The answer is that the remote host is not reachable or is not responding to ARP requests. When a ping to a remote host fails but the local interface ping succeeds, it confirms the interface is up and IP configuration is correct, yet the router cannot complete Layer 2 address resolution. On a Juniper device, the router sends an ARP request to map the destination IP to a MAC address on the local subnet; without a successful ARP reply, the echo request cannot be transmitted. This scenario tests your understanding of the OSI model’s separation between Layer 3 reachability and Layer 2 connectivity, a common trap on the JNCIA-Junos exam where candidates mistakenly blame interface or routing issues. Remember the memory tip: “Ping self works, ping neighbor fails? Check ARP tables, not routes.”
JNCIA-JUNOS Junos OS Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of junos os fundamentals. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Refer to the exhibit.
root@router> show configuration interfaces
ge-0/0/0 {
unit 0 {
family inet {
address 192.168.1.1/24;
}
}
}
root@router> show interfaces ge-0/0/0 terse
Interface Admin Link Proto Local Remote
ge-0/0/0 up up inet 192.168.1.1/24
Refer to the exhibit. An operator tries to ping 192.168.1.2 from this router and fails. The router can ping itself (192.168.1.1). What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Refer to the exhibit.
root@router> show configuration interfaces
ge-0/0/0 {
unit 0 {
family inet {
address 192.168.1.1/24;
}
}
}
root@router> show interfaces ge-0/0/0 terse
Interface Admin Link Proto Local Remote
ge-0/0/0 up up inet 192.168.1.1/24
A
Reverse path forwarding (RPF) check is dropping the echo request.
Why wrong: No evidence of RPF; it's not shown in the config.
B
The remote host is not reachable or is not responding to ARP requests.
Ping failing to a host on the same subnet suggests ARP resolution failure or remote host down.
C
The interface is administratively down.
Why wrong: The output shows 'Admin: up'.
D
Proxy ARP is not configured on the interface.
Why wrong: Proxy ARP is for different subnets; not needed for direct communication.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The remote host is not reachable or is not responding to ARP requests.
The router can ping its own interface (192.168.1.1), confirming that the interface is up and IP is configured correctly. The failure to ping 192.168.1.2 indicates that the router cannot reach the remote host, most likely because the host is down, not connected, or not responding to ARP requests. ARP resolution is required for the router to map the destination IP to a MAC address on the local subnet; without a successful ARP reply, the router cannot send the echo request.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Reverse path forwarding (RPF) check is dropping the echo request.
Why it's wrong here
No evidence of RPF; it's not shown in the config.
✓
The remote host is not reachable or is not responding to ARP requests.
Why this is correct
Ping failing to a host on the same subnet suggests ARP resolution failure or remote host down.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The interface is administratively down.
Why it's wrong here
The output shows 'Admin: up'.
✗
Proxy ARP is not configured on the interface.
Why it's wrong here
Proxy ARP is for different subnets; not needed for direct communication.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse a local connectivity issue (like a down interface or RPF) with a remote host unreachability, but the ability to ping the local interface proves the interface is operational and the problem lies with the destination host or its ARP response.
Trap categories for this question
Command / output trap
No evidence of RPF; it's not shown in the config.
Detailed technical explanation
How to think about this question
When a router pings a destination on the same subnet, it first checks its ARP cache for the destination IP. If no entry exists, it sends an ARP request (broadcast) and waits for an ARP reply. If the remote host is offline or does not respond, the router will not have the MAC address needed to build the Ethernet frame, and the ping fails. The 'ping' command in Junos uses ICMP echo requests, which are encapsulated in IP packets and then in Ethernet frames; without a MAC address, the frame cannot be transmitted.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Junos OS Fundamentals — This question tests Junos OS Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The remote host is not reachable or is not responding to ARP requests. — The router can ping its own interface (192.168.1.1), confirming that the interface is up and IP is configured correctly. The failure to ping 192.168.1.2 indicates that the router cannot reach the remote host, most likely because the host is down, not connected, or not responding to ARP requests. ARP resolution is required for the router to map the destination IP to a MAC address on the local subnet; without a successful ARP reply, the router cannot send the echo request.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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