ECMP (equal-cost multipath)
Beginner
When there are several equally short routes, the switch picks one for each conversation by a fixed rule. That spreads the traffic out.
Novice
Equal-cost multipath routing: a switch with several equally good next hops hashes fields of each packet’s header (addresses and ports) to pick one. All packets of a flow hash the same way, so they stay in order on one path.
Expert
Static per-flow hashing over the 5-tuple (or a dedicated entropy field). Balanced only in expectation: with few large flows, collisions leave some links doubled up and others idle, and repeated hashing at each tier can correlate badly (hash polarization).
Explained in Scale-out networking (Systems).
See also: Elephant flow, Packet spraying.