Fattree

Fat-tree, sometimes written fattree, is a network topology used in data centers. It is a form of Clos network designed to provide high aggregate bandwidth and scalable switching fabric by making bandwidth increase toward the root. In a common instantiation, called a k-ary fat-tree, each switch has k ports and the network is built from several layers of switches organized into pods consisting of edge and aggregation switches, plus a core layer. Each edge switch connects to a number of hosts and to several aggregation switches; each aggregation switch connects upward to core switches as well as to edge switches within its pod. The core layer contains (k/2) x (k/2) core switches, with each core switch connecting to one aggregation switch in every pod. The design yields k^3/4 hosts in total for even k, and provides multiple equal-cost paths between any pair of hosts.

Routing in fat-trees commonly uses ECMP to spread traffic across many parallel paths, helping achieve high bisection bandwidth and fault tolerance. The topology is well suited to data centers that require scalable bandwidth with commodity switches, because capacity grows with the number of switches and ports rather than being bottlenecked at a single layer.

Advantages include high aggregate bandwidth, scalability, load balancing across many paths, and tolerance to failures; disadvantages include complex cabling and management, and the need for careful traffic engineering and routing configuration. Fat-tree variants and related Clos-based designs have influenced modern data center networks, often implemented with software-defined networking overlays or equal-cost multipath routing.