Equal Cost Multi Path

Key Points

• Uses more than one equal-cost route
• Can improve utilization and redundancy
• Common in routed IP networks
• Often used with load sharing or multipath forwarding
• Distributes traffic across paths according to a hashing or load-sharing strategy
• Used in data centers, enterprise networks, and service provider backbones

Definition

Equal Cost Multi Path is a routing method that allows multiple routes with the same cost to be used for forwarding traffic, enabling multipath routing and load sharing.

Concept

Equal Cost Multi Path is a networking term used when a router forwards traffic across more than one path that has the same routing cost. It exists to improve utilization, resilience, or traffic distribution without choosing only one next hop. It is used in IP networks, data centers, enterprise routing, and service provider systems. ECMP can spread flows across paths while preserving destination reachability.

Explainer

Equal Cost Multi Path is a routing behavior in which a router uses multiple next hops that have the same computed cost toward a destination. It works by keeping several equal-cost paths in the forwarding decision set and distributing traffic across them according to a hashing or load-sharing strategy. It is used in data centers, enterprise networks, service provider backbones, and other routed environments that support multipath forwarding. Constraints include path symmetry, flow hashing behavior, topology consistency, and the need for equal or nearly equal metrics. Failure modes include imbalanced traffic distribution, path polarization, packet ordering issues in some designs, and hidden differences between paths that appear equal in the routing table. Tradeoffs involve better utilization and resilience versus more complex troubleshooting and potential flow-level unevenness. Equal Cost Multi Path matters because it can increase capacity use and redundancy without requiring a single preferred path.