- A
The OSPF Hello and Dead intervals are mismatched
Why wrong: Mismatched Hello/Dead intervals prevent the routers from reaching the Two-Way state, not ExStart. They would be stuck in Down or Init states.
- B
The OSPF area IDs are different
Why wrong: Different area IDs would prevent the routers from becoming neighbors entirely; they would not progress to ExStart. OSPF neighbors must be in the same area to form adjacency.
- C
The MTU is mismatched between the two routers
If the MTU size differs, the DBD packets exchanged in ExStart may be too large for one interface, causing the router to reject them and remain stuck in ExStart state.
- D
The network type is misconfigured (e.g., one side is broadcast, the other is point-to-point)
Why wrong: Network type mismatch can cause issues, but typically the OSPF state advances beyond ExStart. The most common cause of ExStart stickiness is MTU mismatch.
Quick Answer
The answer is an MTU mismatch between the two routers. When OSPF neighbors become stuck in the EXSTART state, it means they have successfully completed the two-way handshake but cannot move forward to the Exchange state because their Database Description (DBD) packets are being dropped or ignored. This happens because OSPF will not process a DBD packet that exceeds the neighbor’s interface MTU, causing the routers to keep renegotiating the master/slave relationship without ever exchanging link-state information. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of OSPF adjacency states and the role of Layer 2 constraints; a common trap is to blame a mismatched area ID or authentication, but those typically cause issues earlier in the process. Remember the memory tip: “EXSTART equals oversized DBD—check your MTU first.”
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Two routers are configured with OSPF in the same area, but they do not form an adjacency. Router A shows OSPF state EXSTART, and Router B shows state EXSTART. Which of the following 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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The MTU is mismatched between the two routers
When OSPF routers are stuck in the EXSTART state, it indicates that they have progressed past the 2-Way and ExStart phases but are unable to exchange Database Description (DBD) packets. The most common cause is an MTU mismatch, because OSPF will not proceed to the Exchange state if a DBD packet exceeds the interface MTU of the neighbor. This causes the routers to continuously renegotiate the master/slave relationship without completing the exchange.
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 Hello and Dead intervals are mismatched
Why it's wrong here
Mismatched Hello/Dead intervals prevent the routers from reaching the Two-Way state, not ExStart. They would be stuck in Down or Init states.
- ✗
The OSPF area IDs are different
- ✓
The MTU is mismatched between the two routers
Why this is correct
If the MTU size differs, the DBD packets exchanged in ExStart may be too large for one interface, causing the router to reject them and remain stuck in ExStart state.
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.
- ✗
The network type is misconfigured (e.g., one side is broadcast, the other is point-to-point)
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the MTU mismatch trap by having candidates confuse it with Hello/Dead interval mismatches, but the key clue is that both routers are stuck in EXSTART, not in INIT or 2-Way.
Detailed technical explanation
How to think about this question
OSPF uses the MTU value carried in the Database Description (DBD) packet to ensure both sides can exchange large LSAs. If the receiving router's interface MTU is smaller than the DBD packet, it will silently drop the packet, causing the sender to retransmit and remain stuck in EXSTART. This behavior is defined in RFC 2328, Section 10.6, and can be verified using 'show ip ospf neighbor' and 'show ip interface' to compare MTU values.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The MTU is mismatched between the two routers — When OSPF routers are stuck in the EXSTART state, it indicates that they have progressed past the 2-Way and ExStart phases but are unable to exchange Database Description (DBD) packets. The most common cause is an MTU mismatch, because OSPF will not proceed to the Exchange state if a DBD packet exceeds the interface MTU of the neighbor. This causes the routers to continuously renegotiate the master/slave relationship without completing the exchange.
What should I do if I get this N10-009 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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Last reviewed: Jun 30, 2026
This N10-009 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the N10-009 exam.
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