This 350-401 practice question tests your understanding of ospf. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer is troubleshooting an OSPF issue where a router is not learning a route to a network that is advertised via a type 5 LSA from an ASBR. The engineer checks the OSPF database and sees the type 5 LSA, but the route is not in the routing table. The forwarding address in the LSA is 0.0.0.0. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ASBR is not reachable via an OSPF internal route.
When a Type 5 LSA has a forwarding address of 0.0.0.0, OSPF routers will use the ASBR as the next hop for the external route. For the route to be installed in the routing table, the ASBR must be reachable via an OSPF intra-area or inter-area route. If the ASBR is not reachable (e.g., no valid OSPF route to the ASBR's router ID), the Type 5 LSA is considered unreachable and is not installed, even though it exists in the OSPF database.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The ASBR is not reachable via an OSPF internal route.
Why this is correct
Correct because OSPF requires the ASBR to be reachable via an intra-area or inter-area route for the type 5 LSA to be installed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The type 5 LSA has a metric of 16777215.
Why it's wrong here
Incorrect because OSPF does not have a maximum metric that prevents installation; even high metrics are installed.
✗
The OSPF process ID on the ASBR is different from the other routers.
Why it's wrong here
Incorrect because OSPF process ID is locally significant and does not affect LSA processing.
✗
The type 5 LSA is being filtered by an outbound route filter.
Why it's wrong here
Incorrect because if the LSA is in the database, it has not been filtered; filtering would prevent the LSA from being in the database.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a Type 5 LSA present in the database automatically guarantees the route is installed, but the trap here is that the forwarding address of 0.0.0.0 requires the ASBR to be reachable via an OSPF internal route, which is a common oversight.
Detailed technical explanation
How to think about this question
OSPF RFC 2328 specifies that for Type 5 LSAs with a forwarding address of 0.0.0.0, the next hop is the originating ASBR itself. The router must have a reachable OSPF route to the ASBR's router ID (typically via an intra-area or inter-area route) to install the external route. This behavior is critical in MPLS VPN or redistribution scenarios where the ASBR may be multiple hops away, and the forwarding address field is used to avoid suboptimal routing.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
OSPF — This question tests OSPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ASBR is not reachable via an OSPF internal route. — When a Type 5 LSA has a forwarding address of 0.0.0.0, OSPF routers will use the ASBR as the next hop for the external route. For the route to be installed in the routing table, the ASBR must be reachable via an OSPF intra-area or inter-area route. If the ASBR is not reachable (e.g., no valid OSPF route to the ASBR's router ID), the Type 5 LSA is considered unreachable and is not installed, even though it exists in the OSPF database.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.