The answer is an MTU mismatch on the directly connected interface. This is correct because OSPF routers use the interface MTU to size their Database Description (DBD) packets during the EXSTART state; if Router A’s MTU is larger than Router B’s, Router B will drop the oversized DBD packet, causing the adjacency to stall in EXSTART rather than progressing to the Exchange state. On the JNCIA-Junos exam, this scenario tests your understanding of OSPF state progression and the specific role of MTU in DBD exchange—a common trap is assuming a router ID mismatch or a hello/dead timer issue, but those would cause problems earlier, in the INIT or 2-Way states. A useful memory tip: “EXSTART is about size, not speed”—if the adjacency gets stuck after the 2-Way state, always check the interface MTU first.
JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. 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.
A network engineer is troubleshooting OSPF adjacencies between two Juniper routers. The routers are directly connected and have matching OSPF configurations except for the router IDs. Router A has router-id 10.0.0.1, Router B has router-id 10.0.0.2. The adjacency remains in the EXSTART state. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The MTU on the interface is mismatched.
The EXSTART state indicates that the routers have progressed past the 2-Way state and are attempting to exchange Database Description (DBD) packets. A common cause for getting stuck in EXSTART is an MTU mismatch, because OSPF uses the interface MTU to determine the maximum size of DBD packets. If Router A's MTU is larger than Router B's, Router B will drop the oversized DBD packet and the adjacency will remain in EXSTART.
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 MTU on the interface is mismatched.
Why this is correct
A mismatch in MTU can prevent OSPF from exchanging DD packets, causing the adjacency to remain in EXSTART.
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 router IDs are not the same.
Why it's wrong here
Router IDs must be unique; they do not need to match.
✗
The area ID is not configured.
Why it's wrong here
Mismatched area IDs would prevent adjacency from forming, but the stem says matching configurations.
✗
The hello and dead intervals are mismatched.
Why it's wrong here
Mismatched hello/dead intervals would cause the adjacency to be stuck in INIT or DOWN, not EXSTART.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume mismatched hello/dead intervals are the cause of any adjacency problem, but those issues manifest earlier (at the 2-Way state), while EXSTART specifically points to MTU or DBD packet exchange problems.
Detailed technical explanation
How to think about this question
During the EXSTART state, the master/slave relationship is negotiated using DBD packets, and the MTU is advertised in the OSPF Hello packet's Options field (specifically, the interface MTU is used to size DBD packets per RFC 2328, Section 10.6). If the receiving router's interface has a smaller MTU, it will silently discard the DBD packet, causing the adjacency to hang. In Junos, you can verify this with 'show ospf neighbor detail' and check for 'DbdOptions' mismatches; the 'mtu' statement under 'set interfaces ge-0/0/0 mtu' can be used to align MTUs.
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.
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The MTU on the interface is mismatched. — The EXSTART state indicates that the routers have progressed past the 2-Way state and are attempting to exchange Database Description (DBD) packets. A common cause for getting stuck in EXSTART is an MTU mismatch, because OSPF uses the interface MTU to determine the maximum size of DBD packets. If Router A's MTU is larger than Router B's, Router B will drop the oversized DBD packet and the adjacency will remain in EXSTART.
What should I do if I get this JNCIA-JUNOS 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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. The OSPF neighbor adjacency repeatedly goes up and down on R1. What is a likely cause?
hard
A.The physical link is flapping
✓ B.MTU mismatch between the two routers
C.OSPF authentication is misconfigured
D.The routers are in different OSPF areas
Why B: An MTU mismatch between OSPF neighbors can cause the adjacency to flap because OSPF includes the interface MTU in the Database Description (DBD) packets. If the MTU values do not match, the receiving router will reject the DBD packet, preventing the exchange of LSAs and causing the neighbor state to reset. This is a common cause of repeated up/down OSPF adjacencies even when the physical link is stable.
Last reviewed: Jun 11, 2026
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This JNCIA-JUNOS practice question is part of Courseiva's free Juniper Networks 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 JNCIA-JUNOS exam.
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