The answer is an OSPF area mismatch on the shared segment. OSPF requires that all routers on the same link agree on the area ID in their Hello packets, as this field is checked during the neighbor discovery process; when R1’s interface is in area 0 and R2’s is in area 1, the Hello packets are rejected, preventing adjacency from forming even though the subnet and timers match. On the CCNA 200-301 v2 exam, this question tests your understanding that area IDs must be identical on a common link, while OSPF process IDs are locally significant and do not need to match—a common trap where candidates mistakenly blame process ID differences. A quick memory tip: think of OSPF areas like rooms in a house—two people in different rooms can’t talk directly through the same door, so keep your area IDs consistent on the same wire.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. A key principle to apply: oSPF routers must assign the same area ID to interfaces on a shared segment to form neighbor adjacencies successfully.. 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.
Exhibit
R1#
interface GigabitEthernet0/0
ip address 10.1.50.1 255.255.255.0
!
router ospf 1
network 10.1.50.0 0.0.0.255 area 0
R2#
interface GigabitEthernet0/0
ip address 10.1.50.2 255.255.255.0
!
router ospf 1
network 10.1.50.0 0.0.0.255 area 1
R1 and R2 are connected via Ethernet and are configured with OSPF, but they fail to form an adjacency. Upon checking the interface configurations, you see that R1’s interface is in OSPF area 0 while R2’s interface is in area 1, and both interfaces use default timers and are in the same subnet. What is the most likely reason?
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 OSPF areas do not match on the shared segment.
The most likely cause is an OSPF area mismatch on the shared segment. R1 and R2 are in the same IP subnet and use default hello/dead timers, so neither subnet mismatch nor timer mismatch is the problem. OSPF can run on Ethernet interfaces. However, OSPF requires neighbors on the same link to agree on the area ID; here they differ, preventing adjacency. Option B is incorrect because OSPF process IDs are locally significant and do not need to match.
Key principle: OSPF routers must assign the same area ID to interfaces on a shared segment to form neighbor adjacencies successfully.
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 OSPF areas do not match on the shared segment.
Why this is correct
This is correct because the same link is placed into area 0 on one side and area 1 on the other.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must assign the same area ID to interfaces on a shared segment to form neighbor adjacencies successfully.
✗
The routers must use the same process ID.
Why it's wrong here
This is wrong because matching or differing local process IDs is not the issue here.
When this WOULD be correct
In a different scenario where the question specifies that the routers are configured with multiple OSPF processes and the exam asks about the necessity of matching process IDs for adjacency, this option would be correct. For example, if the question involved multiple OSPF instances on the same router, it would be relevant.
✗
The interfaces are in different IPv4 subnets.
Why it's wrong here
This is wrong because both are in 10.1.50.0/24.
When this WOULD be correct
In a different question setup, if the exam asked about OSPF adjacency issues where the interfaces of R1 and R2 are indeed in different IPv4 subnets, this option would be correct. For example, if the question specified that R1's interface is in 192.168.1.0/24 and R2's interface is in 192.168.2.0/24, then this option would apply.
✗
OSPF cannot run on Ethernet interfaces.
Why it's wrong here
This is wrong because OSPF commonly runs on Ethernet.
When this WOULD be correct
In a different exam scenario, if the question stated that R1 and R2 were configured on a non-Ethernet interface type, such as a serial link, and asked about OSPF compatibility, then this option could be correct if the context implied that OSPF was not supported on that specific interface type.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The OSPF areas do not match on the shared segment.Correct answer▾
Why this is correct
This is correct because the same link is placed into area 0 on one side and area 1 on the other.
✗The routers must use the same process ID.Wrong answer — click to see why▾
Why this is wrong here
OSPF process IDs are locally significant to each router; matching process IDs between neighbors is not a requirement for adjacency.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that the routers are configured with multiple OSPF processes and the exam asks about the necessity of matching process IDs for adjacency, this option would be correct. For example, if the question involved multiple OSPF instances on the same router, it would be relevant.
Why candidates choose this
Candidates may find this option tempting because they might confuse OSPF process IDs with the requirement for matching OSPF configurations, leading them to believe that process IDs play a critical role in forming adjacencies.
✗The interfaces are in different IPv4 subnets.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because OSPF can form adjacencies between routers as long as their interfaces are in the same IPv4 subnet, regardless of the OSPF process IDs. The issue in this scenario is related to OSPF area mismatches, not subnet differences.
★ When this WOULD be the correct answer
In a different question setup, if the exam asked about OSPF adjacency issues where the interfaces of R1 and R2 are indeed in different IPv4 subnets, this option would be correct. For example, if the question specified that R1's interface is in 192.168.1.0/24 and R2's interface is in 192.168.2.0/24, then this option would apply.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of OSPF requirements, believing that adjacency cannot be formed if routers are on different subnets, which is a common misconception.
✗OSPF cannot run on Ethernet interfaces.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF can indeed run on Ethernet interfaces; it is a common practice in networking. The issue with R1 and R2 not forming an adjacency is likely due to mismatched OSPF areas, not the interface type.
★ When this WOULD be the correct answer
In a different exam scenario, if the question stated that R1 and R2 were configured on a non-Ethernet interface type, such as a serial link, and asked about OSPF compatibility, then this option could be correct if the context implied that OSPF was not supported on that specific interface type.
Why candidates choose this
Candidates may find this option tempting because they might recall that certain routing protocols have limitations on specific interface types, leading to confusion about OSPF's compatibility with Ethernet.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common mistake is believing that OSPF process IDs must match between routers to form an adjacency; process IDs are locally significant and do not need to match.
Detailed technical explanation
How to think about this question
Open Shortest Path First (OSPF) is a link-state routing protocol that forms neighbor adjacencies between routers on shared network segments. For two routers to establish an OSPF adjacency, they must agree on several parameters, including the subnet, hello and dead intervals, authentication, and crucially, the OSPF area assigned to the interface. The OSPF area is a logical grouping that segments the network to optimize routing and reduce overhead. Routers on the same physical segment must place that interface in the same OSPF area to exchange link-state advertisements and form a neighbor relationship.
The OSPF adjacency formation process involves routers exchanging hello packets to discover neighbors and negotiate parameters. If the routers detect a mismatch in the OSPF area ID on their connected interfaces, they will not progress beyond the initial hello exchange, preventing adjacency formation. This area mismatch acts as a fundamental barrier because OSPF treats each area as a separate routing domain, and adjacency is only valid within the same area. Cisco IOS requires exact area ID matches on both ends of a link for adjacency to form.
A common exam trap is to overlook the OSPF area mismatch and instead focus on other parameters like process IDs or IP subnet mismatches. While process IDs are locally significant and can differ without affecting adjacency, the area ID must match exactly. In practical network troubleshooting, an area mismatch can cause silent failures where routers appear connected at Layer 2 and share IP subnets but never become OSPF neighbors. Recognizing this subtle but critical configuration error is essential for both exam success and real-world OSPF troubleshooting.
KKey Concepts to Remember
OSPF routers must assign the same area ID to interfaces on a shared segment to form neighbor adjacencies successfully.
OSPF process IDs are local to each router and do not need to match for adjacency to form between neighbors.
OSPF adjacency formation requires matching hello and dead intervals, subnet, authentication, and area ID on connected interfaces.
Routers on the same IP subnet but with different OSPF area assignments will not establish an OSPF neighbor relationship.
An OSPF area mismatch prevents the exchange of link-state advertisements, blocking routing updates between neighbors.
OSPF adjacency failures due to area mismatches often appear as silent failures with no neighbor state progression beyond 'Init' or '2-Way'.
Correct OSPF configuration requires consistent area assignment on both ends of a link to ensure proper routing domain segmentation.
Exam candidates should prioritize verifying OSPF area consistency before troubleshooting other parameters like process ID or interface type.
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
OSPF routers must assign the same area ID to interfaces on a shared segment to form neighbor adjacencies successfully.
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 200-301 question in full detail.
Review oSPF routers must assign the same area ID to interfaces on a shared segment to form neighbor adjacencies successfully., then practise related 200-301 questions on the same topic to reinforce the concept.
IP Routing — This question tests IP Routing — OSPF routers must assign the same area ID to interfaces on a shared segment to form neighbor adjacencies successfully..
What is the correct answer to this question?
The correct answer is: The OSPF areas do not match on the shared segment. — The most likely cause is an OSPF area mismatch on the shared segment. R1 and R2 are in the same IP subnet and use default hello/dead timers, so neither subnet mismatch nor timer mismatch is the problem. OSPF can run on Ethernet interfaces. However, OSPF requires neighbors on the same link to agree on the area ID; here they differ, preventing adjacency. Option B is incorrect because OSPF process IDs are locally significant and do not need to match.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must assign the same area ID to interfaces on a shared segment to form neighbor adjacencies successfully., then practise related 200-301 questions on the same topic to reinforce the concept.
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?
OSPF routers must assign the same area ID to interfaces on a shared segment to form neighbor adjacencies successfully.
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