There are a number of drivers that favor moving away from the common three and four-tier data center networks with Layer 2 and Layer 3 switching, toward a network based on Layer 2 with just edge and core switches (Get it?… ‘See what we mean by “flat?”).
Drivers for Flat Networks
One of the primary drivers for this change is virtualization. Virtualized environments, and clouds, benefit from the fluidity with which virtual machines can move across the network (and across data centers). Some common virtualization technologies are limited though, to Layer 2 environments within a common VLAN. The presence of a Layer 3 hop ads a great deal of complexity in this regard.
One reason we’ve come to know and love the three and four tier networks common in data centers is that we were forced into this architecture by product capabilities, namely port speed and port density. We’ve learned to build a hierarchy of switching because of the building blocks we had in terms of core switches, access switches, TOR switches, et. al. We must admit though, that driving packets up and down these trees of switches comes with complexity and cost.