Question

A service provider engineer configures a remote branch area as a standard OSPF stub area to reduce the LSDB size on low-memory routers. After the change, users in the stub area report they can no longer reach external networks redistributed into OSPF from a BGP-speaking ASBR located in area 0. Internal and inter-area routes still work fine. What is the correct explanation for this behavior?

Accepted Answer

Stub areas block Type 5 external LSAs, so the ABR injects a default route (0.0.0.0/0) via a Type 3 LSA to reach external destinations, but reachability fails if the default route is not being used.

Answer

Stub areas block Type 3 summary LSAs, so inter-area routes are lost and external routes must be re-advertised as intra-area LSAs.

Answer

Stub areas convert Type 5 LSAs into Type 7 LSAs, which the low-memory routers cannot process, dropping the external routes.

Answer

Stub areas require the ASBR to reside inside the stub area, otherwise all external routes are silently discarded at the ABR.