JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Exhibit
Mar 15 10:00:00 R1 rpd[2345]: RPD_OSPF_NBRDOWN: OSPF neighbor 10.0.0.2 (ge-0/0/0) state changed from Full to Down (event: KillNbr)
Mar 15 10:00:01 R1 rpd[2345]: RPD_OSPF_NBRUP: OSPF neighbor 10.0.0.2 (ge-0/0/0) state changed from Down to Full
Refer to the exhibit. The OSPF neighbor adjacency repeatedly goes up and down on R1. What is a likely cause?
Mar 15 10:00:00 R1 rpd[2345]: RPD_OSPF_NBRDOWN: OSPF neighbor 10.0.0.2 (ge-0/0/0) state changed from Full to Down (event: KillNbr)
Mar 15 10:00:01 R1 rpd[2345]: RPD_OSPF_NBRUP: OSPF neighbor 10.0.0.2 (ge-0/0/0) state changed from Down to Full
A
The physical link is flapping
Why wrong: Link flapping would show interface up/down log messages.
B
MTU mismatch between the two routers
MTU mismatch causes larger OSPF packets to be dropped, leading to neighbor down events.
C
OSPF authentication is misconfigured
Why wrong: Authentication failure would result in authentication error messages.
D
The routers are in different OSPF areas
Why wrong: Area mismatch prevents the neighbor from reaching Full state at all.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MTU mismatch between the two routers
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.
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 physical link is flapping
Why it's wrong here
Link flapping would show interface up/down log messages.
✓
MTU mismatch between the two routers
Why this is correct
MTU mismatch causes larger OSPF packets to be dropped, leading to neighbor down events.
Related concept
Read the scenario before looking for a memorised answer.
✗
OSPF authentication is misconfigured
Why it's wrong here
Authentication failure would result in authentication error messages.
✗
The routers are in different OSPF areas
Why it's wrong here
Area mismatch prevents the neighbor from reaching Full state at all.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume physical link flapping (Option A) is the cause, but OSPF adjacency flapping can occur with a stable link due to Layer 3 mismatches like MTU, which is a subtle but classic JNCIA-JUNOS exam topic.
Trap categories for this question
Command / output trap
Link flapping would show interface up/down log messages.
Detailed technical explanation
How to think about this question
OSPF uses the interface MTU value in the Database Description (DBD) packets during the ExStart/Exchange states. If the MTU on one interface is smaller than the DBD packet size, the packet is silently dropped, causing the neighbor to time out and restart the adjacency process. This behavior is defined in RFC 2328, Section 10.6, and can be observed with 'show log messages' or 'monitor traffic interface' on Junos. A common real-world scenario is when one router has an MTU of 1500 and the other has 1492 (e.g., due to PPPoE overhead), leading to persistent flapping.
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: MTU mismatch between the two routers — 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.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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