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)

Question

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

• A
  11% (6)
• B
  6% (3)
• C
  60% (32)
• E
  23% (12)

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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