The answer is an OSPF hello timer mismatch, because two routers on the same subnet must agree on hello and dead interval values to form an adjacency. OSPF uses these timers to maintain neighbor relationships; if one router sends hellos every 5 seconds while the other expects them every 10 seconds, they will not recognize each other as valid neighbors, causing the adjacency to fail. On the CCNA 200-301 v2 exam, this scenario tests your understanding that OSPF adjacency requires matching parameters beyond just IP connectivity—a common trap is assuming ping success guarantees OSPF neighbor formation. Remember that mismatched timers prevent the routers from reaching the 2-Way state, even if they share the same area and subnet. A helpful memory tip: "Hello must harmonize, or the adjacency dies."
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 routers must have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies.. 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.
Two routers are in the same OSPF area and on the same subnet, but they do not form an adjacency. One interface uses a hello interval of 10 seconds and the other uses 5 seconds. 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
✓
The OSPF hello timers do not match.
An OSPF timer mismatch is the most likely cause. In plain language, the routers are trying to discover each other, but they are speaking on different timing expectations. OSPF neighbors on the same segment must agree on certain parameters before they trust each other enough to form an adjacency, and the hello and dead timers are among those critical values.
This is a very common OSPF troubleshooting issue because the interfaces may still be reachable with ping and may even appear correctly addressed and placed in the same area. But OSPF is stricter than simple IP reachability. If the hello interval does not match, the adjacency usually fails before it becomes fully operational.
Key principle: OSPF routers must have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies.
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 hello timers do not match.
Why this is correct
This is correct because OSPF neighbors must agree on hello timing in order to form an adjacency.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies.
✗
The routers need matching hostnames.
Why it's wrong here
This is wrong because hostnames do not determine OSPF adjacency formation.
When this WOULD be correct
In a different scenario where the question asks about OSPF configuration issues related to hostname resolution, a candidate might be asked why two routers with mismatched hostnames cannot communicate properly. In that case, if hostname resolution is a factor, this option could be correct.
✗
The subnet must be changed to /24 before OSPF can work.
Why it's wrong here
This is wrong because OSPF can operate on many subnet sizes; the issue described is timer mismatch.
When this WOULD be correct
In a different scenario where the question states that two routers are on different subnets, and OSPF is configured, a candidate might choose this option if they believe that changing the subnet to /24 would allow the routers to communicate and form an adjacency.
✗
The routers must both use static routes first.
Why it's wrong here
This is wrong because static routes are not required for OSPF adjacency formation.
When this WOULD be correct
In a different scenario, if a question states that two routers are configured to use OSPF but are unable to establish an adjacency due to missing static routes that are required for initial connectivity, then 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.
✓The OSPF hello timers do not match.Correct answer▾
Why this is correct
This is correct because OSPF neighbors must agree on hello timing in order to form an adjacency.
✗The routers need matching hostnames.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF does not require matching hostnames for routers to form an adjacency; it relies on matching OSPF parameters like hello and dead intervals.
★ When this WOULD be the correct answer
In a different scenario where the question asks about OSPF configuration issues related to hostname resolution, a candidate might be asked why two routers with mismatched hostnames cannot communicate properly. In that case, if hostname resolution is a factor, this option could be correct.
Why candidates choose this
Candidates may mistakenly believe that hostnames play a critical role in OSPF adjacency formation, especially if they are familiar with other protocols where hostnames are significant for communication.
✗The subnet must be changed to /24 before OSPF can work.Wrong answer — click to see why▾
Why this is wrong here
The subnet mask does not affect OSPF adjacency formation as long as both routers are in the same subnet. OSPF can operate correctly with any valid subnet mask that includes both routers.
★ When this WOULD be the correct answer
In a different scenario where the question states that two routers are on different subnets, and OSPF is configured, a candidate might choose this option if they believe that changing the subnet to /24 would allow the routers to communicate and form an adjacency.
Why candidates choose this
Candidates may mistakenly believe that subnet configuration is critical for OSPF adjacency, leading them to choose this option due to a lack of understanding of OSPF's requirements for adjacency beyond subnetting.
✗The routers must both use static routes first.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because OSPF can operate without static routes; adjacency issues are typically related to OSPF configuration parameters rather than routing methods.
★ When this WOULD be the correct answer
In a different scenario, if a question states that two routers are configured to use OSPF but are unable to establish an adjacency due to missing static routes that are required for initial connectivity, then this option would be correct.
Why candidates choose this
Candidates might choose this option due to a misunderstanding of OSPF's requirements, mistakenly believing that static routes are necessary for OSPF to function properly in all cases.
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 exam trap is assuming that because two routers can ping each other on the same subnet, their OSPF adjacency will form automatically. The trap lies in overlooking the importance of matching OSPF hello and dead timers. Many candidates focus only on IP addressing or area configuration and miss that OSPF requires exact timer agreement. This leads to confusion when adjacency never forms despite correct IP setup. The exam tests your understanding that OSPF is a protocol with strict neighbor parameter requirements, not just IP reachability.
Detailed technical explanation
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that relies on periodic hello packets to discover and maintain neighbor relationships on a shared network segment. Each router interface configured for OSPF sends hello packets at a configured hello interval, and neighbors must agree on this interval to recognize each other as valid peers. The hello interval controls how often these packets are sent, and the dead interval defines how long a router waits without receiving a hello before declaring the neighbor down.
For two OSPF routers on the same subnet and area to form an adjacency, they must match several parameters exactly, including hello and dead intervals, area ID, subnet mask, and authentication if configured. If the hello intervals differ, one router will send hellos at a different rate than the other expects, causing the adjacency process to fail. This mismatch prevents the routers from establishing a stable neighbor relationship, even though basic IP connectivity like ping may still work.
This scenario is a classic exam trap because it is easy to assume that IP reachability guarantees OSPF adjacency. However, OSPF requires strict parameter matching beyond just IP addressing. The practical impact is that network engineers must verify and align OSPF timers on all routers in the same area and subnet to ensure proper adjacency formation and routing table synchronization. Ignoring timer mismatches can lead to persistent routing issues despite apparent network connectivity.
KKey Concepts to Remember
OSPF routers must have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies.
OSPF hello packets are sent periodically to discover and maintain neighbor relationships, and mismatched timers cause adjacency failures despite IP reachability.
OSPF adjacency formation requires agreement on several parameters including hello interval, dead interval, area ID, subnet mask, and authentication settings.
OSPF uses hello intervals to detect neighbor liveness, and if one router’s hello interval differs, it will not recognize the other as a valid neighbor.
OSPF adjacency failures due to timer mismatches are common troubleshooting issues because interfaces may still ping but never fully synchronize routing information.
Hostname matching is not required for OSPF adjacency; OSPF neighbors identify each other by router IDs and interface parameters, not device names.
OSPF does not require static routes to form adjacencies; it dynamically discovers neighbors using hello packets and forms adjacencies based on protocol parameters.
OSPF supports multiple subnet masks and does not require a /24 subnet; adjacency depends on matching network parameters, not specific subnet sizes.
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 have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies.
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 have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies., then practise related 200-301 questions on the same topic to reinforce the concept.
IP Routing — This question tests IP Routing — OSPF routers must have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies..
What is the correct answer to this question?
The correct answer is: The OSPF hello timers do not match. — An OSPF timer mismatch is the most likely cause. In plain language, the routers are trying to discover each other, but they are speaking on different timing expectations. OSPF neighbors on the same segment must agree on certain parameters before they trust each other enough to form an adjacency, and the hello and dead timers are among those critical values.
This is a very common OSPF troubleshooting issue because the interfaces may still be reachable with ping and may even appear correctly addressed and placed in the same area. But OSPF is stricter than simple IP reachability. If the hello interval does not match, the adjacency usually fails before it becomes fully operational.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies., 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 have matching hello and dead intervals on interfaces within the same area and subnet to successfully form neighbor adjacencies.
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