EqualCost

Equalcost is a general term used to describe a situation in which multiple options have identical total cost according to a chosen metric. The concept appears in fields such as routing, graph theory, and resource allocation, where cost is defined to reflect factors like distance, delay, bandwidth, or policy requirements. When several options share the same cost, they are considered equally viable under the given criteria.

In networking, equal-cost multipath routing (ECMP) is a common application of the idea. ECMP allows routers to forward packets onto multiple next hops that have the same route metric. This can improve throughput and resilience by distributing traffic across several paths rather than concentrating it on a single route. Route metrics are generated by interior gateway protocols such as OSPF or IS-IS, and, in some cases, BGP. Traffic distribution is typically achieved through hashing policies that map flows, destinations, or packets to one of the available paths. This can preserve packet order for individual flows while increasing overall link utilization.

Implementation considerations include the choice of hashing method, path asymmetry, and failure handling. If paths differ significantly in bandwidth or reliability, simple equal-cost routing may lead to suboptimal performance or uneven utilization. In graph theory, equal-cost paths are those that tie for the shortest distance or weight between two nodes, and they form the basis for many shortest-path and exploration algorithms. See also shortest path, multipath routing, and ECMP.