" Congestion within switches incurred by hotspots in the network can cause catastrophic drop-off in overall bandwidth

Conventional large-scale Ethernet deployments have relied upon three-tier architectures of switch, distribution (or aggregation) and core deployments in order to control particular network operations. Such designs have inhibited scalability due to systemic constraints in the architecture: Network resources soon become over-committed, especially in the presence of device-level (east-west) communication.

Such constraints have necessitated a move away from a three-tier model to a based on leaf-switches, providing access to devices (storage and server), and spine-switches, creating a rich multi-path fabric in which potentially all the available bandwidth can be used to sustain device level communication irrespective of the location of these devices.

In practice however, such networks cannot deliver complete isotropy due to the inability to manage congestion as transmission and receive-flows change rapidly in operation. Typically, congestion within the network is formed through either egress port buffering, whereby the volume of traffic attempting to access an attached device is greater than the available bandwidth over the given egress interface, or within the network, when the aggregate traffic volume taking a particular path is greater than the available bandwidth on that path.