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Question 496 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: An engineer is troubleshooting an OSPF adjacency…
This 200-301 practice question tests your understanding of 200-301 exam topics. 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.
Exhibit
R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface R1# R1# show running-config | section router ospf 1 router ospf 1 passive-interface default network 10.0.0.0 0.255.255.255 area 0 !
An engineer is troubleshooting an OSPF adjacency between two directly connected routers, R1 and R2. R1 is configured with a passive-interface default under the OSPF process, and the interface connecting to R2 is not explicitly set to no passive-interface. The engineer runs a show ip ospf neighbor command on R1 and sees no neighbors. What is the most likely reason for the missing adjacency?
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 passive-interface default command is preventing OSPF hellos on the interface.
The passive-interface default command under the OSPF process makes all interfaces passive by default, meaning they will not send or receive OSPF hello packets. Since the interface connecting to R2 is not explicitly configured with the no passive-interface command, it remains passive and cannot form an adjacency. The correct fix is to add the no passive-interface command for the specific interface (e.g., GigabitEthernet0/0) under the OSPF process. Option A is wrong because the network statement is correctly configured to include the subnet. Option C is wrong because the router-id is not required to be manually set for adjacency formation. Option D is wrong because there is no evidence of an authentication mismatch and the symptom is consistent with passive interfaces.
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 network statement does not match the interface IP address.
Why it's wrong here
The network 10.0.0.0 0.255.255.255 area 0 matches any IP in the 10.0.0.0/8 range, which likely includes the interface IP.
- ✓
The passive-interface default command is preventing OSPF hellos on the interface.
Why this is correct
The passive-interface default makes all interfaces passive, and without a no passive-interface command for the specific interface, OSPF hellos are not sent, preventing neighbor discovery.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The router-id is not configured, so OSPF cannot form an adjacency.
Why it's wrong here
A router-id is automatically selected from the highest loopback or physical interface IP; lack of manual configuration does not prevent adjacency formation.
- ✗
There is an OSPF authentication mismatch between R1 and R2.
Why it's wrong here
The running-config does not show any authentication configuration, and the symptom is consistent with passive interfaces, not authentication issues.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The passive-interface default command is preventing OSPF hellos on the interface.Correct answer▾
Why this is correct
The passive-interface default makes all interfaces passive, and without a no passive-interface command for the specific interface, OSPF hellos are not sent, preventing neighbor discovery.
✗The network statement does not match the interface IP address.Wrong answer — click to see why▾
Why this is wrong here
The network statement is correctly configured; the issue is not with mismatched network statements.
✗The router-id is not configured, so OSPF cannot form an adjacency.Wrong answer — click to see why▾
Why this is wrong here
OSPF will automatically select a router-id; this is not the cause of the missing adjacency.
✗There is an OSPF authentication mismatch between R1 and R2.Wrong answer — click to see why▾
Why this is wrong here
No authentication is configured, so a mismatch cannot occur.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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
The running-config does not show any authentication configuration, and the symptom is consistent with passive interfaces, not authentication issues.
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 200-301 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 200-301 question test?
OSPF neighbours must agree on key parameters.
What is the correct answer to this question?
The correct answer is: The passive-interface default command is preventing OSPF hellos on the interface. — The passive-interface default command under the OSPF process makes all interfaces passive by default, meaning they will not send or receive OSPF hello packets. Since the interface connecting to R2 is not explicitly configured with the no passive-interface command, it remains passive and cannot form an adjacency. The correct fix is to add the no passive-interface command for the specific interface (e.g., GigabitEthernet0/0) under the OSPF process. Option A is wrong because the network statement is correctly configured to include the subnet. Option C is wrong because the router-id is not required to be manually set for adjacency formation. Option D is wrong because there is no evidence of an authentication mismatch and the symptom is consistent with passive interfaces.
What should I do if I get this 200-301 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 200-301 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.
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