CCNA IP Routing Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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
In OSPF, the passive-interface command suppresses both outgoing and incoming Hello packets on the specified interface, so no neighbor relationship can be established on that link. Because Serial0/0/0 has been marked passive under the OSPF process, R1 never sends or processes Hellos even though the network statement includes that IP. Issuing no passive-interface Serial0/0/0 in router configuration mode re-enables Hello traffic on exactly that interface, allowing R1 and R2 to form a full OSPF adjacency. This is the only option that directly addresses the root cause of the missing neighbor.
- ✗
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 existing network statement already matches R1's Serial0/0/0 address, so the interface is already participating in the OSPF process. Changing the wildcard mask to 0.0.0.255 would cover a larger range but does not alter the passive-interface configuration, which is evaluated independently of the network statement. Because passive-interface suppresses Hello packets on Serial0/0/0 regardless of how the matching network statement is written, hellos will still be suppressed and no adjacency will form. This change merely alters the set of interfaces declared in area 0, not the Hello behavior on the serial link.
- ✗
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, so R1 may use process 1 while R2 uses process 100; they do not need to match for a neighbor relationship to form. Re-entering the network statement after changing the process ID also does not remove the passive-interface configuration on Serial0/0/0, so the link remains silent. OSPF adjacency only requires agreement on parameters such as area ID, Hello/Dead timers, and authentication, none of which are affected by the process ID. Changing the process ID forces you to reconfigure the entire OSPF process, but it cannot solve the passive interface issue.
- ✗
Issue 'clear ip ospf process' on R1 to restart OSPF and reattempt neighbor discovery.
Why it's wrong here
The clear ip ospf process command restarts OSPF, resetting the routing table and rerunning SPF, but it does not modify the router's configuration. Since Serial0/0/0 is still configured as passive under OSPF, the interface will continue to suppress Hello packets after the restart, so R1 and R2 never exchange Hellos and the neighbor state remains Down. This command also causes a temporary routing disruption on all OSPF routes, which is undesirable in a production environment. Restarting the process without lifting the passive-interface restriction cannot establish the missing adjacency.
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
In OSPF, the passive-interface command suppresses both outgoing and incoming Hello packets on the specified interface, so no neighbor relationship can be established on that link. Because Serial0/0/0 has been marked passive under the OSPF process, R1 never sends or processes Hellos even though the network statement includes that IP. Issuing no passive-interface Serial0/0/0 in router configuration mode re-enables Hello traffic on exactly that interface, allowing R1 and R2 to form a full OSPF adjacency. This is the only option that directly addresses the root cause of the missing neighbor.
✗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?”
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
OSPF
OSPF is a link-state routing protocol used to find the best path for data packets to travel across IP networks, like a smart GPS that recalculates routes when traffic changes.
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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.