Weighted ECMP
Distributing traffic across several paths in proportion to what they can actually carry, instead of splitting evenly between a hundred-gigabit link and a ten-gigabit one.
Ordinary ECMP hashes each flow into one of N equal buckets, which rests on the assumption that the paths are equivalent. The moment they aren't, the result is wrong in a predictable way: the narrow link congests while the wide one runs at half speed, and total throughput is set by the weakest. The weighted variant fixes this by letting a path appear multiple times in the hash table in proportion to its capacity. Information about that capacity is propagated in BGP with a dedicated link bandwidth community. Two limitations remain: the hash table's size bounds how precise the weights can be, and per-flow hashing still balances poorly when traffic consists of a few large flows.