Switch
Beginner
A hub chip that can connect any chip plugged into it to any other, so they don’t each need wires to everyone.
Novice
A chip with many ports that forwards traffic from any input port to any output port. Connecting every accelerator to switches gives every pair the same bandwidth and distance.
Expert
In scale-up, usually a single-stage, high-radix crossbar per plane, with each accelerator spreading its links across several parallel switch planes. Adds a hop of latency, power and cost, and caps one-level domains at the switch radix; can also do in-network reduction and multicast.