CCNA IP Routing Practice Question
Two OSPF routers connected over Ethernet fail to become neighbors. Their interfaces are up/up and in the same IPv4 subnet. One router uses area 0 and the other uses area 1 on the connecting interfaces. What is the most likely cause?
⚠ Common exam trap
A common exam trap is assuming that OSPF process IDs must match between neighbors for adjacency to form. Many candidates mistakenly focus on process ID alignment, but OSPF process IDs are locally significant and do not need to match. The real cause of adjacency failure in this scenario is the mismatch in OSPF area IDs on the shared link. This subtle difference often leads to confusion, causing candidates to overlook the critical role of area consistency in neighbor formation and select incorrect answers related to process IDs or router IDs.
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
✓
Mismatched OSPF areas on the shared link
OSPF neighbors on the same link must agree on key parameters, including the area assigned to that interface. A mismatch prevents the adjacency from forming.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Mismatched OSPF process IDs
Why it's wrong here
OSPF process ID is local to each router and only identifies an instance of the OSPF routing process; it is not included in OSPF packets. Two routers can run different process IDs (e.g., 1 and 100) and still form an adjacency because the process ID has no meaning on the wire. Thus a mismatch would not affect neighbor formation.
When this WOULD be correct
In a scenario where two OSPF routers are configured with the same subnet but have different OSPF process IDs and are also in the same area, a question could ask why they are unable to establish a neighbor relationship. In this case, mismatched process IDs would be the correct answer.
- ✓
Mismatched OSPF areas on the shared link
Why this is correct
OSPF neighbors must agree on the area ID configured on the link. If one router places the interface in area 0 and the other in area 1, Hello packets are rejected because the area ID field doesn't match, preventing neighbor state from advancing beyond INIT. This is a fundamental adjacency requirement, separate from process ID or router ID issues.
- ✗
Missing default routes
Why it's wrong here
Default routes (0.0.0.0/0) are not a prerequisite for OSPF neighbor discovery or the exchange of link-state advertisements. OSPF forms adjacency and builds the LSDB independent of whether any default route exists in the routing table; a default route only matters for routing decisions after adjacency is up. Therefore, their absence does not explain a failure to become neighbors.
When this WOULD be correct
In a scenario where two routers are configured to use OSPF but are unable to reach each other due to missing default routes, a question might ask about the impact of routing on OSPF neighbor formation. In this case, if the routers cannot route packets to each other, the absence of a default route could prevent them from becoming neighbors.
- ✗
Different router IDs
Why it's wrong here
OSPF requires each router to have a unique Router ID; specifically, duplicates prevent adjacency but different IDs are normal and necessary. The Router ID is used to identify the router in LSAs and OSPF protocol exchanges, so two routers must have distinct RIDs to correctly establish neighbor relationships. If they were the same, that would be a problem; being different is expected.
When this WOULD be correct
In a different question setup, if two OSPF routers were configured in the same area but had mismatched router IDs, the question could ask about issues in OSPF route selection or redundancy. In this case, the option would be correct if the question focused on how router IDs affect OSPF operations.
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.
✓Mismatched OSPF areas on the shared linkCorrect answer▾
Why this is correct
OSPF neighbors must agree on the area ID configured on the link. If one router places the interface in area 0 and the other in area 1, Hello packets are rejected because the area ID field doesn't match, preventing neighbor state from advancing beyond INIT. This is a fundamental adjacency requirement, separate from process ID or router ID issues.
✗Mismatched OSPF process IDsWrong answer — click to see why▾
Why this is wrong here
Mismatched OSPF process IDs do not prevent OSPF routers from forming neighbor relationships if they are in the same subnet. The routers can still exchange OSPF packets; however, they will not share routing information if they are in different areas.
★ When this WOULD be the correct answer
In a scenario where two OSPF routers are configured with the same subnet but have different OSPF process IDs and are also in the same area, a question could ask why they are unable to establish a neighbor relationship. In this case, mismatched process IDs would be the correct answer.
Why candidates choose this
Candidates may mistakenly believe that OSPF process IDs are critical for establishing neighbor relationships, leading them to choose this option without fully considering the area configuration requirements.
✗Missing default routesWrong answer — click to see why▾
Why this is wrong here
This option is wrong because missing default routes does not affect OSPF neighbor adjacency directly; OSPF neighbors can still form without default routes as long as they have valid OSPF configurations and are in the same subnet.
★ When this WOULD be the correct answer
In a scenario where two routers are configured to use OSPF but are unable to reach each other due to missing default routes, a question might ask about the impact of routing on OSPF neighbor formation. In this case, if the routers cannot route packets to each other, the absence of a default route could prevent them from becoming neighbors.
Why candidates choose this
Candidates may choose this option because they might confuse general routing issues with OSPF neighbor formation, thinking that routing paths are essential for neighbor relationships, leading to the assumption that missing default routes could be a factor.
✗Different router IDsWrong answer — click to see why▾
Why this is wrong here
Different router IDs do not prevent OSPF routers from forming neighbor relationships; they are only used for OSPF route selection and identification. In this scenario, the routers are in different OSPF areas, which is the actual reason for the failure to become neighbors.
★ When this WOULD be the correct answer
In a different question setup, if two OSPF routers were configured in the same area but had mismatched router IDs, the question could ask about issues in OSPF route selection or redundancy. In this case, the option would be correct if the question focused on how router IDs affect OSPF operations.
Why candidates choose this
Candidates may confuse router IDs with OSPF neighbor relationships, thinking that differing IDs indicate a fundamental misconfiguration. This misunderstanding can lead them to incorrectly select this option when they see OSPF-related issues.
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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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.