The answer is a subnet mask mismatch on the shared link. This is correct because OSPF requires that neighboring routers agree on the subnet mask of the connecting interface for the network type to be considered the same subnet; on a broadcast multi-access segment like GigabitEthernet, a mismatch causes each router to calculate a different network address, preventing the Hello packets from being accepted and blocking adjacency formation. On the CCNA 200-301 v2 exam, this concept tests your understanding of OSPF neighbor requirements and Layer 3 addressing fundamentals—a common trap is to focus on IP address conflicts or mismatched OSPF areas, but the mask mismatch is a subtle yet frequent cause of failure. Remember the memory tip: “Mask must match to make the adjacency hatch,” ensuring both routers see the same network boundary on the wire.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. A key principle to apply: oSPF routers must have matching IP subnet masks on shared interfaces 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 G0/1: 10.1.12.1 255.255.255.0
R2 G0/1: 10.1.12.2 255.255.255.252
Both interfaces are up and in area 0
R1 is not forming an OSPF adjacency with R2 on GigabitEthernet0/1. Which mismatch below 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 routers are using different IP subnet masks on the shared link
For OSPF neighbors to form, key settings on a shared segment must match. A subnet mask mismatch on a broadcast network often prevents proper neighbor formation because the routers do not agree on the local network.
Key principle: OSPF routers must have matching IP subnet masks on shared interfaces 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 routers are using different IP subnet masks on the shared link
Why this is correct
That mismatch can stop OSPF adjacency on the segment.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must have matching IP subnet masks on shared interfaces to form neighbor adjacencies successfully.
✗
OSPF cannot run on GigabitEthernet interfaces
Why it's wrong here
It absolutely can.
When this WOULD be correct
In a different question, if it were stated that OSPF is being configured on a legacy interface type that does not support OSPF, such as a serial interface with specific limitations, then this option could be correct. For example, if the question specified that OSPF was being attempted on a non-supported interface type, this would be the right answer.
✗
Both routers are in area 0
Why it's wrong here
That part is correct, not the problem.
When this WOULD be correct
In a different scenario, if the question stated that both routers were configured to use different OSPF versions or protocols that are not supported on GigabitEthernet interfaces, then this option could be correct. For example, if one router was using a legacy protocol incompatible with GigabitEthernet, it would prevent OSPF from forming an adjacency.
In a different scenario where the question specifies that both routers are configured to use the same router ID for redundancy or load balancing, and they are incorrectly set to the same value, this option would be correct. For example, if the question states that both routers are configured in a way that they must share the same router ID for a specific OSPF design.
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 routers are using different IP subnet masks on the shared linkCorrect answer▾
Why this is correct
That mismatch can stop OSPF adjacency on the segment.
✗OSPF cannot run on GigabitEthernet interfacesWrong answer — click to see why▾
Why this is wrong here
OSPF fully supports GigabitEthernet interfaces; it can run on any Ethernet interface type, including FastEthernet, GigabitEthernet, and TenGigabitEthernet. The interface speed does not affect OSPF's ability to form an adjacency.
★ When this WOULD be the correct answer
In a different question, if it were stated that OSPF is being configured on a legacy interface type that does not support OSPF, such as a serial interface with specific limitations, then this option could be correct. For example, if the question specified that OSPF was being attempted on a non-supported interface type, this would be the right answer.
Why candidates choose this
Students might confuse OSPF's network type (broadcast vs. point-to-point) with interface speed, or incorrectly think that OSPF has limitations on high-speed interfaces.
✗Both routers are in area 0Wrong answer — click to see why▾
Why this is wrong here
Both routers being in area 0 is correct for forming an adjacency; OSPF requires that routers on the same link share the same area ID. This option describes a correct configuration, not a problem.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that both routers were configured to use different OSPF versions or protocols that are not supported on GigabitEthernet interfaces, then this option could be correct. For example, if one router was using a legacy protocol incompatible with GigabitEthernet, it would prevent OSPF from forming an adjacency.
Why candidates choose this
A test-taker might think that area 0 is reserved for backbone only and that other routers cannot be in area 0, but in fact, any router can be in area 0 as long as it is contiguous.
✗The router IDs must match for adjacency to formWrong answer — click to see why▾
Why this is wrong here
OSPF requires that router IDs be unique within the OSPF domain; identical router IDs would cause conflicts and prevent adjacency. However, the question states that adjacency is not forming, and identical router IDs would be a cause, but the option says 'must match' which is incorrect—they must be unique.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that both routers are configured to use the same router ID for redundancy or load balancing, and they are incorrectly set to the same value, this option would be correct. For example, if the question states that both routers are configured in a way that they must share the same router ID for a specific OSPF design.
Why candidates choose this
Students may confuse the concept of matching parameters (like area ID, subnet mask) with router IDs, thinking that router IDs need to match for adjacency, when in fact they must be unique.
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 confusing the need for matching router IDs with adjacency formation. While router IDs must be unique within an OSPF domain, they do not need to match for adjacency to form. Another trap is assuming OSPF cannot run on GigabitEthernet interfaces, which is incorrect because OSPF supports all standard interface types. The most tempting mistake is overlooking the subnet mask mismatch, which silently prevents adjacency by making routers believe they are on different networks, even if other parameters like area ID match. This subtle misconfiguration is often missed during troubleshooting.
Detailed technical explanation
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that requires neighbors on a shared network segment to agree on several key parameters before forming an adjacency. One critical parameter is the IP subnet mask on the interface connecting the routers. Both routers must have interfaces configured within the same IP subnet and mask to recognize each other as neighbors and exchange OSPF Hello packets successfully. If the subnet masks differ, the routers perceive the link as belonging to different networks, preventing neighbor discovery and adjacency formation.
The OSPF adjacency formation process involves routers exchanging Hello packets on interfaces configured for OSPF. These Hello packets include network information such as the subnet mask, area ID, and Hello/Dead intervals. If any of these parameters mismatch, including the subnet mask, the routers will not establish adjacency. Cisco IOS enforces this strictly because OSPF uses the subnet mask to determine the network boundary. Therefore, a subnet mask mismatch on a shared Ethernet segment is a common cause of adjacency failure.
A frequent exam trap is assuming that router IDs must match for adjacency, which is incorrect since router IDs must be unique to identify routers distinctly. Another trap is thinking OSPF cannot run on GigabitEthernet interfaces, which is false as OSPF supports all common interface types. In practical networks, subnet mask mismatches often occur due to manual misconfiguration or inconsistent addressing schemes, leading to silent adjacency failures that can disrupt routing convergence and network stability.
KKey Concepts to Remember
OSPF routers must have matching IP subnet masks on shared interfaces to form neighbor adjacencies successfully.
OSPF Hello packets include subnet mask information that routers use to verify network consistency before adjacency formation.
A subnet mask mismatch causes routers to perceive different networks, preventing OSPF neighbor discovery on the shared link.
OSPF requires unique router IDs for each router; identical router IDs prevent proper OSPF operation but do not cause adjacency failure due to mask mismatch.
OSPF can run on GigabitEthernet interfaces without restriction, supporting high-speed Ethernet links in routing domains.
OSPF adjacency formation depends on matching parameters such as area ID, Hello/Dead intervals, and subnet masks on the shared segment.
A subnet mask mismatch is a common cause of OSPF adjacency failure on broadcast networks like Ethernet.
Routers in the same OSPF area with consistent subnet masks and interface configurations will successfully form adjacencies.
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 IP subnet masks on shared interfaces 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 have matching IP subnet masks on shared interfaces 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 have matching IP subnet masks on shared interfaces to form neighbor adjacencies successfully..
What is the correct answer to this question?
The correct answer is: The routers are using different IP subnet masks on the shared link — For OSPF neighbors to form, key settings on a shared segment must match. A subnet mask mismatch on a broadcast network often prevents proper neighbor formation because the routers do not agree on the local network.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must have matching IP subnet masks on shared interfaces 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 have matching IP subnet masks on shared interfaces to form neighbor adjacencies successfully.
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