The answer is that OSPF is disabled on the interface at the leaf switch. This is the most likely cause because when a point-to-point link is up at Layer 1 but the OSPF interface state shows DOWN in the output of show ospf interface, it indicates that OSPF is not actively running on that specific interface, often due to an explicit disable command or a passive interface configuration that prevents OSPF from sending hellos. On the JNCIA-Junos exam, this scenario tests your understanding that OSPF adjacency troubleshooting must distinguish between Layer 1/2 connectivity and the OSPF process itself—a common trap is to assume an MTU or network type mismatch, but those would leave the interface state UP while preventing adjacency formation. Remember that if the interface is up/up but OSPF shows DOWN, the protocol is not enabled there; a quick memory tip is “Layer 1 up, OSPF down? Check the protocol config, not the cable.”
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 a network engineer for a large enterprise deploying a new data center using a spine-and-leaf architecture with Juniper QFX5100 switches. The underlay network uses OSPF for loopback reachability, and the overlay uses EBGP for EVPN. The leaf switches are configured as VTEPs (Virtual Tunnel Endpoints). One of the leaf switches, leaf-03, cannot establish OSPF adjacency with its spine switch, spine-01. The interfaces are up/up and the IP addresses are correctly configured. 'show ospf neighbor' on leaf-03 returns nothing. 'show ospf interface' shows the interface is in state DOWN. Both switches are configured with the same OSPF area (0.0.0.0) and the same hello interval (10 seconds). The MTU on both sides is 1500. Authentication is not configured. The spine switch has multiple OSPF neighbors from other leaves. The network is in production and other leaf switches are working fine. What is the most likely cause of the problem?
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
✓
OSPF is disabled on the interface at the leaf switch.
Option B is correct because if the OSPF interface is down on leaf-03 but up/up at Layer 1, the most likely cause is that OSPF is disabled on that interface, or there is an OSPF passive interface configuration. Option A is wrong - router ID duplication would cause both to have issues, but other leaves are fine. Option C is wrong - if network type mismatch, the interface would still be up but adjacency would not form, but the interface state would be up (not down). Option D is wrong - MTU mismatch would be at L3, but interface state would be up with adjacency failing.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 network type is not consistent between the two switches.
Why it's wrong here
Network type mismatch would show interface state as up but adjacency fails.
✗
The interface MTU is misconfigured on one side.
Why it's wrong here
MTU mismatch would not cause OSPF interface to be down; adjacency would fail but interface state remains up.
✓
OSPF is disabled on the interface at the leaf switch.
Why this is correct
If OSPF is not enabled on the interface, the OSPF interface state will be down.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
A duplicate router ID would cause problems, but other leaves work with spine-01.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Command / output trap
Network type mismatch would show interface state as up but adjacency fails.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related JNCIA-JUNOS OSPF questions on adjacency and route selection.
Networking Fundamentals — This question tests Networking Fundamentals — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: OSPF is disabled on the interface at the leaf switch. — Option B is correct because if the OSPF interface is down on leaf-03 but up/up at Layer 1, the most likely cause is that OSPF is disabled on that interface, or there is an OSPF passive interface configuration. Option A is wrong - router ID duplication would cause both to have issues, but other leaves are fine. Option C is wrong - if network type mismatch, the interface would still be up but adjacency would not form, but the interface state would be up (not down). Option D is wrong - MTU mismatch would be at L3, but interface state would be up with adjacency failing.
What should I do if I get this JNCIA-JUNOS question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related JNCIA-JUNOS OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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. Two routers are connected via a point-to-point Ethernet link. They are configured with IP addresses in the same subnet, but OSPF does not form an adjacency. The link is up/up. What is a likely cause?
hard
A.The routers are in different OSPF areas
B.OSPF authentication is configured on one router but not the other
C.The routers have the same router ID
✓ D.The OSPF hello and dead intervals are mismatched
Why D: OSPF requires that Hello and Dead intervals match between neighbors on a point-to-point link to form an adjacency. Even though the link is up/up and IP addresses are in the same subnet, mismatched timers prevent the routers from agreeing on neighbor state, so no adjacency forms.
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Question Discussion
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