The answer is an MTU mismatch between the two routers' interfaces. When OSPF neighbors reach the EXSTART state, they attempt to negotiate the size of Database Description (DBD) packets, and if the MTU values differ, the larger DBD packet will be dropped, preventing the routers from agreeing on the master/slave relationship and leaving the adjacency stuck. On the CCNA 200-301 v2 exam, this scenario tests your understanding of OSPF state progression and the specific role of MTU in the EXSTART and EXCHANGE phases—a common trap is to blame a network type mismatch, which would actually block earlier states like 2-WAY. Remember that OSPF can discover neighbors and exchange Hello packets despite an MTU mismatch, but the adjacency halts precisely when DBD sizing is negotiated. A useful memory tip: MTU mismatch kills the exchange, so think "MTU stops the DBD chew."
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: oSPF routers must have matching interface MTU values to successfully negotiate Database Description packets during adjacency formation.. 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 Gi0/0:
ip address 10.10.12.1 255.255.255.252
ip ospf 10 area 0
ip mtu 1500
R2 Gi0/0:
ip address 10.10.12.2 255.255.255.252
ip ospf 10 area 0
ip mtu 1400
Both interfaces are up/up.
show ip ospf neighbor on both routers:
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 EXSTART 00:00:31 10.10.12.2 Gi0/0
R1 and R2 should form an OSPF adjacency on their shared GigabitEthernet link, but they remain stuck in EXSTART. 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 Gi0/0:
ip address 10.10.12.1 255.255.255.252
ip ospf 10 area 0
ip mtu 1500
R2 Gi0/0:
ip address 10.10.12.2 255.255.255.252
ip ospf 10 area 0
ip mtu 1400
Both interfaces are up/up.
show ip ospf neighbor on both routers:
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 EXSTART 00:00:31 10.10.12.2 Gi0/0
A
The routers are using different OSPF areas.
Why wrong: Both sides are in area 0, so area mismatch is not the problem.
B
The interface MTU values do not match.
This is the classic cause of OSPF neighbors getting stuck in EXSTART or EXCHANGE.
C
One side is configured as passive-interface.
Why wrong: A passive interface would stop hello packets and usually prevent the neighbor from appearing like this.
D
The subnet mask on the link is incorrect.
Why wrong: Both ends are in the same /30, so the addressing shown is consistent.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface MTU values do not match.
The MTU values do not match. OSPF neighbors can discover each other and even move through earlier states, but an MTU mismatch commonly leaves them stuck in EXSTART or EXCHANGE because the routers do not agree on database description packet sizing. Area mismatch, network mismatch, and passive-interface issues usually prevent a much earlier stage of adjacency formation.
Key principle: OSPF routers must have matching interface MTU values to successfully negotiate Database Description packets during adjacency formation.
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
Both sides are in area 0, so area mismatch is not the problem.
When this WOULD be correct
In a different scenario where the question specifies that both routers are in the same OSPF area but are unable to establish adjacency, this option would be correct if the routers were configured to use different OSPF area IDs, preventing adjacency formation.
✓
The interface MTU values do not match.
Why this is correct
This is the classic cause of OSPF neighbors getting stuck in EXSTART or EXCHANGE.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF routers must have matching interface MTU values to successfully negotiate Database Description packets during adjacency formation.
✗
One side is configured as passive-interface.
Why it's wrong here
A passive interface would stop hello packets and usually prevent the neighbor from appearing like this.
When this WOULD be correct
In a different scenario, if the question stated that OSPF adjacency was not forming at all and one router was configured with a passive-interface command on the shared link, then this option would be correct as it would directly prevent OSPF from establishing a neighbor relationship.
✗
The subnet mask on the link is incorrect.
Why it's wrong here
Both ends are in the same /30, so the addressing shown is consistent.
When this WOULD be correct
In a different scenario, if the question were about OSPF failing to establish an adjacency due to incorrect IP addressing, including a mismatched subnet mask, this option would be correct. For example, if R1 and R2 were on different subnets, OSPF would not form an adjacency.
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 interface MTU values do not match.Correct answer▾
Why this is correct
This is the classic cause of OSPF neighbors getting stuck in EXSTART or EXCHANGE.
✗The routers are using different OSPF areas.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF adjacencies can still form between routers in different areas, as long as they are not directly connected. The EXSTART state issue is more likely related to mismatched MTU settings rather than area configuration.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that both routers are in the same OSPF area but are unable to establish adjacency, this option would be correct if the routers were configured to use different OSPF area IDs, preventing adjacency formation.
Why candidates choose this
Candidates may choose this option due to a common misconception that OSPF requires routers to be in the same area for adjacency, leading them to overlook other factors like MTU mismatches that can cause EXSTART issues.
✗One side is configured as passive-interface.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because configuring one side as passive-interface prevents OSPF from forming an adjacency, but the question specifies that R1 and R2 are stuck in EXSTART, which indicates they are attempting to establish adjacency.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that OSPF adjacency was not forming at all and one router was configured with a passive-interface command on the shared link, then this option would be correct as it would directly prevent OSPF from establishing a neighbor relationship.
Why candidates choose this
Candidates might choose this option because they recall that passive-interface settings can disrupt OSPF adjacencies, leading them to mistakenly assume it applies in this situation without considering the specific EXSTART state.
✗The subnet mask on the link is incorrect.Wrong answer — click to see why▾
Why this is wrong here
The subnet mask on the link being incorrect would typically lead to connectivity issues or routing problems, but it would not specifically cause OSPF to be stuck in EXSTART. OSPF can still establish a neighbor relationship if the subnet mask is mismatched, as long as the routers can reach each other.
★ When this WOULD be the correct answer
In a different scenario, if the question were about OSPF failing to establish an adjacency due to incorrect IP addressing, including a mismatched subnet mask, this option would be correct. For example, if R1 and R2 were on different subnets, OSPF would not form an adjacency.
Why candidates choose this
Candidates may choose this option because they associate subnet mask issues with general connectivity problems, leading them to believe it could affect OSPF adjacency formation.
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 selecting area mismatch or passive-interface as the cause of OSPF adjacency stuck in ExStart. While area mismatch prevents neighbor formation entirely, and passive-interface stops hello packets, these issues cause earlier failures, not ExStart stalls. The ExStart state specifically involves negotiating database description packets, which requires matching MTU values. Candidates often overlook MTU mismatches because neighbors appear in the topology, misleading them to suspect other configuration errors. Understanding that MTU mismatch allows neighbor discovery but blocks database synchronization is key to avoiding this trap.
Trap categories for this question
Command / output trap
Both ends are in the same /30, so the addressing shown is consistent.
Detailed technical explanation
How to think about this question
OSPF (Open Shortest Path First) is a link-state routing protocol that forms neighbor adjacencies to exchange routing information. The adjacency process includes several states: Down, Init, Two-Way, ExStart, Exchange, Loading, and Full. The ExStart state is where routers negotiate the master-slave relationship and sequence numbers for Database Description (DBD) packets. A critical factor in this negotiation is the interface MTU (Maximum Transmission Unit), which must match on both ends to ensure proper packet exchange.
When OSPF neighbors have mismatched MTU values on their shared interface, they can discover each other and progress through initial states but get stuck in ExStart or Exchange. This happens because the routers cannot agree on the size of DBD packets, preventing the adjacency from reaching the Full state. Cisco routers strictly enforce MTU matching to avoid fragmentation and ensure reliable OSPF database synchronization.
The exam trap here is assuming that adjacency failure is due to area mismatch, passive interface, or subnet mask issues, which typically prevent neighbor discovery or initial hello exchanges. In practice, MTU mismatches allow neighbors to see each other but block database synchronization, causing the adjacency to stall in ExStart. Network engineers must verify and align MTU settings on both ends of the link to resolve this issue and achieve full OSPF adjacency.
KKey Concepts to Remember
OSPF routers must have matching interface MTU values to successfully negotiate Database Description packets during adjacency formation.
An OSPF adjacency stuck in the ExStart state usually indicates a problem with DBD packet negotiation, often caused by MTU mismatches.
OSPF neighbors in different areas cannot form adjacencies, but area mismatch typically prevents neighbor discovery before ExStart.
A passive-interface configuration stops OSPF hello packets, preventing neighbor formation rather than causing adjacency to stall in ExStart.
Subnet mask mismatches prevent routers from recognizing each other as neighbors, blocking adjacency formation at the hello stage.
OSPF adjacency states progress sequentially, and failure to move beyond ExStart usually points to issues with packet size negotiation or authentication.
Cisco routers enforce strict MTU matching on OSPF interfaces to avoid fragmentation and ensure reliable link-state database synchronization.
Troubleshooting OSPF adjacency issues requires verifying interface parameters like MTU, area ID, and passive-interface status to isolate the root cause.
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 interface MTU values to successfully negotiate Database Description packets during adjacency formation.
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 interface MTU values to successfully negotiate Database Description packets during adjacency formation., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Services and Security — This question tests Network Services and Security — OSPF routers must have matching interface MTU values to successfully negotiate Database Description packets during adjacency formation..
What is the correct answer to this question?
The correct answer is: The interface MTU values do not match. — The MTU values do not match. OSPF neighbors can discover each other and even move through earlier states, but an MTU mismatch commonly leaves them stuck in EXSTART or EXCHANGE because the routers do not agree on database description packet sizing. Area mismatch, network mismatch, and passive-interface issues usually prevent a much earlier stage of adjacency formation.
What should I do if I get this 200-301 question wrong?
Review oSPF routers must have matching interface MTU values to successfully negotiate Database Description packets during adjacency formation., 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 interface MTU values to successfully negotiate Database Description packets during adjacency formation.
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