- A
Router IDs are identical
Why wrong: Duplicate router IDs can create OSPF problems, but the exhibit does not point to that issue. The detail the question emphasizes is the mismatch in hello and dead timer values. On exam-style troubleshooting questions, the best answer is usually the one directly supported by the data provided.
- B
OSPF timers do not match
Correct. This is correct. OSPF neighbors must agree on timer values such as hello and dead intervals. If they do not, the devices will not form a full adjacency even if other parts of the link appear healthy.
- C
The interface cost is too low
Why wrong: OSPF cost affects route preference after the adjacency has already formed. It does not normally stop the neighbor relationship from coming up in the first place. So even though cost is an OSPF concept, it is not the cause that best matches the symptoms shown here.
- D
The subnet mask is invalid because it is a transit link
Why wrong: Transit links can absolutely use ordinary subnet masks, and the problem described is not about the mask choice. The key mismatch shown is the different OSPF timer configuration between the two routers.
Quick Answer
The answer is an OSPF timer mismatch. This adjacency failure occurs because OSPF neighbors on the same segment must agree on both the Hello and Dead intervals before they can form a neighbor relationship; if one router sends Hellos every 10 seconds expecting a Dead interval of 40, but the neighbor uses 30 and 120, neither router will recognize the other’s Hellos as valid, so the adjacency never establishes. On the CCNA 200-301 v2 exam, this is a classic troubleshooting scenario that tests your understanding of the OSPF neighbor state machine—specifically that timer agreement is a prerequisite for moving past the INIT state, while metrics like cost only affect routing decisions after adjacency is formed. A common trap is assuming that because both interfaces are up and IP connectivity exists, OSPF should work, but the mismatch silently blocks neighbor formation. Memory tip: think of it as two dancers expecting different tempos—they can see each other, but they’ll never synchronize.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: oSPF neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment.. 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 router interface is configured for OSPF, but neighbors do not form. The engineer checks the interface and sees Hello 10 and Dead 40. The neighbor on the same segment uses Hello 30 and Dead 120.
What is the most likely cause of the OSPF adjacency failure?
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
OSPF timers do not match
The adjacency fails because the OSPF hello and dead timers do not match. OSPF neighbors on the same segment must agree on several key parameters before they accept each other as neighbors, and the timer settings are one of those required matches. In plain terms, each router is expecting to hear OSPF hellos at one rhythm, but the other side is speaking at a different rhythm, so the relationship never becomes valid. This is different from cost, which matters later during path selection after neighbors are already exchanging information. The timer mismatch is a classic CCNA troubleshooting point because the interfaces can be up and IP connectivity can even appear normal while the OSPF adjacency still fails.
Key principle: OSPF neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Router IDs are identical
Why it's wrong here
Duplicate router IDs can create OSPF problems, but the exhibit does not point to that issue. The detail the question emphasizes is the mismatch in hello and dead timer values. On exam-style troubleshooting questions, the best answer is usually the one directly supported by the data provided.
When this WOULD be correct
In a different question, if two routers on the same OSPF segment are configured with identical Router IDs, the exam could ask about the implications of this configuration. In that case, the answer would be correct as identical Router IDs can prevent OSPF adjacencies from forming.
- ✓
OSPF timers do not match
Why this is correct
Correct. This is correct. OSPF neighbors must agree on timer values such as hello and dead intervals. If they do not, the devices will not form a full adjacency even if other parts of the link appear healthy.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment.
- ✗
The interface cost is too low
Why it's wrong here
OSPF cost affects route preference after the adjacency has already formed. It does not normally stop the neighbor relationship from coming up in the first place. So even though cost is an OSPF concept, it is not the cause that best matches the symptoms shown here.
When this WOULD be correct
In a different scenario, if a question stated that two routers were unable to establish OSPF adjacency due to mismatched interface costs, it could be correct if the cost was configured incorrectly on one of the routers, preventing it from being selected as the best route.
- ✗
The subnet mask is invalid because it is a transit link
Why it's wrong here
Transit links can absolutely use ordinary subnet masks, and the problem described is not about the mask choice. The key mismatch shown is the different OSPF timer configuration between the two routers.
When this WOULD be correct
In a different scenario, if a question states that two routers are configured with OSPF on a transit link but have mismatched subnet masks, this could lead to adjacency failures. For example, if one router is configured with a /30 mask and the other with a /24 mask, this option would be correct.
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.
✓OSPF timers do not matchCorrect answer▾
Why this is correct
Correct. This is correct. OSPF neighbors must agree on timer values such as hello and dead intervals. If they do not, the devices will not form a full adjacency even if other parts of the link appear healthy.
✗Router IDs are identicalWrong answer — click to see why▾
Why this is wrong here
This option is wrong because OSPF allows multiple routers to have the same Router ID, but they can still form adjacencies if other parameters are correctly configured. The issue in this scenario is related to mismatched OSPF timers, not Router IDs.
★ When this WOULD be the correct answer
In a different question, if two routers on the same OSPF segment are configured with identical Router IDs, the exam could ask about the implications of this configuration. In that case, the answer would be correct as identical Router IDs can prevent OSPF adjacencies from forming.
Why candidates choose this
Candidates may choose this option due to a common misconception that Router IDs must be unique for OSPF operations, leading them to overlook the actual cause of adjacency issues related to timer mismatches.
✗The interface cost is too lowWrong answer — click to see why▾
Why this is wrong here
This option is incorrect because the interface cost does not directly affect the formation of OSPF adjacencies; it only influences the route selection process once adjacencies are established.
★ When this WOULD be the correct answer
In a different scenario, if a question stated that two routers were unable to establish OSPF adjacency due to mismatched interface costs, it could be correct if the cost was configured incorrectly on one of the routers, preventing it from being selected as the best route.
Why candidates choose this
Candidates might choose this option because they understand that interface cost impacts routing decisions and may mistakenly believe it also affects adjacency formation, leading to confusion about the OSPF process.
✗The subnet mask is invalid because it is a transit linkWrong answer — click to see why▾
Why this is wrong here
The subnet mask being invalid is not the cause of the OSPF adjacency failure in this scenario, as the issue lies with the mismatch of OSPF timers rather than an incorrect subnet mask. Both routers are on the same segment and should have compatible subnet configurations.
★ When this WOULD be the correct answer
In a different scenario, if a question states that two routers are configured with OSPF on a transit link but have mismatched subnet masks, this could lead to adjacency failures. For example, if one router is configured with a /30 mask and the other with a /24 mask, this option would be correct.
Why candidates choose this
Candidates may choose this option because they understand that subnet mask mismatches can cause connectivity issues, leading them to incorrectly associate it with OSPF adjacency problems without considering the specific timer mismatch in this case.
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
Be cautious of confusing OSPF timer mismatches with other common OSPF configuration issues like network types or authentication.
Trap categories for this question
Command / output trap
OSPF cost affects route preference after the adjacency has already formed. It does not normally stop the neighbor relationship from coming up in the first place. So even though cost is an OSPF concept, it is not the cause that best matches the symptoms shown here.
Detailed technical explanation
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that relies on neighbor relationships to exchange routing information. These neighbor relationships form through a process involving Hello packets, which are sent at regular intervals defined by the Hello timer. The Dead timer defines how long a router waits without receiving Hello packets before declaring a neighbor down. Both timers must match between neighbors on the same segment for adjacency to form successfully. OSPF requires that neighbors agree on several parameters, including Hello and Dead intervals, area ID, authentication, and subnet mask. If any of these parameters differ, the routers will not establish a full adjacency. The Hello timer controls how often Hello packets are sent, while the Dead timer determines the timeout period. A mismatch in these timers causes routers to expect different Hello packet intervals, preventing them from recognizing each other as neighbors. A common exam trap is confusing OSPF cost or router ID issues with adjacency failures caused by timer mismatches. While cost affects route selection after adjacency, and router ID conflicts can cause instability, the specific symptom of mismatched Hello and Dead timers directly prevents adjacency formation. In practical networks, this mismatch can cause interfaces to appear up with IP connectivity intact, yet OSPF neighbors never form, making timer synchronization critical for OSPF troubleshooting.
KKey Concepts to Remember
- OSPF neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment.
- The Hello timer controls how frequently OSPF Hello packets are sent to discover and maintain neighbor relationships.
- The Dead timer defines how long a router waits without receiving Hello packets before declaring a neighbor down.
- OSPF adjacency formation requires agreement on several parameters including timers, area ID, authentication, and subnet mask.
- A mismatch in OSPF Hello or Dead timers causes routers to expect different Hello intervals, preventing neighbor recognition.
- OSPF cost influences route selection after adjacency formation and does not affect the ability to establish neighbor relationships.
- Router ID conflicts can cause OSPF instability but do not directly prevent adjacency formation caused by timer mismatches.
- Proper synchronization of OSPF timers is critical for adjacency formation even when IP connectivity and interfaces appear operational.
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 neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment.
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.
Review oSPF neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment., then practise related 200-301 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this 200-301 question test?
IP Routing — This question tests IP Routing — OSPF neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment..
What is the correct answer to this question?
The correct answer is: OSPF timers do not match — The adjacency fails because the OSPF hello and dead timers do not match. OSPF neighbors on the same segment must agree on several key parameters before they accept each other as neighbors, and the timer settings are one of those required matches. In plain terms, each router is expecting to hear OSPF hellos at one rhythm, but the other side is speaking at a different rhythm, so the relationship never becomes valid. This is different from cost, which matters later during path selection after neighbors are already exchanging information. The timer mismatch is a classic CCNA troubleshooting point because the interfaces can be up and IP connectivity can even appear normal while the OSPF adjacency still fails.
What should I do if I get this 200-301 question wrong?
Review oSPF neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment., 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 neighbors must have matching Hello and Dead timer values to successfully form an adjacency on the same network segment.
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Last reviewed: May 17, 2026
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