In the topology shown in the figure, the entire network runs the OSPFv3 protocol. Without external routes being imported, which type of LSA will not be generated on R1?

Question

Exhibit

Options

• AInter-Area-Prefix-LSA
• BLink-LSA
• CIntra-Area-Prefix-LSA
• DInter-Area-Router LSA

• A
  17% (6)
• B
  6% (2)
• C
  6% (2)
• D
  72% (26)

Explanation

Inter-Area-Router-LSA (Type 4) is correct because this LSA exists solely to advertise the location of an ASBR (AS Boundary Router) to routers in other areas - it is only generated by an ABR when an ASBR exists in the network. Since no external routes are being imported, there is no ASBR, and therefore no Type 4 LSA will be produced on R1.

Why the distractors are wrong:

• A (Inter-Area-Prefix-LSA / Type 3): If R1 is an ABR, it generates these to advertise prefixes between areas - this happens in any multi-area OSPF setup, regardless of external routes.
• B (Link-LSA / Type 8): OSPFv3 splits prefix info from topology info; every router generates a Link-LSA on each of its links to share its link-local address and IPv6 prefixes - this is a core OSPFv3 function that always occurs.
• C (Intra-Area-Prefix-LSA / Type 9): Every OSPFv3 router generates these to advertise IPv6 prefixes within its area, again independent of external route imports.

Memory tip: Type 4 = "Finding the ASBR." If there's no ASBR (no external routes imported), the "finder" LSA has nothing to find - so Type 4 is never generated.

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