This 200-301 practice question tests your understanding of ip routing. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 running-config | section router ospf
router ospf 1
network 10.1.1.0 0.0.0.3 area 0
passive-interface default
no passive-interface GigabitEthernet0/0
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
(no output)
R1# show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Se0/0/0 1 0 10.1.1.1/30 64 P2P 0/0
Gi0/0 1 0 192.168.1.1/24 1 DR 0/0
A network administrator configures OSPF on two routers, R1 and R2, connected via their Serial0/0/0 interfaces (IP addresses 10.1.1.1/30 and 10.1.1.2/30). They verify that both routers use the same OSPF process ID and area 0, but R1's 'show ip ospf neighbor' shows no adjacencies. Given the partial exhibit from R1, what is the most likely cause of the adjacency failure and its correct solution?
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.
R1# show running-config | section router ospf
router ospf 1
network 10.1.1.0 0.0.0.3 area 0
passive-interface default
no passive-interface GigabitEthernet0/0
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
(no output)
R1# show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Se0/0/0 1 0 10.1.1.1/30 64 P2P 0/0
Gi0/0 1 0 192.168.1.1/24 1 DR 0/0
A
Configure 'no passive-interface Serial0/0/0' under router ospf 1 on R1.
This command lifts the passive restriction on Serial0/0/0, allowing OSPF hello packets to be sent and received, enabling the adjacency to form.
B
Replace the network statement with 'network 10.1.1.0 0.0.0.255 area 0' to cover a larger range.
Why wrong: The current network statement already matches the interface IP because 10.1.1.1 is within 10.1.1.0/24. Changing the wildcard mask does not alter the passive-interface status, so hellos remain suppressed.
C
Change the OSPF process ID on R1 to match R2, using 'router ospf 100' and re-entering the network command.
Why wrong: OSPF process IDs are locally significant and do not need to match between routers for adjacency. The mismatch of process IDs does not cause the failure shown.
D
Issue 'clear ip ospf process' on R1 to restart OSPF and reattempt neighbor discovery.
Why wrong: Clearing the OSPF process resets the protocol state, but the configuration still has the interface set to passive. After the reset, no hellos will be sent, and adjacency will not be reestablished.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure 'no passive-interface Serial0/0/0' under router ospf 1 on R1.
The most likely cause is that R1's Serial0/0/0 interface is configured as a passive interface under OSPF. When an interface is set as passive, OSPF does not send Hello packets out of it, preventing neighbor discovery and adjacency formation. The solution is to use the 'no passive-interface Serial0/0/0' command under router ospf 1 on R1, which allows Hello packets to be transmitted and the adjacency to establish.
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.
✓
Configure 'no passive-interface Serial0/0/0' under router ospf 1 on R1.
Why this is correct
This command lifts the passive restriction on Serial0/0/0, allowing OSPF hello packets to be sent and received, enabling the adjacency to form.
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.
✗
Replace the network statement with 'network 10.1.1.0 0.0.0.255 area 0' to cover a larger range.
Why it's wrong here
The current network statement already matches the interface IP because 10.1.1.1 is within 10.1.1.0/24. Changing the wildcard mask does not alter the passive-interface status, so hellos remain suppressed.
✗
Change the OSPF process ID on R1 to match R2, using 'router ospf 100' and re-entering the network command.
Why it's wrong here
OSPF process IDs are locally significant and do not need to match between routers for adjacency. The mismatch of process IDs does not cause the failure shown.
✗
Issue 'clear ip ospf process' on R1 to restart OSPF and reattempt neighbor discovery.
Why it's wrong here
Clearing the OSPF process resets the protocol state, but the configuration still has the interface set to passive. After the reset, no hellos will be sent, and adjacency will not be reestablished.
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.
✓Configure 'no passive-interface Serial0/0/0' under router ospf 1 on R1.Correct answer▾
Why this is correct
This command lifts the passive restriction on Serial0/0/0, allowing OSPF hello packets to be sent and received, enabling the adjacency to form.
✗Replace the network statement with 'network 10.1.1.0 0.0.0.255 area 0' to cover a larger range.Wrong answer — click to see why▾
Why this is wrong here
A larger wildcard mask does not override the passive-interface setting; adjacency still fails.
✗Change the OSPF process ID on R1 to match R2, using 'router ospf 100' and re-entering the network command.Wrong answer — click to see why▾
Why this is wrong here
Adjacency depends on area and authentication, not on the router-local process ID; passive-interface is the real issue.
✗Issue 'clear ip ospf process' on R1 to restart OSPF and reattempt neighbor discovery.Wrong answer — click to see why▾
Why this is wrong here
The root cause is a configuration that blocks hellos, not a transient state; the reset is ineffective.
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: answer the scenario, not the keyword
Cisco often tests the misconception that OSPF process IDs must match between routers, leading candidates to choose option C, when in fact process IDs are locally significant and only area IDs and authentication must match.
Trap categories for this question
Command / output trap
OSPF process IDs are locally significant and do not need to match between routers for adjacency. The mismatch of process IDs does not cause the failure shown.
Detailed technical explanation
How to think about this question
OSPF uses Hello packets (multicast to 224.0.0.5) to discover neighbors and maintain adjacencies. The 'passive-interface' command suppresses these Hellos, effectively making the interface a stub network from OSPF's perspective. In real-world scenarios, passive interfaces are often used on LAN segments to prevent unnecessary OSPF overhead, but on point-to-point serial links, they must be disabled to allow neighbor formation. The 'show ip ospf interface serial0/0/0' command would reveal the passive state by showing 'No Hellos' or a passive flag.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
IP Routing — This question tests IP Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure 'no passive-interface Serial0/0/0' under router ospf 1 on R1. — The most likely cause is that R1's Serial0/0/0 interface is configured as a passive interface under OSPF. When an interface is set as passive, OSPF does not send Hello packets out of it, preventing neighbor discovery and adjacency formation. The solution is to use the 'no passive-interface Serial0/0/0' command under router ospf 1 on R1, which allows Hello packets to be transmitted and the adjacency to establish.
What should I do if I get this 200-301 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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