Unequal cost multipath (UCMP) allows traffic to be distributed according to the bandwidth ratio of multiple unequal-cost paths that point to the same destination with the same precedence. All paths carry proportional traffic according to bandwidth ratio to achieve optimal load balancing.
When equal-cost routes have multiple outbound interfaces that connect to both high-speed links and low-speed links, equal cost multipath (ECMP) evenly distributes traffic among links to a destination, regardless of the difference between link bandwidths. When the link bandwidths differ greatly, low-bandwidth links may be congested, whereas high-bandwidth links may be idle. To fully utilize bandwidths of different links, traffic must be balanced according to the bandwidth ratio of these links.