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 administrator is implementing OSPF on a Juniper MX router. During verification, the OSPF adjacency does not come up. The interfaces have correct IP addresses and are up. 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 interface MTU does not match between the two routers.
In OSPF, the interface MTU must match between neighbors for the adjacency to form. When a router receives a Database Description (DBD) packet with the 'More' bit set but the packet size exceeds the receiving interface's MTU, the packet is silently dropped, preventing the adjacency from progressing beyond the ExStart state. This is a common issue on Juniper MX routers when connecting to devices with different MTU configurations.
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 router ID is not in the same subnet as the interface.
The interface MTU does not match between the two routers.
Why this is correct
MTU mismatch prevents OSPF from forming adjacency.
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 VLAN tag is missing on the interface.
Why it's wrong here
Missing VLAN tag would cause Layer 2 issues, not OSPF specific.
✗
The max-lsa limit has been reached.
Why it's wrong here
Max-lsa stops receiving new LSAs, not adjacency formation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume OSPF adjacency issues are always due to IP addressing or authentication mismatches, overlooking the MTU mismatch which is a subtle but common cause, especially in multi-vendor environments where default MTU values may differ.
Detailed technical explanation
How to think about this question
OSPF adjacency formation requires that the MTU of the interface match between neighbors because the DBD packets exchanged during the ExStart and Exchange states are padded to the interface MTU. If the MTU differs, the receiving router will drop the oversized DBD packet, causing the adjacency to stall in the ExStart state. On Juniper devices, you can verify this with 'show ospf neighbor' and check for a state of 'ExStart' or 'Exchange' without progressing; the 'show interfaces <interface> extensive' command displays the MTU value.
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 interface MTU does not match between the two routers. — In OSPF, the interface MTU must match between neighbors for the adjacency to form. When a router receives a Database Description (DBD) packet with the 'More' bit set but the packet size exceeds the receiving interface's MTU, the packet is silently dropped, preventing the adjacency from progressing beyond the ExStart state. This is a common issue on Juniper MX routers when connecting to devices with different MTU configurations.
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.
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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.
Question Discussion
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