Simple definition
ECMP (Equal-Cost Multi-Path) allows multiple equal-cost routes to the same destination to be used at the same time.
Technical definition
When a routing table contains several valid next hops with an equivalent metric, the device can install them simultaneously and distribute new flows using a selection algorithm, often based on a hash of addresses and ports. Exact behavior depends on the platform and routing protocol.
How it works / role
Its role is to use multiple available paths in parallel to increase aggregate capacity and provide resilience without keeping every alternate link completely idle.
What is it used for?
Distribute traffic across multiple WAN links, data-center paths, core-network links, or dynamically learned routes when they have equal cost.
Practical example
A router has two equal-cost OSPF paths to a remote network. Both routes are active and new sessions are distributed across the next hops according to the device algorithm.
Common issues
- Equal-cost paths have very different real capacities
- Traffic asymmetry conflicts with a stateful firewall
- Hashing places several large flows on the same link
- ECMP is incorrectly assumed to balance every individual packet
Key takeaway: ECMP usually distributes flows rather than every packet; verify the platform algorithm and any traffic-symmetry requirements.