JNCIA-JUNOS Junos Configuration Basics Practice Question
This JNCIA-JUNOS practice question tests your understanding of junos configuration basics. 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.
user@router> show configuration interfaces ge-0/0/0
unit 0 {
family inet {
address 10.0.0.1/24;
}
family inet6 {
address 2001:db8::1/64;
}
}
Refer to the exhibit. An engineer configures interface ge-0/0/0 with both IPv4 and IPv6 addresses. The engineer notices that the interface is operationally up, but the IPv6 address is not pingable from a directly connected host. The host has an IPv6 address in the same subnet. 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.
user@router> show configuration interfaces ge-0/0/0
unit 0 {
family inet {
address 10.0.0.1/24;
}
family inet6 {
address 2001:db8::1/64;
}
}
A
The interface MTU is too small for IPv6 packets.
Why wrong: MTU affects both IPv4 and IPv6; if IPv4 works, MTU is likely sufficient.
B
The interface does not have IPv6 neighbor discovery enabled.
IPv6 requires neighbor discovery to be enabled; without it, the router may not respond to NS/NA messages.
C
The IPv6 address is not configured under the correct unit.
Why wrong: The exhibit shows the IPv6 address under unit 0, which is correct.
D
The interface has an IPv4 address, which prevents IPv6 from working.
Why wrong: Junos supports dual-stack; IPv4 and IPv6 can coexist on the same interface.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface does not have IPv6 neighbor discovery enabled.
IPv6 neighbor discovery (ND) is required for address resolution and duplicate address detection on a link. If ND is not enabled on the interface, the router cannot respond to Neighbor Solicitation messages from the host, making the IPv6 address unreachable even though the interface is operationally up. In Junos, ND is enabled by default on IPv6-enabled interfaces, but it can be disabled with the 'no-neighbor-discovery' configuration statement.
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.
✗
The interface MTU is too small for IPv6 packets.
Why it's wrong here
MTU affects both IPv4 and IPv6; if IPv4 works, MTU is likely sufficient.
✓
The interface does not have IPv6 neighbor discovery enabled.
Why this is correct
IPv6 requires neighbor discovery to be enabled; without it, the router may not respond to NS/NA messages.
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 IPv6 address is not configured under the correct unit.
Why it's wrong here
The exhibit shows the IPv6 address under unit 0, which is correct.
✗
The interface has an IPv4 address, which prevents IPv6 from working.
Why it's wrong here
Junos supports dual-stack; IPv4 and IPv6 can coexist on the same interface.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume IPv6 works automatically once an address is configured, overlooking that neighbor discovery is a separate protocol that must be enabled for basic connectivity on multi-access links.
Trap categories for this question
Command / output trap
The exhibit shows the IPv6 address under unit 0, which is correct.
Detailed technical explanation
How to think about this question
IPv6 neighbor discovery uses ICMPv6 messages (Neighbor Solicitation and Neighbor Advertisement) to map IPv6 addresses to MAC addresses, replacing ARP in IPv4. When ND is disabled, the router cannot learn the host's MAC address or respond to NS messages, so the host cannot resolve the router's IPv6 address to a link-layer address. In Junos, this is controlled at the interface level under 'family inet6' with the 'no-neighbor-discovery' statement, which is sometimes used in point-to-point links where ND is unnecessary.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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 Configuration Basics — This question tests Junos Configuration Basics — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The interface does not have IPv6 neighbor discovery enabled. — IPv6 neighbor discovery (ND) is required for address resolution and duplicate address detection on a link. If ND is not enabled on the interface, the router cannot respond to Neighbor Solicitation messages from the host, making the IPv6 address unreachable even though the interface is operationally up. In Junos, ND is enabled by default on IPv6-enabled interfaces, but it can be disabled with the 'no-neighbor-discovery' configuration statement.
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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