CCNA IP Routing Practice Question
Exhibit
R1# interface GigabitEthernet0/0 ip address 10.1.50.1 255.255.255.0 ! router ospf 1 network 10.1.50.0 0.0.0.255 area 0 R2# interface GigabitEthernet0/0 ip address 10.1.50.2 255.255.255.0 ! router ospf 1 network 10.1.50.0 0.0.0.255 area 1
R1 and R2 are connected via Ethernet and are configured with OSPF, but they fail to form an adjacency. Upon checking the interface configurations, you see that R1’s interface is in OSPF area 0 while R2’s interface is in area 1, and both interfaces use default timers and are in the same subnet. What is the most likely reason?
⚠ Common exam trap
A common mistake is believing that OSPF process IDs must match between routers to form an adjacency; process IDs are locally significant and do not need to 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
✓
The OSPF areas do not match on the shared segment.
The most likely cause is an OSPF area mismatch on the shared segment. R1 and R2 are in the same IP subnet and use default hello/dead timers, so neither subnet mismatch nor timer mismatch is the problem. OSPF can run on Ethernet interfaces. However, OSPF requires neighbors on the same link to agree on the area ID; here they differ, preventing adjacency. Option B is incorrect because OSPF process IDs are locally significant and do not need to match.
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 OSPF areas do not match on the shared segment.
Why this is correct
OSPF requires both neighbors to agree on the area ID for a given link; a mismatch prevents the formation of adjacency because the Hello packets carry the area ID and a mismatch causes the receiving router to drop them. Even if subnets and timers match, an area mismatch leaves the neighbor state stuck in Down or Attempt.
- ✗
The routers must use the same process ID.
Why it's wrong here
The OSPF process ID is locally significant only; it does not appear in OSPF packets or influence neighbor adjacency. Two routers can use different process IDs and still become fully adjacent as long as other parameters (area, authentication, timers, network type, subnet) match. Therefore a mismatched process ID would not prevent adjacency and is not the reason in this exhibit.
When this WOULD be correct
In a different scenario where the question specifies that the routers are configured with multiple OSPF processes and the exam asks about the necessity of matching process IDs for adjacency, this option would be correct. For example, if the question involved multiple OSPF instances on the same router, it would be relevant.
- ✗
The interfaces are in different IPv4 subnets.
Why it's wrong here
The exhibit shows both interfaces have addresses in the same /24 subnet, so the subnet mask and network prefix match. OSPF Hello packets include the network mask; if they differed, the adjacency would fail, but here that condition is satisfied so this is not the cause. Because the IP addresses are consistent, this cannot be the reason R1 is not forming an OSPF neighbor relationship.
When this WOULD be correct
In a different question setup, if the exam asked about OSPF adjacency issues where the interfaces of R1 and R2 are indeed in different IPv4 subnets, this option would be correct. For example, if the question specified that R1's interface is in 192.168.1.0/24 and R2's interface is in 192.168.2.0/24, then this option would apply.
- ✗
OSPF cannot run on Ethernet interfaces.
Why it's wrong here
OSPF is designed to operate over broadcast multiaccess networks such as Ethernet, using multicast Hello packets to 224.0.0.5 and DR/BDR election for efficiency. Ethernet is a supported and typical OSPF network type (broadcast), so the interface type is not the problem. This statement is false as a general restriction.
When this WOULD be correct
In a different exam scenario, if the question stated that R1 and R2 were configured on a non-Ethernet interface type, such as a serial link, and asked about OSPF compatibility, then this option could be correct if the context implied that OSPF was not supported on that specific interface type.
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 OSPF areas do not match on the shared segment.Correct answer▾
Why this is correct
OSPF requires both neighbors to agree on the area ID for a given link; a mismatch prevents the formation of adjacency because the Hello packets carry the area ID and a mismatch causes the receiving router to drop them. Even if subnets and timers match, an area mismatch leaves the neighbor state stuck in Down or Attempt.
✗The routers must use the same process ID.Wrong answer — click to see why▾
Why this is wrong here
OSPF process IDs are locally significant to each router; matching process IDs between neighbors is not a requirement for adjacency.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that the routers are configured with multiple OSPF processes and the exam asks about the necessity of matching process IDs for adjacency, this option would be correct. For example, if the question involved multiple OSPF instances on the same router, it would be relevant.
Why candidates choose this
Candidates may find this option tempting because they might confuse OSPF process IDs with the requirement for matching OSPF configurations, leading them to believe that process IDs play a critical role in forming adjacencies.
✗The interfaces are in different IPv4 subnets.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because OSPF can form adjacencies between routers as long as their interfaces are in the same IPv4 subnet, regardless of the OSPF process IDs. The issue in this scenario is related to OSPF area mismatches, not subnet differences.
★ When this WOULD be the correct answer
In a different question setup, if the exam asked about OSPF adjacency issues where the interfaces of R1 and R2 are indeed in different IPv4 subnets, this option would be correct. For example, if the question specified that R1's interface is in 192.168.1.0/24 and R2's interface is in 192.168.2.0/24, then this option would apply.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of OSPF requirements, believing that adjacency cannot be formed if routers are on different subnets, which is a common misconception.
✗OSPF cannot run on Ethernet interfaces.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF can indeed run on Ethernet interfaces; it is a common practice in networking. The issue with R1 and R2 not forming an adjacency is likely due to mismatched OSPF areas, not the interface type.
★ When this WOULD be the correct answer
In a different exam scenario, if the question stated that R1 and R2 were configured on a non-Ethernet interface type, such as a serial link, and asked about OSPF compatibility, then this option could be correct if the context implied that OSPF was not supported on that specific interface type.
Why candidates choose this
Candidates may find this option tempting because they might recall that certain routing protocols have limitations on specific interface types, leading to confusion about OSPF's compatibility with Ethernet.
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
Segment
A segment is a division of a larger network, such as a collision domain, broadcast domain, or a portion of a TCP data stream, used to organize traffic and improve performance.
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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.