The answer is a subnet mismatch between the two OSPF interfaces. OSPFv2 requires that neighboring interfaces share an identical subnet and mask to form an adjacency; if R1’s GigabitEthernet0/1 is configured with 192.168.1.1/30 and R2’s interface belongs to a different subnet, such as 192.168.2.0/30, the routers will not process each other’s Hello packets, leaving the neighbor state stuck in DOWN. On the CCNA 200-301 v2 exam, this question tests your understanding of a fundamental OSPF neighbor formation rule—matching subnet—and often appears with a show cdp neighbors detail output to trick you into focusing on Layer 2 connectivity instead of the Layer 3 mismatch. A common trap is assuming that any two IP addresses on the same network segment will work, but OSPF is strict about the prefix length. Remember the mnemonic: “Same subnet, same mask, or the adjacency won’t last.”
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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# show cdp neighbors detail
-------------------------
Device ID: R2
Entry address(es):
IP address: 10.1.1.2
Platform: Cisco C2900, Capabilities: Router
Interface: GigabitEthernet0/1, Port ID (outgoing port): GigabitEthernet0/0
Holdtime: 140 sec
Version:
Cisco IOS Software, Version 15.4(3)M3, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2015 by Cisco Systems, Inc.
Compiled Wed 02-Jul-15 10:45 by prod_rel_team
Advertisement Version: 2
Duplex: full
Total cdp entries displayed : 1
Refer to the exhibit. A network engineer is troubleshooting an OSPFv2 neighbor adjacency that remains in the DOWN state between R1 and R2. The engineer issues the show cdp neighbors detail command on R1 and confirms that R1's GigabitEthernet0/1 interface is configured with IP address 192.168.1.1/30. What is the most likely cause of the problem?
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.
R1# show cdp neighbors detail
-------------------------
Device ID: R2
Entry address(es):
IP address: 10.1.1.2
Platform: Cisco C2900, Capabilities: Router
Interface: GigabitEthernet0/1, Port ID (outgoing port): GigabitEthernet0/0
Holdtime: 140 sec
Version:
Cisco IOS Software, Version 15.4(3)M3, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2015 by Cisco Systems, Inc.
Compiled Wed 02-Jul-15 10:45 by prod_rel_team
Advertisement Version: 2
Duplex: full
Total cdp entries displayed : 1
A
The OSPF area IDs configured on R1 and R2 do not match.
Why wrong: While area mismatch can prevent OSPF adjacency, no evidence in the exhibit or scenario indicates an area mismatch. The CDP output does not contain OSPF area information, and the IP subnet discrepancy is a more fundamental issue.
B
There is an MTU mismatch between the two routers.
Why wrong: MTU mismatch can cause OSPF adjacency to stall, but the exhibit does not provide MTU values. The discovered IP addresses being on different subnets is a confirmed misconfiguration.
C
The IP addresses on the connected interfaces are in different subnets.
The CDP entry for R2 shows an IP address of 10.1.1.2, while the local interface on R1 has 192.168.1.1/30, which is a different subnet. OSPF requires both ends of a link to share a common subnet.
D
OSPF authentication is configured incorrectly on one of the routers.
Why wrong: Authentication mismatch would indeed break OSPF adjacency, but no authentication parameters are shown in the output or scenario. The IP subnet mismatch is the clear cause.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The IP addresses on the connected interfaces are in different subnets.
The correct answer is C because OSPFv2 requires that neighboring interfaces share a common subnet to form an adjacency. The show cdp neighbors detail output confirms R1's GigabitEthernet0/1 is configured with IP address 192.168.1.1/30, but if R2's interface is on a different subnet (e.g., 192.168.2.0/30), the routers will not be able to communicate OSPF Hello packets, leaving the neighbor state in DOWN. This is a fundamental OSPF neighbor formation requirement, and a subnet mismatch will prevent the adjacency from progressing past the DOWN state.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 area IDs configured on R1 and R2 do not match.
Why it's wrong here
While area mismatch can prevent OSPF adjacency, no evidence in the exhibit or scenario indicates an area mismatch. The CDP output does not contain OSPF area information, and the IP subnet discrepancy is a more fundamental issue.
✗
There is an MTU mismatch between the two routers.
Why it's wrong here
MTU mismatch can cause OSPF adjacency to stall, but the exhibit does not provide MTU values. The discovered IP addresses being on different subnets is a confirmed misconfiguration.
✓
The IP addresses on the connected interfaces are in different subnets.
Why this is correct
The CDP entry for R2 shows an IP address of 10.1.1.2, while the local interface on R1 has 192.168.1.1/30, which is a different subnet. OSPF requires both ends of a link to share a common subnet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
OSPF authentication is configured incorrectly on one of the routers.
Why it's wrong here
Authentication mismatch would indeed break OSPF adjacency, but no authentication parameters are shown in the output or scenario. The IP subnet mismatch is the clear cause.
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 IP addresses on the connected interfaces are in different subnets.Correct answer▾
Why this is correct
The CDP entry for R2 shows an IP address of 10.1.1.2, while the local interface on R1 has 192.168.1.1/30, which is a different subnet. OSPF requires both ends of a link to share a common subnet.
✗The OSPF area IDs configured on R1 and R2 do not match.Wrong answer — click to see why▾
Why this is wrong here
Area mismatch is a valid cause but not supported by the given data; the IP addresses are on different subnets, which logically precedes area negotiation.
✗There is an MTU mismatch between the two routers.Wrong answer — click to see why▾
Why this is wrong here
MTU is not shown; candidates may recall that mismatched MTU can cause OSPF problems but the scenario explicitly reveals an IP addressing inconsistency.
✗OSPF authentication is configured incorrectly on one of the routers.Wrong answer — click to see why▾
Why this is wrong here
Authentication issues are common OSPF traps, but they are not indicated here; the IP address difference is explicitly observed.
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
Cisco often tests the specific OSPF neighbor state where a subnet mismatch causes the adjacency to remain in DOWN, tricking candidates into thinking it would cause a later state issue like EXSTART/EXCHANGE, which is actually associated with MTU or area ID mismatches.
Trap categories for this question
Command / output trap
While area mismatch can prevent OSPF adjacency, no evidence in the exhibit or scenario indicates an area mismatch. The CDP output does not contain OSPF area information, and the IP subnet discrepancy is a more fundamental issue.
Scenario analysis trap
While area mismatch can prevent OSPF adjacency, no evidence in the exhibit or scenario indicates an area mismatch. The CDP output does not contain OSPF area information, and the IP subnet discrepancy is a more fundamental issue.
Detailed technical explanation
How to think about this question
OSPFv2 uses the IP subnet of the interface to determine if a neighbor is on the same link; if the source IP of a received Hello packet is not in the same subnet as the receiving interface, the router silently discards the packet, preventing any neighbor state progression. This behavior is defined in RFC 2328, Section 9.2, where the router checks that the IP address in the Hello packet belongs to a network directly connected to the receiving interface. In real-world scenarios, a common misconfiguration is using a /30 on one side and a /29 on the other, which still results in a subnet mismatch if the network addresses differ.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The IP addresses on the connected interfaces are in different subnets. — The correct answer is C because OSPFv2 requires that neighboring interfaces share a common subnet to form an adjacency. The show cdp neighbors detail output confirms R1's GigabitEthernet0/1 is configured with IP address 192.168.1.1/30, but if R2's interface is on a different subnet (e.g., 192.168.2.0/30), the routers will not be able to communicate OSPF Hello packets, leaving the neighbor state in DOWN. This is a fundamental OSPF neighbor formation requirement, and a subnet mismatch will prevent the adjacency from progressing past the DOWN state.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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