This 200-301 practice question tests your understanding of ip routing. 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.
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
!
A
The network statement does not match the interface IP address.
Why wrong: 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.
B
The passive-interface default command is preventing OSPF hellos on the interface.
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.
C
The router-id is not configured, so OSPF cannot form an adjacency.
Why wrong: A router-id is automatically selected from the highest loopback or physical interface IP; lack of manual configuration does not prevent adjacency formation.
D
There is an OSPF authentication mismatch between R1 and R2.
Why wrong: The running-config does not show any authentication configuration, and the symptom is consistent with passive interfaces, not authentication issues.
The answer is that the passive-interface default command is preventing OSPF hellos on the interface. When you configure `passive-interface default` under the OSPF process, every interface on the router is set to passive, meaning OSPF Hellos are not sent or received on those interfaces unless you explicitly override them with `no passive-interface` on the specific link. Since R1’s interface connecting to R2 was not configured with that override, R1 never sends Hellos, and the adjacency cannot form. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how OSPF neighbor discovery works and the common pitfall of assuming a network statement alone enables adjacency. A frequent trap is thinking a missing router-id or authentication is the cause, but a passive interface blocks Hellos regardless of those settings. Memory tip: “Default passive kills the hello—only a ‘no passive’ lets it flow.”
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 OSPF Hellos are not sent unless explicitly overridden with `no passive-interface`. Since the interface to R2 was not configured with `no passive-interface`, R1 does not send Hellos, preventing adjacency. Option A is less likely because even if the network statement is correct, a passive interface still blocks Hellos. Option C is incorrect because OSPF automatically selects a router-id if not configured (highest loopback or interface IP), and a missing router-id does not prevent Hellos. Option D is unsupported by the scenario; no authentication mismatch is indicated.
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 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
Read the scenario before looking for a memorised answer.
✗
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 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. Therefore, the network statement is not the issue.
Why candidates choose this
Students often think that a missing or incorrect network statement is the primary cause of OSPF adjacency failures, but in this scenario the network statement is correctly configured.
✗The router-id is not configured, so OSPF cannot form an adjacency.Wrong answer — click to see why▾
Why this is wrong here
A router-id is automatically selected from the highest loopback or physical interface IP if not manually configured. Lack of manual configuration does not prevent adjacency formation; OSPF will still function.
Why candidates choose this
Many students believe that a router-id must be explicitly configured for OSPF to work, but OSPF can dynamically select a router-id from available IP addresses.
✗There is an OSPF authentication mismatch between R1 and R2.Wrong answer — click to see why▾
Why this is wrong here
The running-config does not show any authentication configuration, and the symptom (no neighbors) is consistent with passive interfaces, not authentication mismatches. An authentication mismatch would typically show neighbors in EXSTART/EXCHANGE state, not missing entirely.
Why candidates choose this
Authentication mismatches are a common cause of OSPF adjacency issues, so students may jump to that conclusion without checking for passive interface configuration.
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 nuance that `passive-interface default` suppresses Hellos on all interfaces unless overridden, leading candidates to overlook the fact that even directly connected routers cannot form an adjacency without Hellos being sent.
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 uses multicast Hellos (224.0.0.5) to discover neighbors and maintain adjacencies. The `passive-interface default` command suppresses the sending of these Hellos on all interfaces, effectively making OSPF unable to discover neighbors on those links. A common real-world scenario is when an engineer uses `passive-interface default` for security or to reduce overhead on LAN interfaces but forgets to explicitly enable Hellos on point-to-point WAN links, causing adjacency failures.
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.
Visual reference
Quick reference
Routing Protocol Comparison
Protocol
Metric
Max Hops
Algorithm
Type
RIP v2
Hop count
15
Bellman-Ford
Distance vector
OSPF
Cost (bandwidth)
Unlimited
Dijkstra (SPF)
Link state
EIGRP
Composite metric
Unlimited
DUAL
Hybrid
IS-IS
Cost
Unlimited
Dijkstra
Link state
BGP
Policy / attributes
Unlimited
Path vector
Path vector
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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: 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 OSPF Hellos are not sent unless explicitly overridden with `no passive-interface`. Since the interface to R2 was not configured with `no passive-interface`, R1 does not send Hellos, preventing adjacency. Option A is less likely because even if the network statement is correct, a passive interface still blocks Hellos. Option C is incorrect because OSPF automatically selects a router-id if not configured (highest loopback or interface IP), and a missing router-id does not prevent Hellos. Option D is unsupported by the scenario; no authentication mismatch is indicated.
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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