CCNA IP Routing Practice Question
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?
⚠ Common exam trap
Be cautious of confusing OSPF timer mismatches with other common OSPF configuration issues like network types or authentication.
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.
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.
- ✗
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
There is no OSPF rule that makes a subnet mask invalid on a transit link; any properly configured mask is acceptable for an interface used as a transit network. The symptoms in the question point to mismatched OSPF hello and dead timers, which prevent neighbor adjacency regardless of the mask. A mask issue would manifest as an OSPF network type or area mismatch, not the timer mismatch described here.
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?”
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
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Key term
OSPF
OSPF is a link-state routing protocol that uses the SPF algorithm to compute the shortest path to each destination within a single autonomous system.
Key term
OSPF
OSPF is a link-state routing protocol used to find the best path for data packets to travel across IP networks, like a smart GPS that recalculates routes when traffic changes.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 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 200-301 exam.