300-410 Question 67
Single answerA network administrator has configured an OSPF stub area on a branch site to reduce the size of the OSPF database and limit external route propagation. The administrator notices that devices within the stub area are unable to reach destinations outside of the OSPF domain. What could be the most likely cause of this issue?
- A
The ABR for the stub area did not inject a default route into the stub area.
- B
The stub area was not configured with 'no-summary' on the Area Border Router (ABR).
- C
The stub area type was mistakenly configured as a totally stubby area on the ABR.
- D
The external routes were blocked because the stub area does not allow Type 3 summary LSAs.
Show answer and explanation
Correct answer: A
Explanation
In OSPF stub areas, external routes (Type 4 and Type 5 LSAs) are not propagated to reduce overhead. To ensure reachability to external destinations, the ABR must inject a default route (0.0.0.0/0) into the stub area using Type 3 LSAs. Without this default route, devices within the stub area cannot route traffic to external networks, resulting in connectivity issues.
- A. Correct.
Correct. In a stub area, the ABR must inject a default route (Type 3 LSA) for devices in the stub area to reach external destinations. Without this default route, the devices in the stub area cannot route traffic outside the OSPF domain.
- B. Incorrect.
Incorrect. The 'no-summary' option is used to create a totally stubby area, which blocks all summary LSAs in addition to external LSAs. However, this is unrelated to the lack of a default route injection in a standard stub area.
- C. Incorrect.
Incorrect. A totally stubby area blocks both summary and external LSAs, but this configuration does not directly explain the lack of reachability in a standard stub area.
- D. Incorrect.
Incorrect. Stub areas allow Type 3 summary LSAs; they block only Type 4 and Type 5 LSAs (external routes). Therefore, this explanation is not valid for the scenario.