300-320 Question #197: Real Exam Question with Answer & Explanation

The correct answer is C: OSPF stub areas are always insulated from external changes. By definition, OSPF stub areas do not accept Type 5 (external AS) LSAs, so any changes to external routes outside the OSPF domain never propagate into the stub area-it is permanently insulated from external routing changes (C). This is a key stability and scalability benefit. Tot

Question

OSPF stub areas are an important tool for the Network designer; which of the following two should be considered when utilizing OSPF stub areas? (Choose two)

Options

AOSPF stub areas increase the size of the LSDB with the addition of Type 3 and 5 LSAs
BOSPF not so stubby areas are particularly useful as a simpler form of summarization
COSPF stub areas are always insulated from external changes
DOSPF totally stubby areas cannot distinguish among ABRs for the best route to destinations
EOSPF stub areas can distinguish among ASBRs for destinations that are external to the OSPF

Explanation

By definition, OSPF stub areas do not accept Type 5 (external AS) LSAs, so any changes to external routes outside the OSPF domain never propagate into the stub area-it is permanently insulated from external routing changes (C). This is a key stability and scalability benefit. Totally stubby areas go further by also blocking Type 3 inter-area LSAs; they receive only a single default route from the ABR. When multiple ABRs exist, all advertise the same default route with no more-specific information, so routers inside the area cannot select the optimal ABR for reaching specific inter-area or external destinations (D). Statement A is false-stub areas reduce LSDB size by eliminating Type 5 LSAs. Statement E is false-stub areas cannot distinguish among ASBRs because external LSAs are blocked entirely.

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