OSPF Adjacency Failure Due to Passive Interface
An administrator has just configured OSPF in a single area between router R1 and router R2, which are directly connected via their Gi0/0 interfaces with IP addresses 10.0.0.1/30 and 10.0.0.2/30. On R1, the command show ip ospf neighbor shows no entries, and a further check on R2 with show ip ospf interface gi0/0 indicates that the interface is passive. Which configuration error is most likely causing the adjacency failure?
Quick Answer
The answer is that R2 has the passive-interface default command configured without a corresponding no passive-interface Gi0/0 command. This is correct because OSPF passive interface prevents adjacency formation by blocking the sending and receiving of Hello packets on that interface, which are essential for neighbor discovery and the establishment of a full adjacency. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how the passive-interface default command globally silences all OSPF interfaces, a common trap where candidates forget to explicitly enable a specific interface with the no passive-interface command. The key distinction is that passive-interface default makes every interface passive, while passive-interface under a specific interface only affects that one. A helpful memory tip: think of “default” as a blanket that covers all interfaces—you must lift it off the one you need by using the “no” form.
⚠ Common exam trap
Cisco often tests the distinction between passive-interface default and the need for explicit no passive-interface commands, as candidates may assume that OSPF will automatically form adjacencies on directly connected interfaces without considering passive configuration.
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
✓
R2 has the passive-interface default command but no no passive-interface command for Gi0/0.
The output on R2 shows the Gi0/0 interface is passive, meaning OSPF will not send or receive hello packets on that interface, preventing neighbor discovery. The passive-interface default command makes all interfaces passive by default, and without a no passive-interface Gi0/0 command, the interface remains passive, blocking adjacency formation. This directly explains why R1's show ip ospf neighbor shows no entries.
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 command on R1 does not cover the 10.0.0.0/30 subnet.
Why it's wrong here
A missing network statement would prevent the subnet from being advertised into OSPF, but it would not automatically make the interface passive nor completely stop hello packets. The symptom of a passive interface is distinct.
- ✓
R2 has the passive-interface default command but no no passive-interface command for Gi0/0.
Why this is correct
The passive-interface default command sets all OSPF interfaces to passive mode. To allow neighbor adjacency on a specific interface, a no passive-interface <interface> command is required. Without it, Gi0/0 remains passive, preventing OSPF hellos and adjacency formation.
- ✗
The OSPF process IDs on R1 and R2 are mismatched.
Why it's wrong here
OSPF process IDs are locally significant and are not exchanged in OSPF packets. Neighbor relationships can form between routers using different process IDs.
- ✗
The hello and dead intervals on R1 and R2 are not the same.
Why it's wrong here
While mismatched hello/dead intervals do prevent adjacency, they would typically result in a state of INIT or EXSTART and would not cause the interface to be marked as passive.
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.
✓R2 has the passive-interface default command but no no passive-interface command for Gi0/0.Correct answer▾
Why this is correct
The passive-interface default command sets all OSPF interfaces to passive mode. To allow neighbor adjacency on a specific interface, a no passive-interface <interface> command is required. Without it, Gi0/0 remains passive, preventing OSPF hellos and adjacency formation.
✗The network command on R1 does not cover the 10.0.0.0/30 subnet.Wrong answer — click to see why▾
Why this is wrong here
This option fails to account for the explicit passive interface state shown on R2. A missing network statement would not cause the interface to be displayed as passive; it would simply not be enrolled in the OSPF process.
✗The OSPF process IDs on R1 and R2 are mismatched.Wrong answer — click to see why▾
Why this is wrong here
This is a common misconception, but process ID mismatch does not affect OSPF neighbor formation. The passive interface status on R2 directly contradicts this as the cause.
✗The hello and dead intervals on R1 and R2 are not the same.Wrong answer — click to see why▾
Why this is wrong here
The passive interface status would not appear if the only issue were interval mismatches; the interface would still be active and sending hellos. This directly conflicts with the given show output.
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
OSPF
OSPF is a link-state routing protocol that uses the SPF algorithm to compute the shortest path to each destination within a single autonomous system.
Key term
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
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Same concept, more angles
1 more way this is tested on 200-301
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. You are connected to R1 via console. The network administrator has attempted to configure OSPFv2 between R1, R2, and R3 but OSPF neighbor adjacencies are failing. Configure R1 to correct all issues so that R1 becomes FULL neighbors with both R2 and R3. Do not modify any other device's configuration.
hard- A.Remove the 'passive-interface default' command and remove the 'passive-interface GigabitEthernet0/0' and 'passive-interface GigabitEthernet0/1' commands under OSPF configuration.
- ✓ B.Add 'no passive-interface GigabitEthernet0/0' and 'no passive-interface GigabitEthernet0/1' under OSPF configuration, but keep 'passive-interface default'.
- C.Change the network statements to include the correct wildcard masks for the subnets on GigabitEthernet0/0 and GigabitEthernet0/1.
- D.Add 'ip ospf network point-to-point' on both GigabitEthernet0/0 and GigabitEthernet0/1 interfaces.
Why B: The core issue is that R1 has 'passive-interface default' under the OSPF process, which prevents all interfaces from sending or receiving OSPF hello packets, breaking neighbor formation. The correct fix is to override this default with 'no passive-interface' on the interfaces that connect to neighbors (GigabitEthernet0/0 and GigabitEthernet0/1), allowing OSPF to form adjacencies while keeping other interfaces passive for security. Option B matches the solution commands and is the intended approach. Option A removes the 'passive-interface default' entirely, which would also allow OSPF on all interfaces, but this is not the best practice and may expose unintended interfaces; therefore, option A is not the recommended solution and is considered incorrect in this exam context.
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.