300-440 Question 108
Single answerA network engineer is designing an OSPF network for an organization. The network includes multiple areas, and the engineer needs to minimize the size of the routing tables in Area 2 while allowing external routes from a connected BGP domain to be redistributed and accessible. Which OSPF area type should the engineer configure Area 2 as?
- A
Backbone area
- B
Stub area
- C
Totally stubby area
- D
NSSA (Not-So-Stubby Area)
Show answer and explanation
Correct answer: D
Explanation
An NSSA (Not-So-Stubby Area) is the best choice for Area 2 because it strikes a balance between minimizing routing table size and allowing external route redistribution. NSSAs allow Type 7 LSAs, which can carry external routes, to be present within the area while filtering out Type 5 LSAs. This makes them suitable for scenarios where external routes need to be redistributed into the OSPF domain but the routing table size must be kept manageable.
- A. Incorrect.
Backbone area (Area 0) is required for inter-area routing in OSPF and cannot be used to minimize routing tables or allow external route redistribution in a specific area.
- B. Incorrect.
Stub areas prevent external routes (Type 5 LSAs) from being advertised into the area, which contradicts the requirement to allow external routes in Area 2.
- C. Incorrect.
A totally stubby area blocks both external routes (Type 5 LSAs) and inter-area routes (Type 3 LSAs), making it unsuitable for the requirement to allow external routes.
- D. Correct.
An NSSA (Not-So-Stubby Area) allows external routes (Type 7 LSAs) to be redistributed into the area while still minimizing routing tables by filtering out Type 5 LSAs. This satisfies the requirement of minimizing routing tables while allowing external routes.