Equal-cost multi-path routing

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ECMP animation using 802.1aq protocol

Equal-cost multi-path routing (ECMP) is a routing strategy where next-hop packet forwarding to a single destination can occur over multiple "best paths" which tie for top place in routing metric calculations. Multi-path routing can be used in conjunction with most routing protocols, because it is a per-hop decision limited to a single router. It can substantially increase bandwidth by load-balancing traffic over multiple paths; however, there may be significant problems in deploying it in practice.[1] RFC 2991 discusses multi-path routing in general.

In 2014, the Institute of Electrical and Electronics Engineers (IEEE) incorporated Equal Cost Multiple Paths (ECMP) or IEEE standard 802.1Qbp into IEEE 802.1Q-2014 for Shortest Path Bridging. Specifying the forward and reverse paths used for unicast and multicast traffic in shortest path bridging as symmetric insuring flows on deterministic paths, resolving configuration complexities, management functionality and performance issues within original standards implementations.[2][3][4][5][6]


In the past, load balancing by per-packet multipath routing was generally deprecated due to the impact of rapidly changing latency, packet reordering and maximum transmission unit (MTU) differences within a network flow, which could disrupt the operation of many Internet protocols, most notably TCP and path MTU discovery. RFC 2992 analyzed one particular multipath routing strategy involving the assignment of flows to bins by hashing flow-related data in the packet header, which is designed to avoid these problems by sending all packets from any particular network flow down a single deterministic path, while balancing multiple flows over multiple paths in general.[7]

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