference: Clos Networks: What's Old Is New Again, What goes around, comes around - Clos Networks are back
http://www.networkworld.com/article/2226122/cisco-subnet/clos-networks--what-s-old-is-new-again.html