The answer is an OSPF MTU mismatch between the directly connected interfaces on R1 and R2. This is the most likely cause because during the OSPF database description (DBD) packet exchange, both routers must agree on the interface MTU value; if they do not match, the adjacency becomes stuck in the ExStart or Exchange state, preventing full neighbor formation. On the CCNA 200-301 v2 exam, this scenario tests your understanding of the OSPF neighbor state machine and the specific role of MTU in the database exchange process—a common trap is assuming a Layer 1 or IP addressing issue since basic ping still works due to ICMP packets being small enough to pass. Remember that OSPF adjacency failure with working connectivity almost always points to a mismatch in MTU or authentication. A helpful memory tip: MTU mismatch keeps OSPF “stuck in ExStart,” so think “MTU = Must Talk Unanimously” for DBD exchange.
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 MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information.. 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 ip ospf interface g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.12.1/30, Area 0
MTU 1500 bytes
R2# show ip ospf interface g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.12.2/30, Area 0
MTU 1400 bytes
R1 and R2 are directly connected and running OSPF. The IP addressing is correct and both routers are in area 0, but they do not form an adjacency. 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.
R1# show ip ospf interface g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.12.1/30, Area 0
MTU 1500 bytes
R2# show ip ospf interface g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.12.2/30, Area 0
MTU 1400 bytes
A
The OSPF MTU values do not match on the connected interfaces.
This is correct because MTU mismatch is a known cause of OSPF adjacency problems during database exchange.
B
The routers must use identical hostnames before OSPF can form neighbors.
Why wrong: This is wrong because hostnames do not determine OSPF adjacency.
C
The routers must use VLAN 1 for OSPF to operate.
Why wrong: This is wrong because OSPF is not dependent on VLAN 1 in this routed-interface scenario.
D
The interfaces need to be configured as trunks instead of routed ports.
Why wrong: This is wrong because OSPF commonly runs over routed interfaces, not switch trunks.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The OSPF MTU values do not match on the connected interfaces.
The most likely cause is an MTU mismatch between the two interfaces. During the OSPF database exchange process, neighbors must agree on the MTU value to successfully exchange DBD packets; a mismatch typically causes the adjacency to become stuck in the ExStart or Exchange state. Basic connectivity (e.g., ping) often still works because ICMP packets are small, but the OSPF adjacency fails due to the MTU discrepancy.
Key principle: OSPF routers must have matching MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information.
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 MTU values do not match on the connected interfaces.
Why this is correct
This is correct because MTU mismatch is a known cause of OSPF adjacency problems during database exchange.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must have matching MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information.
✗
The routers must use identical hostnames before OSPF can form neighbors.
Why it's wrong here
This is wrong because hostnames do not determine OSPF adjacency.
When this WOULD be correct
In a different scenario where the question specifies that OSPF adjacency formation is contingent upon matching router configurations, including hostnames for identification purposes in a lab setup, this option could be correct.
✗
The routers must use VLAN 1 for OSPF to operate.
Why it's wrong here
This is wrong because OSPF is not dependent on VLAN 1 in this routed-interface scenario.
When this WOULD be correct
In a different scenario where the question specifies that OSPF is only configured to operate on VLAN 1, and both routers are incorrectly configured on different VLANs, this option would be correct. The question would need to emphasize that OSPF requires VLAN 1 for adjacency.
✗
The interfaces need to be configured as trunks instead of routed ports.
Why it's wrong here
This is wrong because OSPF commonly runs over routed interfaces, not switch trunks.
When this WOULD be correct
In a different scenario where the question specifies that OSPF is being used in a network with VLANs and requires trunking for inter-VLAN routing, this option could be correct. For example, if the question states that OSPF must be configured on trunk interfaces to communicate between different VLANs, then this option would apply.
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 MTU values do not match on the connected interfaces.Correct answer▾
Why this is correct
This is correct because MTU mismatch is a known cause of OSPF adjacency problems during database exchange.
✗The routers must use identical hostnames before OSPF can form neighbors.Wrong answer — click to see why▾
Why this is wrong here
OSPF neighbor formation does not depend on hostnames; it relies on matching parameters such as area ID, authentication, hello/dead intervals, and network type. Hostnames are only used for local identification and do not affect OSPF adjacency.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPF adjacency formation is contingent upon matching router configurations, including hostnames for identification purposes in a lab setup, this option could be correct.
Why candidates choose this
Students might confuse the requirement for matching OSPF router IDs (which are typically IP addresses) with hostnames, or they may think that consistent naming is necessary for network protocols to communicate.
✗The routers must use VLAN 1 for OSPF to operate.Wrong answer — click to see why▾
Why this is wrong here
OSPF can operate over any VLAN or routed interface; there is no requirement to use VLAN 1. The adjacency issue is unrelated to VLAN numbering, and OSPF works independently of VLAN assignments on routed ports.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPF is only configured to operate on VLAN 1, and both routers are incorrectly configured on different VLANs, this option would be correct. The question would need to emphasize that OSPF requires VLAN 1 for adjacency.
Why candidates choose this
Some might think that VLAN 1 is the default VLAN and therefore necessary for routing protocols, but this is incorrect because OSPF runs at Layer 3 and does not depend on specific VLANs.
✗The interfaces need to be configured as trunks instead of routed ports.Wrong answer — click to see why▾
Why this is wrong here
OSPF is designed to run on routed interfaces (Layer 3 interfaces) and does not require trunking. Trunk ports are used for carrying multiple VLANs between switches, not for OSPF adjacency between routers.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that OSPF is being used in a network with VLANs and requires trunking for inter-VLAN routing, this option could be correct. For example, if the question states that OSPF must be configured on trunk interfaces to communicate between different VLANs, then this option would apply.
Why candidates choose this
Test-takers may confuse the need for trunking in switch-to-switch connections with router-to-router connections, or they may think that OSPF requires a specific encapsulation like dot1q, which is not true for point-to-point links.
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
Don't overlook MTU settings when OSPF adjacency issues arise, especially when basic connectivity is confirmed.
Trap categories for this question
Scenario analysis trap
This is wrong because OSPF is not dependent on VLAN 1 in this routed-interface scenario.
Detailed technical explanation
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that establishes neighbor adjacencies to exchange routing information within the same area. For two routers to form an OSPF adjacency, several parameters must match, including area ID, hello and dead intervals, authentication settings, and importantly, the MTU (Maximum Transmission Unit) size on the interfaces. The MTU defines the largest packet size that can be transmitted without fragmentation. If the MTU values differ, OSPF adjacency formation can stall during the database exchange phase because routers reject packets that exceed their MTU size.
During OSPF neighbor establishment, routers first exchange hello packets to discover neighbors and verify basic parameters. Once neighbors are recognized, they proceed to exchange link-state databases. If the MTU sizes do not match, the routers detect this mismatch and prevent the adjacency from fully forming to avoid packet loss or fragmentation issues. This behavior is specific to OSPF and is a common troubleshooting point when IP connectivity and area configurations appear correct but adjacency fails.
The exam trap here is assuming that IP connectivity and matching area IDs guarantee OSPF adjacency. In reality, OSPF adjacency depends on multiple interface parameters, and MTU mismatch is a subtle but critical factor. Practically, routers can ping each other because ICMP packets are smaller and less sensitive to MTU differences, but OSPF adjacency fails because OSPF packets during database exchange are larger and require matching MTU. Understanding this nuance is essential for CCNA candidates to diagnose adjacency issues effectively.
KKey Concepts to Remember
OSPF routers must have matching MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information.
OSPF adjacency formation involves multiple parameters including area ID, hello/dead intervals, authentication, and MTU size, all of which must align.
An MTU mismatch prevents OSPF routers from completing the database exchange phase, causing adjacency to remain in the INIT or EXSTART state.
Basic IP connectivity and correct subnetting do not guarantee OSPF adjacency if interface parameters like MTU differ between neighbors.
OSPF hello packets verify neighbor presence, but the adjacency fully forms only after successful database synchronization, which MTU mismatch can block.
Routers do not need identical hostnames for OSPF adjacency; hostname differences do not affect OSPF neighbor relationships.
OSPF operates over routed interfaces and does not require VLAN 1 or trunk configurations to form adjacencies on point-to-point links.
Understanding MTU mismatch as a cause of adjacency failure helps avoid misdiagnosing OSPF neighbor issues that appear as connectivity problems.
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 MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information.
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 MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information., 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 MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information..
What is the correct answer to this question?
The correct answer is: The OSPF MTU values do not match on the connected interfaces. — The most likely cause is an MTU mismatch between the two interfaces. During the OSPF database exchange process, neighbors must agree on the MTU value to successfully exchange DBD packets; a mismatch typically causes the adjacency to become stuck in the ExStart or Exchange state. Basic connectivity (e.g., ping) often still works because ICMP packets are small, but the OSPF adjacency fails due to the MTU discrepancy.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must have matching MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information., 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 MTU values on connected interfaces to successfully form neighbor adjacencies and exchange routing information.
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