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Equal-cost multi-path routing

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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. Multipath routing can be used in conjunction with most routing protocols, since it is a per-hop decision that is limited to a single router. It potentially offers substantial increases in bandwidth by load-balancing traffic over multiple paths; however, there can be significant problems in its deployment in practice.[1] RFC 2991 discusses multipath routing in general.

In 2014 the IEEE approved IEEE 802.1Qbp (part of IEEE 802.1) for Shortest Path Bridging which defines ECMP as an IEEE standard. The forward and reverse paths used for unicast and multicast traffic in IEEE 802.1aq networks are symmetric resolving configuration complexities, management functionalities and performance issues within original RFC implementations.[2][3][4][5]

Load balancing by per-packet multipath routing is generally deprecated due to the impact of rapidly changing latency, packet reordering and maximum transmission unit (MTU) differences within a network flow, which can disrupt the operation of many Internet protocols, most notably TCP and path MTU discovery. RFC 2992 analyzes 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.[6]

In many situations, ECMP may not offer any real advantage over best-path routing: for example, if the multiple best next-hop paths to a destination re-converge downstream into a single low-bandwidth path (a common scenario), it will merely add complexity to the traffic paths to that destination without improving available bandwidth. ECMP may also interact negatively with other routing algorithms where the physical topology of the system differs from the logical topology, for example in systems that employ VLANs at layer 2, or virtual circuit-based architectures such as ATM or MPLS.

See also

References

  1. ^ "Multipath Issues in Unicast and Multicast Next-Hop Selection". Retrieved 16 December 2013.
  2. ^ "IEEE Standard for Local and Metropolitan Area Networks---Virtual Bridged Local Area Networks - Amendment: Equal Cost Multiple Paths (ECMP)".
  3. ^ "Alcatel-Lucent, Avaya, Huawei, Solana and Spirent Showcase Shortest Path Bridging Interoperability". Huawei. 7 September 2011. Retrieved 11 September 2011.
  4. ^ An improved shortest path bridging protocol for Ethernet backbone network. IEEE Xplore. 3 March 2011. doi:10.1109/ICOIN.2011.5723169. ISBN 978-1-61284-661-3. ISSN 1976-7684. Retrieved 11 September 2011.
  5. ^ "Lab Testing Summary Report; Data Center Configuration with SPB" (PDF). Miercom. September 2011. Retrieved 25 December 2011.
  6. ^ "Analysis of an Equal-Cost Multi-Path Algorithm".


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