The answer is an OSPF Hello/Dead timer mismatch. This is correct because OSPF requires that neighboring routers agree on key interface parameters before forming an adjacency, and the Hello and Dead intervals are among the most critical. If one router advertises a Hello interval of 10 seconds with a Dead interval of 40 seconds, while the other uses 5 and 20 seconds respectively, the routers will reject each other’s Hello packets and never become neighbors, even though they can ping each other. On the CCNA 200-301 v2 exam, this scenario tests your ability to look beyond basic IP connectivity and verify OSPF-specific parameters; it is a classic trap because the interfaces appear otherwise healthy. A reliable memory tip is “Hello must match, or the neighbor won’t hatch”—always check the timers first when OSPF neighbors fail to form on a directly connected link.
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 match hello and dead interval timers on their interfaces 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.
Exhibit
R1#
interface GigabitEthernet0/0
ip address 10.10.12.1 255.255.255.0
ip ospf hello-interval 10
ip ospf dead-interval 40
!
router ospf 1
network 10.10.12.0 0.0.0.255 area 0
R2#
interface GigabitEthernet0/0
ip address 10.10.12.2 255.255.255.0
ip ospf hello-interval 5
ip ospf dead-interval 20
!
router ospf 1
network 10.10.12.0 0.0.0.255 area 0
A network engineer configures OSPF between R1 and R2, but the routers never become neighbors on GigabitEthernet0/0. Based on the exhibit, 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.
Clue: "never"
Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The OSPF interface timers do not match.
The most likely cause is a hello/dead timer mismatch. In practical terms, both routers are on the same subnet and both are trying to run OSPF in area 0, so the obvious items look correct. But OSPF does not form adjacency based on IP reachability alone. It also checks whether key interface parameters agree. Hello and dead intervals are among those required parameters.
On R1, the hello interval is 10 seconds and the dead interval is 40 seconds. On R2, the hello interval is 5 seconds and the dead interval is 20 seconds. That mismatch is enough to prevent the neighbor relationship from forming. This is a common CCNA troubleshooting pattern because the interfaces can still ping each other, which makes the failure look less obvious at first glance.
Key principle: OSPF routers must match hello and dead interval timers on their interfaces 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 routers are using different OSPF areas.
Why it's wrong here
This is wrong because both routers place the shared subnet into area 0.
When this WOULD be correct
In a different scenario, if the question specified that both routers must be in the same OSPF area for neighbor adjacency and the configurations showed different area IDs, then this option would be correct. For example, if R1 is in area 0 and R2 is in area 1, they would not become neighbors.
✓
The OSPF interface timers do not match.
Why this is correct
This is correct because OSPF neighbors on the same segment must agree on hello/dead intervals.
Clue confirmation
The clue words "most likely", "never" in the question point toward this answer.
Related concept
OSPF routers must match hello and dead interval timers on their interfaces to successfully form neighbor adjacencies.
✗
The routers must use the same hostname to become neighbors.
Why it's wrong here
This is wrong because hostnames do not affect OSPF neighbor formation.
When this WOULD be correct
In a different exam scenario, if the question specified that OSPF neighbor relationships depend on hostname resolution via DNS and that both routers are configured to use hostnames instead of IP addresses, then this option would be correct.
✗
The subnet mask prevents multicast OSPF traffic.
Why it's wrong here
This is wrong because a /24 mask does not prevent normal OSPF multicast behavior here.
When this WOULD be correct
In a different scenario where the question specifies that the routers are configured with different subnet masks that create separate broadcast domains, this option would be correct. For example, if R1 is configured with a /24 mask and R2 with a /30 mask, OSPF would fail to establish neighbors due to the inability to reach each other.
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 interface timers do not match.Correct answer▾
Why this is correct
This is correct because OSPF neighbors on the same segment must agree on hello/dead intervals.
✗The routers are using different OSPF areas.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF routers can still form neighbor relationships even if they are in different areas, as long as they are configured to communicate across area boundaries. The issue in the question is likely related to OSPF timers or other configuration mismatches.
★ When this WOULD be the correct answer
In a different scenario, if the question specified that both routers must be in the same OSPF area for neighbor adjacency and the configurations showed different area IDs, then this option would be correct. For example, if R1 is in area 0 and R2 is in area 1, they would not become neighbors.
Why candidates choose this
Candidates may mistakenly believe that OSPF requires routers to be in the same area to establish neighbor relationships, leading them to choose this option due to a misunderstanding of OSPF area functionality.
✗The routers must use the same hostname to become neighbors.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because OSPF does not require routers to have the same hostname to establish neighbor relationships; it relies on IP addresses and OSPF configurations instead.
★ When this WOULD be the correct answer
In a different exam scenario, if the question specified that OSPF neighbor relationships depend on hostname resolution via DNS and that both routers are configured to use hostnames instead of IP addresses, then this option would be correct.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of OSPF requirements, mistakenly believing that hostname consistency is necessary for neighbor formation, especially if they confuse OSPF with other protocols that may have such requirements.
✗The subnet mask prevents multicast OSPF traffic.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF can still form neighbors even if the subnet mask is not the same, as long as the interfaces are in the same broadcast domain and can reach each other. The issue in this scenario is related to OSPF timers, not the subnet mask.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that the routers are configured with different subnet masks that create separate broadcast domains, this option would be correct. For example, if R1 is configured with a /24 mask and R2 with a /30 mask, OSPF would fail to establish neighbors due to the inability to reach each other.
Why candidates choose this
Candidates might choose this option due to a common misconception that OSPF requires identical subnet masks for neighbor relationships, leading them to overlook the actual cause of the issue in the given context.
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 frequent exam trap is assuming that OSPF adjacency forms as long as routers are in the same area and can ping each other. Candidates often overlook the necessity for matching hello and dead intervals, which are critical for neighbor discovery and maintenance. Because routers can still exchange ICMP packets, it may appear that the link is fully operational, leading to the mistaken belief that OSPF should work. This misunderstanding causes candidates to select incorrect answers related to area mismatches or subnet masks, ignoring the timer mismatch that actually prevents adjacency.
Detailed technical explanation
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that establishes neighbor relationships between routers on a common network segment. These relationships are formed through the exchange of hello packets, which are multicast messages sent at regular intervals defined by the hello timer. The dead interval timer defines how long a router waits without receiving a hello before declaring the neighbor down. Both timers must match on connected interfaces for OSPF to form an adjacency.
When two routers attempt to become OSPF neighbors, they first verify that they share the same area ID and subnet mask, ensuring they are in the same routing domain and network segment. Next, they compare hello and dead intervals; if these timers differ, the routers will not recognize each other as valid neighbors. This timer mismatch causes hello packets to be ignored, preventing the routers from progressing beyond the initial discovery phase.
A common exam trap is assuming that IP connectivity or matching area IDs alone guarantee OSPF adjacency. In reality, OSPF requires strict parameter consistency, including timer values. Even if routers can ping each other, a hello/dead interval mismatch will block neighbor formation. This subtlety often confuses candidates because basic IP reachability tests succeed, masking the underlying protocol disagreement.
KKey Concepts to Remember
OSPF routers must match hello and dead interval timers on their interfaces to successfully form neighbor adjacencies.
OSPF hello packets are multicast messages used to discover and maintain neighbor relationships on a shared network segment.
OSPF area IDs must be consistent between routers on the same link to ensure proper adjacency formation and routing information exchange.
OSPF neighbor relationships do not depend on router hostnames; hostnames are local identifiers and do not affect protocol operation.
OSPF uses multicast addresses 224.0.0.5 and 224.0.0.6 to send hello and database description packets, which require proper subnet masks to function.
Mismatch in OSPF interface timers causes routers to reject each other's hello packets, preventing adjacency despite IP connectivity.
OSPF adjacency formation requires agreement on several parameters including area ID, hello/dead intervals, authentication, and subnet mask.
OSPF routers can ping each other but still fail to form neighbor relationships if key protocol parameters like timers do not match.
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 match hello and dead interval timers on their interfaces 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 match hello and dead interval timers on their interfaces 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 match hello and dead interval timers on their interfaces to successfully form neighbor adjacencies..
What is the correct answer to this question?
The correct answer is: The OSPF interface timers do not match. — The most likely cause is a hello/dead timer mismatch. In practical terms, both routers are on the same subnet and both are trying to run OSPF in area 0, so the obvious items look correct. But OSPF does not form adjacency based on IP reachability alone. It also checks whether key interface parameters agree. Hello and dead intervals are among those required parameters.
On R1, the hello interval is 10 seconds and the dead interval is 40 seconds. On R2, the hello interval is 5 seconds and the dead interval is 20 seconds. That mismatch is enough to prevent the neighbor relationship from forming. This is a common CCNA troubleshooting pattern because the interfaces can still ping each other, which makes the failure look less obvious at first glance.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must match hello and dead interval timers on their interfaces 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", "never". 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 match hello and dead interval timers on their interfaces to successfully form neighbor adjacencies.
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