Fat Tree
A multistage network topology that increases aggregate capacity toward upper levels to avoid the root bottleneck of a uniform tree.
The defining property of a fat-tree is not its visual tree shape; aggregate capacity should not collapse as traffic moves toward upper levels. In datacenters this is commonly built from many equal-capacity links combined with ECMP rather than physically wider links.
Oversubscription
Calling a topology a fat-tree does not automatically make it non-blocking. Uplink/downlink ratios, switch radix, ECMP distribution, and residual capacity after failures determine the real oversubscription.
Heavy east-west traffic can still create local hot spots when multiple flows hash onto the same path, so link-utilization distribution and incast behavior matter alongside aggregate throughput.
Source
- https://ieeexplore.ieee.org/document/6312192