- A
The interface MTU size is mismatched.
Why wrong: MTU mismatch usually leads to adjacency issues in the EXCHANGE state, not INIT. The INIT state indicates that hello packets are being received but not acknowledged, which is not typical of MTU problems.
- B
The OSPF network type is mismatched.
Why wrong: Both routers default to broadcast network type, so there is no mismatch. A mismatch between broadcast and point-to-point could cause INIT, but that is not the case here.
- C
The OSPF authentication keys do not match.
MD5 authentication requires matching keys on both routers. Since the keys differ, R1 will not accept R2's hello packets and vice versa, causing the adjacency to remain stuck in INIT.
- D
The router IDs are not properly configured.
Why wrong: Router IDs are unique and correctly configured. Duplicate or invalid router IDs would cause issues in EXCHANGE or LOADING, not INIT. The routers can still send and receive hello packets with mismatched router IDs.
Quick Answer
The answer is an OSPF authentication mismatch caused by differing keys. This is correct because OSPF authentication requires that the authentication type and key match exactly on both ends of a link; the MD5 digest computed from the key is included in every Hello packet, and if the digests do not match, the receiving router silently drops the packet, preventing the adjacency from progressing past the INIT state. On the JNCIA-Junos exam, this scenario tests your understanding that OSPF adjacency failures often stem from mismatched parameters beyond just hello and dead intervals—authentication keys are a common trap because they are easy to misconfigure. A key memory tip is “same type, same key, or no adjacency for thee”—always verify both the authentication-type and the authentication-key values are identical on neighboring interfaces.
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.
You are troubleshooting an OSPF adjacency issue between two Juniper MX series routers, R1 and R2, both running Junos 18.2. They are directly connected via a Gigabit Ethernet link. R1's interface ge-0/0/0 is configured with IP 192.168.1.1/24, and R2's ge-0/0/0 is configured with IP 192.168.1.2/24. Both interfaces are in OSPF area 0.0.0.0. The physical connectivity is confirmed up, and both interfaces are administratively enabled. You have checked that the hello and dead intervals match (hello 10, dead 40), the area IDs are identical, and the subnet masks are /24. No firewall filters are blocking OSPF. The OSPF configuration on R1 includes 'authentication-type md5' and 'authentication-key juniper123' under the interface. On R2, the configuration also includes 'authentication-type md5' but the key is 'juniper321'. Both routers have loopback addresses as router IDs: 1.1.1.1 for R1 and 2.2.2.2 for R2. No explicit OSPF network type is set, so the default is broadcast. Despite these configurations, the OSPF adjacency state on both routers remains in INIT. What is the most likely cause of this issue?
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 OSPF authentication keys do not match.
Option C is correct because OSPF authentication keys must match exactly for the adjacency to form. R1 uses key 'juniper123' while R2 uses 'juniper321', causing the MD5 digest computed on each Hello packet to differ. Since OSPF authentication is validated per-packet, mismatched keys prevent the routers from moving past the INIT state.
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 interface MTU size is mismatched.
Why it's wrong here
MTU mismatch usually leads to adjacency issues in the EXCHANGE state, not INIT. The INIT state indicates that hello packets are being received but not acknowledged, which is not typical of MTU problems.
- ✗
The OSPF network type is mismatched.
Why it's wrong here
Both routers default to broadcast network type, so there is no mismatch. A mismatch between broadcast and point-to-point could cause INIT, but that is not the case here.
- ✓
The OSPF authentication keys do not match.
Why this is correct
MD5 authentication requires matching keys on both routers. Since the keys differ, R1 will not accept R2's hello packets and vice versa, causing the adjacency to remain stuck in INIT.
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 properly configured.
Why it's wrong here
Router IDs are unique and correctly configured. Duplicate or invalid router IDs would cause issues in EXCHANGE or LOADING, not INIT. The routers can still send and receive hello packets with mismatched router IDs.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often overlook authentication key consistency because they focus on matching hello/dead intervals and area IDs, assuming authentication is correctly configured if the type matches.
Detailed technical explanation
How to think about this question
OSPF authentication (RFC 2328, Appendix D) uses the configured key to compute an MD5 digest over the OSPF packet payload plus a shared secret. Even a single character difference in the key produces a completely different digest, causing the receiving router to silently drop the packet. In a production network, this is a common misconfiguration when keys are manually entered across multiple devices, and the adjacency remains stuck in INIT with 'auth type mismatch' or 'bad authentication' log messages.
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 JNCIA-JUNOS question test?
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 OSPF authentication keys do not match. — Option C is correct because OSPF authentication keys must match exactly for the adjacency to form. R1 uses key 'juniper123' while R2 uses 'juniper321', causing the MD5 digest computed on each Hello packet to differ. Since OSPF authentication is validated per-packet, mismatched keys prevent the routers from moving past the INIT state.
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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Last reviewed: Jun 24, 2026
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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