In OSPF design, what types of routes are allowed into a Not-So-Stubby Area (NSSA) by default, and how are external routes handled inside that area?

Introduction / Context:
This question targets your understanding of advanced OSPF area types, specifically the Not-So-Stubby Area (NSSA). NSSAs are used when you want most of the benefits of a stub area, such as limiting external routes, while still allowing an Autonomous System Boundary Router inside the area to redistribute external routes into OSPF. Knowing exactly which routes are allowed in and how they are represented is essential for correct design and troubleshooting.

Given Data / Assumptions:
- We are working with OSPF as the Interior Gateway Protocol.
- The area in question has been configured as an NSSA, not a totally stubby or regular area.
- There may be an ASBR inside the NSSA that redistributes external routes from another domain (for example, from EIGRP or static routes).
- We want to know which routes are allowed into the NSSA and how external routes are represented inside the area.

Concept / Approach:
In a normal OSPF area, ABRs flood Type 1 and Type 2 LSAs (intra-area), Type 3 LSAs (inter-area summaries), and Type 5 LSAs (external routes). In a stub area, Type 5 externals are not allowed, and typically a default route is injected instead. An NSSA is a special case: it still blocks Type 5 LSAs from entering the area, but it allows an internal ASBR to redistribute external routes into the NSSA as Type 7 LSAs. These Type 7 LSAs are then translated back into Type 5 LSAs by an ABR when leaving the NSSA. Intra-area and inter-area routes are still allowed into the NSSA like any normal area.

Step-by-Step Solution:
Step 1: Recall that stub areas block external Type 5 LSAs and often receive a default route instead.Step 2: Understand that NSSAs behave similarly in that they do not accept Type 5 LSAs from the backbone or other areas.Step 3: Identify the special feature of an NSSA: an ASBR inside the area may redistribute external routes into OSPF using Type 7 LSAs.Step 4: Recognize that Type 7 LSAs exist only inside the NSSA and are translated into Type 5 LSAs by an ABR when they leave the NSSA.Step 5: Confirm that intra-area (Type 1 and Type 2) and inter-area (Type 3) routes are still allowed into an NSSA, just as in a normal area.Step 6: Choose the option that states intra-area and inter-area routes are allowed and that external routes redistributed inside the NSSA are carried as Type 7 LSAs.

Verification / Alternative check:
In a lab, you can configure one OSPF area as NSSA and place an ASBR inside it that redistributes static routes. By examining the LSDB on routers inside the area, you will see those external destinations represented as Type 7 LSAs. On ABRs, you will see translation to Type 5 LSAs when advertising toward other non-stub areas. You will also observe that standard intra-area and inter-area routes still appear inside the NSSA, confirming that they are allowed.

Why Other Options Are Wrong:
Option A incorrectly claims that inter-area routes are blocked; NSSAs still receive Type 3 summaries. Option C suggests that only external Type 5 LSAs are allowed, which directly contradicts NSSA behavior. Option D incorrectly states that NSSA behaves exactly like a normal area and allows Type 5 LSAs unchanged, which is not true. Option E describes something closer to a totally stubby area, where a default route substitutes for many specifics, but that is not the definition of an NSSA.

Common Pitfalls:
Students often confuse stub, totally stubby, and NSSA behavior, especially regarding Type 5 and Type 7 LSAs. Another pitfall is assuming that NSSA means “no external routes at all,” when in fact it allows external routes to originate inside the area as Type 7 LSAs. Remembering that NSSA stands for Not-So-Stubby helps you recall that it is mostly stub-like but still supports local redistribution via special LSAs.

Final Answer:
By default, an NSSA allows intra-area and inter-area routes, and any external routes redistributed inside the NSSA are advertised as Type 7 LSAs, with no Type 5 LSAs entering the NSSA from other areas.