- A
OSPF is load balancing across both paths equally.
Why wrong: Option A is wrong because the costs are not equal (20 vs 40), so load balancing would not occur.
- B
The OSPF network type on R1-R3 is broadcast, causing DR/BDR election issues.
Why wrong: Option D is wrong because all adjacencies are full, so DR/BDR is not an issue.
- C
The subnet behind R4 is an external route (type 5 LSA) and the cost to the ASBR via R2 is lower than via R3 due to a mismatched reference bandwidth or manual cost setting.
Option C is correct because external routes are evaluated based on cost to ASBR plus external cost; if the cost to ASBR via R2 is lower (e.g., due to different reference bandwidth), the path via R2 may be preferred.
- D
The R1-R3 link is flapping, causing OSPF to prefer the stable path via R2.
Why wrong: Option B is wrong because all adjacencies are full, indicating stability.
Quick Answer
The answer is a mismatched OSPF external route cost calculation, where the subnet behind R4 is redistributed as a Type 5 LSA and the path via R2 has a lower total metric despite higher intra-area costs. This occurs because OSPF external route path selection combines the cost to reach the ASBR (R4) with the external cost advertised in the Type 5 LSA; if R4’s redistribution uses a lower reference bandwidth or a manually set metric for the external route, the total cost via R2 (40 + low external cost) can beat the cost via R3 (20 + high external cost). On the ENCOR 350-401 exam, this tests your understanding that OSPF does not compare intra-area costs alone for external routes—it always adds the ASBR reachability cost to the external metric, a common trap when engineers assume lower OSPF interface costs guarantee a better path. Remember the mnemonic: “External adds ASBR—check the redistributed metric, not just the link cost.”
CCNP Network Assurance Practice Question
This 350-401 practice question tests your understanding of network assurance. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 company has a core network running OSPF in a single area (area 0). The network consists of four routers: R1, R2, R3, and R4. R1 is connected to R2 and R3. R2 is also connected to R4. R3 is also connected to R4. All links are GigabitEthernet with OSPF cost based on bandwidth (reference bandwidth 100 Gbps). The network engineer notices that traffic from R1 to a subnet behind R4 is taking a suboptimal path: R1 -> R2 -> R4, instead of R1 -> R3 -> R4 which has lower cost. Upon checking OSPF neighbor states, all adjacencies are full. The engineer verifies that the cost on the R1-R3 link is 10 and the cost on the R3-R4 link is 10, while the R1-R2 link cost is 20 and the R2-R4 link cost is 20. The total cost via R2 is 40, via R3 is 20. However, the routing table on R1 shows the next-hop as R2 for the subnet behind R4. 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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The subnet behind R4 is an external route (type 5 LSA) and the cost to the ASBR via R2 is lower than via R3 due to a mismatched reference bandwidth or manual cost setting.
Option C is correct because the subnet behind R4 is likely redistributed into OSPF as an external route (type 5 LSA). The cost to reach the ASBR (R4) via R2 is 40, while via R3 it is 20, but if the external route's advertised cost is lower via R2 (e.g., due to a mismatched reference bandwidth or manual cost setting on R4's redistribution), OSPF will prefer the path with the lower total cost (intra-area cost to ASBR + external cost). In this scenario, the external cost via R2 could be 0 or very low, making the total cost via R2 (40 + low) lower than via R3 (20 + high), overriding the intra-area cost advantage.
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.
- ✗
OSPF is load balancing across both paths equally.
Why it's wrong here
Option A is wrong because the costs are not equal (20 vs 40), so load balancing would not occur.
- ✗
The OSPF network type on R1-R3 is broadcast, causing DR/BDR election issues.
Why it's wrong here
Option D is wrong because all adjacencies are full, so DR/BDR is not an issue.
- ✓
The subnet behind R4 is an external route (type 5 LSA) and the cost to the ASBR via R2 is lower than via R3 due to a mismatched reference bandwidth or manual cost setting.
Why this is correct
Option C is correct because external routes are evaluated based on cost to ASBR plus external cost; if the cost to ASBR via R2 is lower (e.g., due to different reference bandwidth), the path via R2 may be preferred.
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 R1-R3 link is flapping, causing OSPF to prefer the stable path via R2.
Why it's wrong here
Option B is wrong because all adjacencies are full, indicating stability.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that OSPF always uses the lowest intra-area cost, but external route types (E1 vs E2) and their metric handling can override intra-area path selection, causing candidates to overlook redistribution and metric-type behavior.
Detailed technical explanation
How to think about this question
OSPF external routes (type 5 LSAs) carry a separate metric (E1 or E2). By default, E2 routes use only the external cost, ignoring the internal cost to the ASBR, which can cause suboptimal path selection if the ASBR advertises a low external cost via a high-cost internal path. Engineers can use 'metric-type 1' (E1) to add internal cost to the external metric, ensuring SPF chooses the shortest total path. This scenario is common when redistributing static or connected routes with a fixed metric, leading to asymmetric 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.
What to study next
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FAQ
Questions learners often ask
What does this 350-401 question test?
Network Assurance — This question tests Network Assurance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The subnet behind R4 is an external route (type 5 LSA) and the cost to the ASBR via R2 is lower than via R3 due to a mismatched reference bandwidth or manual cost setting. — Option C is correct because the subnet behind R4 is likely redistributed into OSPF as an external route (type 5 LSA). The cost to reach the ASBR (R4) via R2 is 40, while via R3 it is 20, but if the external route's advertised cost is lower via R2 (e.g., due to a mismatched reference bandwidth or manual cost setting on R4's redistribution), OSPF will prefer the path with the lower total cost (intra-area cost to ASBR + external cost). In this scenario, the external cost via R2 could be 0 or very low, making the total cost via R2 (40 + low) lower than via R3 (20 + high), overriding the intra-area cost advantage.
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.
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Last reviewed: Jun 11, 2026
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.
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