CCNA IP Routing Practice Question
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?
⚠ Common exam trap
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.
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.
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 statement 'network 10.0.0.0 0.255.255.255 area 0' uses a wildcard mask of 0.255.255.255, meaning it matches any interface whose IP address begins with 10. Since R1 and R2 are in the 10.0.0.0/8 range, their interfaces are correctly included in the OSPF process. Therefore, the interface IP is not excluded by the network statement; instead, the problem is that the interface is passive, which stops OSPF hello packets from being sent regardless of network statement matching.
- ✓
The passive-interface default command is preventing OSPF hellos on the interface.
Why this is correct
The passive-interface default command configures OSPF so that every interface is passive unless explicitly overridden with 'no passive-interface'. A passive interface does not send OSPF hello packets, but the connected subnet is still advertised into the OSPF database. Without a 'no passive-interface' statement applied to the link between R1 and R2, neither router will send hellos, and an adjacency can never form—exactly matching the symptom of no neighbors.
- ✗
The router-id is not configured, so OSPF cannot form an adjacency.
Why it's wrong here
Even without a manually configured router-id, OSPF automatically selects one by taking the highest IP address among loopback interfaces, or if no loopback exists, the highest IP on an active physical interface. This auto-selected router ID is sufficient to build a Router ID and participate in the SPF algorithm, so the absence of an explicit router-id command does not block adjacency formation. A missing router-id would not prevent hello packet transmission, which is the actual failure point here.
- ✗
There is an OSPF authentication mismatch between R1 and R2.
Why it's wrong here
OSPF authentication is not configured on either router; the running-config lacks commands such as ip ospf authentication or area authentication, so both use null authentication by default. If authentication were mismatched, you would see hello packets rejected and neighbor state stuck in EXSTART/EXCHANGE, not a complete absence of neighbors. The given symptom—no HELLOs sent at all—is better explained by a passive interface, not an authentication mismatch.
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 command configures OSPF so that every interface is passive unless explicitly overridden with 'no passive-interface'. A passive interface does not send OSPF hello packets, but the connected subnet is still advertised into the OSPF database. Without a 'no passive-interface' statement applied to the link between R1 and R2, neither router will send hellos, and an adjacency can never form—exactly matching the symptom of no neighbors.
✗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?”
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.
Go deeper
Related to this question
Learn chapter
OSPFv3 Single-Area Configuration for IPv6
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
Key term
OSPF neighbor
An OSPF neighbor is another router that has been directly discovered through OSPF Hello packets and is willing to exchange routing information to build a network topology map.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco 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 200-301 exam.