ECMP
Equal-Cost Multi-Path — when several paths to the same destination have the same cost, the router spreads traffic across all of them at once. The foundation for load balancing and bandwidth scaling in modern data centers (spine-leaf/Clos topologies).
Mechanic: the router hashes the packet's flow key (source/dest IP + ports) → picks one of the equal-cost paths, so all packets in the same flow take the same path (avoiding reordering) but different flows spread out. Win: use all the parallel bandwidth between switches without a single bottleneck link. Per-flow hashing can give uneven distribution with a few large "elephant flows". Central to data-center fabrics where each leaf has several uplinks to the spine layer. Related: LACP (link aggregation at L2), flowlet switching (finer splitting). Improved ECMP hashing is a recurring optimization area.