With the increased focus on massive data transfers and instantaneous data travel in the network, the aging three-tier design within a data center is being replaced with what is being called the Leaf-Spine design. It is also referred to as leaf and spine topology, in this design there are switches found at the top of each rack that connect to the servers in the rack,with a server connecting into each switch for redundancy. People refer to this as a top-of-rack (ToR) design because the switches physically reside at the top of the rack.
The Leaf layer consists of access switches that connect to devices like servers, firewalls, load balancers, and edge routers. The Spine layer (made up of switches that perform routing) is the backbone of the network, where every Leaf switch is interconnected with each and every Spine switch.
With Leaf-Spine, the network uses Layer 2 and Layer 3 switching.
All routes are configured in an active state through the use of Equal-Cost Multipathing (ECMP). This allows all connections to be utilized at the same time while still remaining stable and avoiding loops within the network.
With traditional Layer 2 switching protocols like Spanning Tree on three-tiered networks, it must be configured on all devices correctly and all of the assumptions that Spanning Tree Protocol (STP) relies on must be taken into account.
The removal of STP between the Access and Aggregation layers using Layer 3 switching results in a much more stable environment.