CCNA IP Routing Practice Question
Exhibit
R1 G0/1: 10.1.12.1 255.255.255.0 R2 G0/1: 10.1.12.2 255.255.255.252 Both interfaces are up and in area 0
R1 is not forming an OSPF adjacency with R2 on GigabitEthernet0/1. Which mismatch below is the most likely cause?
⚠ Common exam trap
A common exam trap is confusing the need for matching router IDs with adjacency formation. While router IDs must be unique within an OSPF domain, they do not need to match for adjacency to form. Another trap is assuming OSPF cannot run on GigabitEthernet interfaces, which is incorrect because OSPF supports all standard interface types. The most tempting mistake is overlooking the subnet mask mismatch, which silently prevents adjacency by making routers believe they are on different networks, even if other parameters like area ID match. This subtle misconfiguration is often missed during troubleshooting.
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 routers are using different IP subnet masks on the shared link
For OSPF neighbors to form, key settings on a shared segment must match. A subnet mask mismatch on a broadcast network often prevents proper neighbor formation because the routers do not agree on the local network.
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 routers are using different IP subnet masks on the shared link
Why this is correct
For OSPF to form an adjacency on a broadcast multiaccess segment, both routers must agree on the subnet mask assigned to the shared link. If the masks differ, each router derives a different network address from the same IP prefix, so OSPF considers the interfaces to be on separate subnets and ignores the hello packets. Consequently, the neighbor relationship never reaches the two-way state, and the adjacency fails to establish. This is a classic mismatch that prevents OSPF from operating correctly.
- ✗
OSPF cannot run on GigabitEthernet interfaces
Why it's wrong here
OSPF is a Layer 3 routing protocol that operates independently of the underlying Layer 2 technology, as long as IP is supported. GigabitEthernet interfaces are fully capable of running OSPF; they are treated as broadcast multiaccess networks, which means OSPF will elect a designated router and backup designated router. There is no inherent limitation that prevents OSPF from running on GigabitEthernet, so this statement is false. The issue must lie elsewhere, such as a subnet mask mismatch.
When this WOULD be correct
In a different question, if it were stated that OSPF is being configured on a legacy interface type that does not support OSPF, such as a serial interface with specific limitations, then this option could be correct. For example, if the question specified that OSPF was being attempted on a non-supported interface type, this would be the right answer.
- ✗
Both routers are in area 0
Why it's wrong here
Both routers residing in area 0 is a correct and necessary condition for OSPF adjacency, because routers must be in the same area to exchange link-state information. Area 0 is the backbone area, and all areas must connect to it, so being in area 0 is expected and not a fault. In fact, if the routers were in different areas, they could not form an OSPF adjacency unless they were on an ABR. Therefore, this option describes a normal configuration, not the cause of the failure.
When this WOULD be correct
In a different scenario, if the question stated that both routers were configured to use different OSPF versions or protocols that are not supported on GigabitEthernet interfaces, then this option could be correct. For example, if one router was using a legacy protocol incompatible with GigabitEthernet, it would prevent OSPF from forming an adjacency.
- ✗
The router IDs must match for adjacency to form
Why it's wrong here
Router IDs must be unique within an OSPF domain, not identical; duplicate router IDs would actually prevent adjacency because OSPF uses the router ID to identify the neighbor and would treat the duplicate as a conflict. For two routers to become adjacent, each must have a distinct router ID, ensuring that the neighbor table correctly maps to unique sources of routing information. Saying the router IDs must match is precisely the opposite of the requirement, making this option incorrect. The actual problem is more likely an interface-level parameter mismatch like subnet masks.
When this WOULD be correct
In a different scenario where the question specifies that both routers are configured to use the same router ID for redundancy or load balancing, and they are incorrectly set to the same value, this option would be correct. For example, if the question states that both routers are configured in a way that they must share the same router ID for a specific OSPF design.
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 routers are using different IP subnet masks on the shared linkCorrect answer▾
Why this is correct
For OSPF to form an adjacency on a broadcast multiaccess segment, both routers must agree on the subnet mask assigned to the shared link. If the masks differ, each router derives a different network address from the same IP prefix, so OSPF considers the interfaces to be on separate subnets and ignores the hello packets. Consequently, the neighbor relationship never reaches the two-way state, and the adjacency fails to establish. This is a classic mismatch that prevents OSPF from operating correctly.
✗OSPF cannot run on GigabitEthernet interfacesWrong answer — click to see why▾
Why this is wrong here
OSPF fully supports GigabitEthernet interfaces; it can run on any Ethernet interface type, including FastEthernet, GigabitEthernet, and TenGigabitEthernet. The interface speed does not affect OSPF's ability to form an adjacency.
★ When this WOULD be the correct answer
In a different question, if it were stated that OSPF is being configured on a legacy interface type that does not support OSPF, such as a serial interface with specific limitations, then this option could be correct. For example, if the question specified that OSPF was being attempted on a non-supported interface type, this would be the right answer.
Why candidates choose this
Students might confuse OSPF's network type (broadcast vs. point-to-point) with interface speed, or incorrectly think that OSPF has limitations on high-speed interfaces.
✗Both routers are in area 0Wrong answer — click to see why▾
Why this is wrong here
Both routers being in area 0 is correct for forming an adjacency; OSPF requires that routers on the same link share the same area ID. This option describes a correct configuration, not a problem.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that both routers were configured to use different OSPF versions or protocols that are not supported on GigabitEthernet interfaces, then this option could be correct. For example, if one router was using a legacy protocol incompatible with GigabitEthernet, it would prevent OSPF from forming an adjacency.
Why candidates choose this
A test-taker might think that area 0 is reserved for backbone only and that other routers cannot be in area 0, but in fact, any router can be in area 0 as long as it is contiguous.
✗The router IDs must match for adjacency to formWrong answer — click to see why▾
Why this is wrong here
OSPF requires that router IDs be unique within the OSPF domain; identical router IDs would cause conflicts and prevent adjacency. However, the question states that adjacency is not forming, and identical router IDs would be a cause, but the option says 'must match' which is incorrect—they must be unique.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that both routers are configured to use the same router ID for redundancy or load balancing, and they are incorrectly set to the same value, this option would be correct. For example, if the question states that both routers are configured in a way that they must share the same router ID for a specific OSPF design.
Why candidates choose this
Students may confuse the concept of matching parameters (like area ID, subnet mask) with router IDs, thinking that router IDs need to match for adjacency, when in fact they must be unique.
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
Interpreting HSRP and VRRP Operational Status
Key term
Subnet
A subnet is a logical subdivision of an IP network, created by partitioning a larger network address space using subnet masks.
Key term
OSPF adjacency
An OSPF adjacency is a logical neighbor relationship formed between two OSPF routers that have completed a series of hello and database exchange processes, enabling them to share routing information and maintain a consistent view of the network topology.
About these practice questions
One of 1,389 original 200-301 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
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.