One application for a central BFD implementation is so BFD can be supported over spoke SDPs used to inter-connect IES or VPRN interfaces. When there are spoke SDPs for inter-connections over an MPLS network between two routers, BFD is used to speed up failure detections between nodes so re-convergence of unicast and multicast routing information can begin as quickly as possible.
The MPLS LSP associated with the spoke SDP can enter or egress from multiple interfaces on the router. BFD for these types of interfaces cannot exist on the IOM/XCM by itself.
Figure: BFD for IES/VPRN over spoke SDP
