Refer to the exhibit.

AS-path prepending is the correct inbound path-control tool for this design. The objective is for traffic entering AS100 to use link 1 during normal operation and link 2 only after a failure. In BGP, the AS_PATH attribute is a primary path-selection attribute used by external autonomous systems; a shorter AS path is normally preferred over a longer path. By prepending AS100 multiple times on advertisements sent from R4 across link 2, AS100 makes the backup path look less attractive to upstream networks while leaving it available for failover. Modifying next-hop on R3 or R4 does not influence how external networks choose between two entry points into AS100. Prepending on R3 would make the preferred link less attractive, which is the opposite of the requirement. The design should also document provider policy because some providers local-preference routes before AS path, but within the choices, prepending on R4 is the appropriate inbound traffic-engineering method. Reference topics: BGP AS_PATH, inbound path control, multihoming, backup Internet edge design.