CCNP Practice Question: Is designing an OSPF network with multiple areas

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  Stub area

  Why this is correct

  A stub area is the correct choice because it filters out Type 5 (external) LSAs, preventing the router from learning external routes. Instead, the ABR automatically injects a default route (0.0.0.0/0) into the stub area, which routers use to forward traffic to destinations outside the OSPF domain. This satisfies the requirement of not learning external routes while still allowing full outbound connectivity.

• ✗

  Not-so-stubby area (NSSA)

  Why it's wrong here

  Not-so-stubby area (NSSA) is incorrect because it explicitly permits the import of external routes from within the area in the form of Type 7 LSAs. These Type 7 LSAs are converted into Type 5 LSAs by the ABR when advertising to the rest of OSPF, meaning external routes are indeed learned and advertised. Therefore, an NSSA does not meet the condition that external routes should not be learned.

• ✗

  Totally stubby area

  Why it's wrong here

  A totally stubby area (TSA) is incorrect because it blocks not only Type 5 external LSAs but also Type 3 inter-area summary LSAs, except for a single default route. This would prevent the router from learning networks in area 0 or any other non-stub area, which is too restrictive if the requirement is only to filter external routes. The question specifically asks to block external routes, implying inter-area routes should still be allowed.

• ✗

  Standard area

  Why it's wrong here

  A standard area (or backbone/normal area) is incorrect because it allows all OSPF LSA types, including Type 5 external LSAs introduced by an ASBR. Routers in a standard area will learn and install external routes in their routing table, directly contradicting the condition that external routes must not be learned. Standard areas are used when full routing information is required, not when external route filtering is needed.