In OSPF, what types of routes are allowed into a stub area by default, and what is the treatment of external routes?

Introduction / Context:
This question examines your understanding of OSPF stub areas, a key feature for controlling the size and complexity of the LSDB in parts of a network that do not need full external routing information. Design decisions about where to use stub areas have a direct impact on scalability and CPU utilization of routers in branch or remote sites.

Given Data / Assumptions:
- We are using OSPF as the IGP.
- A particular area is configured as a stub area (not NSSA and not totally stubby in this basic scenario).
- We want to know which routes are allowed into that stub area by default.
- We specifically care about the handling of external routes from outside the OSPF domain or from ASBRs.

Concept / Approach:
In a normal OSPF area, routers may receive intra-area routes (Type 1 and 2 LSAs), inter-area summary routes (Type 3 LSAs), and external routes (Type 5 LSAs). A stub area is designed to shield its routers from the flood of external Type 5 LSAs. Instead, the ABR injects a default route into the stub area, allowing devices inside the area to reach external destinations without needing to know all the specifics. Intra-area and inter-area information is still allowed, so internal connectivity remains detailed where needed.

Step-by-Step Solution:
Step 1: Recall that a stub area blocks external Type 5 LSAs from entering the area to keep the LSDB smaller.Step 2: Understand that the ABR usually injects a default route (0.0.0.0/0) into the stub area so that stub routers have a path to external networks.Step 3: Recognize that intra-area LSAs (Type 1 and Type 2) remain fully present within the stub area because they describe the internal topology.Step 4: Confirm that inter-area summary routes (Type 3 LSAs) from other areas are allowed so that stub routers can reach internal destinations outside their own area without needing external specifics.Step 5: Select the option stating that intra-area and inter-area routes are allowed, while external Type 5 LSAs are blocked and replaced by a default route.

Verification / Alternative check:
In a lab, configure area 1 as a stub area and observe the LSDB on routers inside that area. You will see Type 1 and Type 2 LSAs representing intra-area connectivity, and Type 3 LSAs summarizing other areas. You will not see Type 5 LSAs for external networks. Instead, you will typically see a Type 3 LSA for the default route injected by the ABR. This behavior matches the textbook definition of a stub area.

Why Other Options Are Wrong:
Option A incorrectly blocks inter-area routes, which would isolate the stub area from the rest of the OSPF domain. Option C claims that only external routes are allowed, which is the opposite of stub behavior. Option D describes a normal area, not a stub. Option E suggests that only a default route is allowed and nothing else, which is closer to the behavior of a totally stubby area, not a standard stub area as generally defined in exam questions.

Common Pitfalls:
Learners often confuse stub, totally stubby, and NSSA behavior, especially regarding which LSA types are blocked. Another common mistake is to think that stub areas cannot reach external networks at all, forgetting about the injected default route. Keeping a clear mental map of which LSAs are allowed in each special area type helps avoid configuration and exam errors.

Final Answer:
By default, an OSPF stub area allows intra-area and inter-area routes but blocks external Type 5 LSAs, relying instead on a default route injected by the ABR to reach external destinations.