Question

An engineer is troubleshooting an OSPF network with three areas: Area 0 (backbone), Area 1, and Area 2. Both Area 1 and Area 2 are non-backbone areas connected to Area 0 through separate ABRs. A user reports that a subnet learned via a Type 3 LSA originating in Area 1 is not being installed by routers in Area 2, even though the ABRs are fully adjacent and the backbone is stable. Which OSPF loop-prevention behavior best explains why the route from Area 1 reaches Area 2 only through the backbone and not directly?

Accepted Answer

An ABR only accepts Type 3 LSAs for the SPF calculation if they are received on a link belonging to Area 0, preventing inter-area routes from being learned across non-backbone areas.

Answer

Type 3 LSAs are flooded throughout the entire OSPF domain, so the route should appear in Area 2 regardless of the backbone; the issue must be an area authentication mismatch.

Answer

OSPF uses split horizon on Type 3 LSAs, so an ABR never re-advertises a summary LSA back into any area from which it was originally received.

Answer

Type 3 LSAs carry a DN (down) bit that, when set, blocks the receiving ABR from installing the route unless the routers share the same RID domain.