The answer is a missing or misconfigured ABR between area 1 and area 0, or area 2 and area 0. This is correct because OSPF inter-area routing strictly requires all traffic between non-backbone areas to pass through the backbone area 0; without a properly functioning ABR on either side, Type 3 summary LSAs cannot be generated or flooded, breaking connectivity between areas 1 and 2 even when area 0 routers can ping each other. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of OSPF’s hierarchical design and the ABR’s role as the mandatory gateway for inter-area prefixes—a common trap is assuming that OSPF supports direct area-to-area routing without area 0. Remember the memory tip: “No ABR, no inter-area—all roads lead through area 0.”
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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 in a service provider network. The network uses OSPFv2 with multiple areas. Routers in area 0 are able to ping each other, but routers in area 1 cannot reach routers in area 2. All routers are OSPF-enabled and have correct network type configurations. 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
✓
Missing or misconfigured ABRs between area 1 and area 0, or area 2 and area 0
In OSPFv2, inter-area communication must traverse area 0 (the backbone). Routers in area 1 and area 2 can only reach each other if Area Border Routers (ABRs) exist between each non-backbone area and area 0, and those ABRs are correctly configured and have full adjacencies. Since area 0 routers can ping each other but area 1 and area 2 cannot, the most likely cause is a missing or misconfigured ABR on either side, preventing the exchange of Type 3 summary LSAs between the areas.
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.
✓
Missing or misconfigured ABRs between area 1 and area 0, or area 2 and area 0
Why this is correct
Inter-area routing must go through area 0; without proper ABRs, areas cannot exchange routes.
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.
✗
Mismatched OSPF area IDs on routers in area 1 and area 2
Why it's wrong here
Area IDs are local and do not need to match across areas.
OSPF network type mismatch preventing adjacency formation
Why it's wrong here
The question states correct network type configurations, so this is not the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that OSPF areas can communicate directly without the backbone, leading candidates to overlook the mandatory role of ABRs and area 0 in inter-area routing.
Detailed technical explanation
How to think about this question
OSPF ABRs must have at least one interface in area 0 and one in a non-backbone area to generate Type 3 summary LSAs. If an ABR is missing or misconfigured (e.g., the backbone interface is passive, or the router is not designated as an ABR due to missing area 0 adjacency), Type 3 LSAs are not injected, causing a black hole for inter-area traffic. In real-world service provider networks, this often occurs during migration or when virtual links are needed but not configured for discontinuous area 0.
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Missing or misconfigured ABRs between area 1 and area 0, or area 2 and area 0 — In OSPFv2, inter-area communication must traverse area 0 (the backbone). Routers in area 1 and area 2 can only reach each other if Area Border Routers (ABRs) exist between each non-backbone area and area 0, and those ABRs are correctly configured and have full adjacencies. Since area 0 routers can ping each other but area 1 and area 2 cannot, the most likely cause is a missing or misconfigured ABR on either side, preventing the exchange of Type 3 summary LSAs between the areas.
What should I do if I get this 350-501 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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