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    Equal-Cost Multipath Support Overview

    Equal-cost multipath (ECMP) is a traffic load-balancing feature that enables traffic to the same destination to be distributed over multiple paths that have the same cost. BGP ECMP support for BGP/MPLS VPNs enables MPLS VPN routes to be included in the list of available equal-cost paths. You can specify that up to 16 equal-cost paths be considered.

    The set of ECMP legs in a network can contain MPLS indirect next hops, either as a leg itself or pointed to by a leg. If the path to any of the MPLS indirect next hops fails, then the routing protocol begins recalculating the set of viable routes as soon as it is notified of the failure. When the recalculation has finished, the protocol then updates the routing table with the new routes.

    From the time the path fails until the routing table is updated, the traffic flowing over the ECMP leg that has the failed MPLS indirect next hop is lost.

    To reduce the amount of lost traffic, the failed path is quickly pruned from the ECMP set as soon as the protocol is notified of the connectivity failure. Traffic for the destination is then forwarded over the remaining equal-cost paths to the destination. When the recalculated set of routes is installed in the routing table, traffic for the destination is forwarded by means of the new route.

    ECMP sets can have an MPLS indirect next hop as one of the legs in the following scenarios:

    • In a BGP-MPLS VPN where a given VPN prefix is learned from multiple PE routers.
    • When multiple RSVP-TE tunnels are created over different paths to the same destination.
    • In a network that connects IPv6 islands across an IPv4 core, where a given IPv6 prefix is learned from multiple egress PEs running IPv6.

    Published: 2014-08-18