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Routing & Switching · Intermediate

Equal-Cost Multipath (ECMP)

Installing and load sharing across multiple routes to the same prefix that have identical cost.

Definition

ECMP lets a router use several best paths simultaneously, typically hashing flows across them to keep packets of a flow on one path. Cisco IOS defaults to 4 paths for most IGPs (configurable with maximum-paths), while BGP defaults to 1 path unless multipath is enabled. Junos installs multiple paths in the RIB but needs a load-balance per-packet policy exported to the forwarding table to use them.

Example

maximum-paths 8 under router ospf 1

Exam tip

Junos 'load-balance per-packet' actually results in per-flow hashing on modern hardware; a common gotcha.

Related terms