Users Guide

1340 BGP
A BGP NEXT_HOP can resolve to an ECMP IGP route. When BGP is
configured to allow ECMP iBGP routes, the BGP NEXT_HOP resolves to
multiple next hops. BGP retains up to the number of resolved next hops
allowed for an iBGP route. For example, in Figure 36-2, R4 receives an iBGP
route from internal peer R1. The BGP NEXT_HOP of this path resolves to an
ECMP OSPF route through R2 and R3. If BGP is configured on R4 to allow
ECMP iBGP routes, then R4 will resolve the path's BGP NEXT_HOP to a
pair of next hops through R2 and R3.
Figure 36-2. ECMP NEXT_HOP Resolution
When iBGP paths are combined into an ECMP route, their next-hop sets are
merged to form the set of next hops for the route. For example, in
Figure 36-3, if R4 learns another route via R5 and R300 with the same
destination as the route in the previous example, and the path from R300 is
equivalent to the path through R100, then R4 will install a route using R2,
R3, and R5 as next hops.