Juniper JUNOSE SOFTWARE FOR E SERIES 11.0.X - BGP AND MPLS CONFIGURATION GUIDE 2009-12-30 Configuration Manual page 411

For information about specifying an address family, see "Multicast Services over
VPNs" on page 412.
Equal-Cost Multipath Support
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:
Consider the simple ECMP scenario for a BGP/MPLS VPN shown in Figure 68 on
page 376.
VPLS If you specify the VPLS address family, you can configure the router to
exchange layer 2 NLRI for a specified VPLS instance. For a description of VPLS,
see "Configuring VPLS" on page 583.
VPWS If you specify the VPWS address family, you can configure the PE router
to exchange layer 2 NLRI for a specified VPWS instance. For a description of
VPWS, see "Configuring VPWS" on page 651.
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.
Chapter 5: Configuring BGP-MPLS Applications
Overview
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