CCNA Network Infrastructure and Connectivity Practice Question
A technician is troubleshooting a connection between two routers, R1 and R2, connected back-to-back using Ethernet cables. Both routers have their interfaces configured and are in an 'up/up' state. R1's interface uses 192.168.1.1/24, and R2's interface uses 192.168.2.1/24. When the technician attempts to ping R2 from R1, the ping fails. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the concept that directly connected devices must share the same subnet, and the trap here is that candidates assume 'up/up' means Layer 3 connectivity is guaranteed, or they mistakenly think a routing protocol is needed to exchange routes between directly connected interfaces.
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 IP addresses assigned to the interfaces belong to different subnets, preventing direct Layer 3 communication.
The ping fails because R1's interface is configured with IP address 192.168.1.1/24, which places it in the 192.168.1.0/24 subnet, while R2's interface uses 192.168.2.1/24, placing it in the 192.168.2.0/24 subnet. For two devices to communicate directly at Layer 3 over a single Ethernet link, their IP addresses must belong to the same subnet. Since these addresses are in different subnets, R1 will see the destination as unreachable and will not even attempt to send an ARP request for R2's MAC address, resulting in a failed ping.
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 Ethernet cable is faulty, causing intermittent physical layer failures.
Why it's wrong here
A faulty Ethernet cable typically causes the interface to fluctuate between up/down states or show excessive CRC errors on the physical layer. In this scenario, both interfaces remain consistently in an up/up state, which means carrier is detected and the physical layer is functioning. While a marginal cable could introduce intermittent errors, it would not produce a total inability to ping while the link status stays stable. Thus, the cable fault is not the root cause of the communication failure.
- ✗
A duplex mismatch exists between R1 and R2, causing one-way communication.
Why it's wrong here
A duplex mismatch causes CRC errors, late collisions, and poor performance, but it does not entirely block all traffic. More importantly, even with matching duplex settings, the different subnets would still prevent any ping from succeeding.
- ✗
A routing protocol is not configured to allow the routers to learn about each other's directly connected networks.
Why it's wrong here
Routers automatically place directly connected subnets into their routing table when the interface IP matches that subnet. Since these routers belong to two different subnets (192.168.1.0/24 and 192.168.2.0/24), they do not have routes to each other’s local network. However, a routing protocol would not help because they are on the same Ethernet segment and would still need to be in the same subnet for neighbor adjacency. The real cause is the IP mismatch, not routing protocol absence.
- ✓
The IP addresses assigned to the interfaces belong to different subnets, preventing direct Layer 3 communication.
Why this is correct
When two devices are in the same broadcast domain and have IP addresses in different subnets, they do not consider each other as local destinations. Router R1 will not attempt to ARP for 192.168.2.1 because it believes that address is in a different network, making the ping fail even though link status is up/up.
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 IP addresses assigned to the interfaces belong to different subnets, preventing direct Layer 3 communication.Correct answer▾
Why this is correct
When two devices are in the same broadcast domain and have IP addresses in different subnets, they do not consider each other as local destinations. Router R1 will not attempt to ARP for 192.168.2.1 because it believes that address is in a different network, making the ping fail even though link status is up/up.
✗The Ethernet cable is faulty, causing intermittent physical layer failures.Wrong answer — click to see why▾
Why this is wrong here
Candidates might suspect a physical issue first, but the up/up status definitively rules out a cable or hardware problem.
✗A duplex mismatch exists between R1 and R2, causing one-way communication.Wrong answer — click to see why▾
Why this is wrong here
Candidates commonly associate ‘cannot ping’ with duplex mismatches, forgetting that the fundamental issue here is the IP subnet mismatch.
✗A routing protocol is not configured to allow the routers to learn about each other's directly connected networks.Wrong answer — click to see why▾
Why this is wrong here
The trap is thinking that a routing protocol is always needed for inter-subnet communication, overlooking that directly connected devices on the same broadcast domain must share a common subnet.
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
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
ARP
Address Resolution Protocol (ARP) is a network protocol used to map a device's IP address to its physical MAC address so data can be delivered correctly on a local network.
Key term
Ethernet
Ethernet is a wired networking technology that connects devices like computers, printers, and servers to each other and to the internet using cables.
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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.