Comprehensive and Detailed In-Depth Explanation:
To determine which routers are unable to import external routes, we need to analyze the behavior of each OSPF area type:
* Stub Area (Area 1 - Contains R6)
* Stub areas do not allow external routes (Type-5 LSAs).
* Instead, the ABR (R2) generates a default route (0.0.0.0/0) for reachability outside the area.
* R6 is in a stub area, so it cannot import external routes. #
* Totally Stub Area (Area 2 - Contains R3, R4)
* Totally stub areas block both external routes (Type-5 LSAs) and inter-area routes (Type-3 LSAs).
* Only a default route (0.0.0.0/0) is injected by the ABR (R2).
* R3 is in a totally stub area, so it cannot import external routes. #
* R4, however, is just inside the totally stub area but is not restricted from generating external routes itself. # (R4 can import external routes)
* NSSA (Area 3 - Contains R5)
* NSSAs allow external routes (Type-7 LSAs) but do not accept standard external routes (Type-5 LSAs) from other areas.
* R5, being in an NSSA, can import external routes but only as Type-7 LSAs. # (R5 can import external routes) Final Answer: # R3, R6 (Option A, C) References:
* HCIP-Datacom-Advanced Routing & Switching Technology V1.0 - OSPF Stub, Totally Stub, and NSSA Areas
* OSPF LSA Types and External Route Restrictions