External routes from another routing domain are not appearing in Area 1. What could be the cause?
- The ASBR is not generating Type 5 LSAs
- The router is not configured as an ASBR
- Area 1 should be configured as NSSA
- Area 1 is configured as a stub area
Explanation: According to the sources, an OSPF stub area is a special area type designed to reduce the size of the Link-State Database (LSDB) by blocking External LSAs (Type 5) from entering the area.
The ABR of a stub area filters these external network advertisements, which are typically redistributed from other routing protocols, and does not permit them to flood into the area.
Instead of receiving individual external routes, routers within the stub area rely on a default route advertised by the ABR to reach destinations outside of the OSPF autonomous system.
Therefore, if external routes are missing from Area 1, it is most likely because the area has been configured with the area stub command.
Related exam: CCNP ENARSI v9 Course Final Exam
