JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking 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.
```
user@switch> show configuration interfaces ge-0/0/1
unit 0 {
family ethernet-switching {
port-mode access;
vlan {
members VLAN10;
}
}
}
user@switch> show vlans
VLAN name Status Ports
VLAN10 Up ge-0/0/1, ge-0/0/2
VLAN20 Up ge-0/0/3
```
Refer to the exhibit. A host connected to ge-0/0/1 cannot reach a host connected to ge-0/0/2 even though both are in VLAN10. 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@switch> show configuration interfaces ge-0/0/1
unit 0 {
family ethernet-switching {
port-mode access;
vlan {
members VLAN10;
}
}
}
user@switch> show vlans
VLAN name Status Ports
VLAN10 Up ge-0/0/1, ge-0/0/2
VLAN20 Up ge-0/0/3
```
A
VLAN10 is not defined in the global VLAN configuration.
Why wrong: The VLAN is shown as active.
B
The hosts are configured with IP addresses on different subnets.
They can't communicate at Layer 2? Actually Layer 2 doesn't care about IP. So this is tricky. Actually within same VLAN, IP subnet must match for Layer 3, but Layer 2 should work. However if hosts are on different subnets, they need a router. But the question says 'cannot reach' - likely they try to ping. If on different subnets, they need default gateway. So the most likely cause is they are on different subnets and no gateway. Alternatively, the switch might have port security. But given typical JNCIA, this is plausible.
C
An IRB interface for VLAN10 is not configured.
Why wrong: IRB is for Layer 3 routing, not needed for intra-VLAN.
D
The interface ge-0/0/2 is configured as a trunk port.
Why wrong: Exhibit shows ge-0/0/2 is in VLAN10 in the VLAN list, implying it's an access port? Actually the VLAN list shows ports, but not its mode. Could be trunk. But not stated.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The hosts are configured with IP addresses on different subnets.
Option B is correct because for two hosts in the same VLAN to communicate at Layer 2, they must be in the same IP subnet. If the hosts are configured with IP addresses on different subnets, they will attempt to route traffic through a default gateway rather than sending ARP requests directly, causing communication failure even though they are in the same broadcast domain.
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.
✗
VLAN10 is not defined in the global VLAN configuration.
The hosts are configured with IP addresses on different subnets.
Why this is correct
They can't communicate at Layer 2? Actually Layer 2 doesn't care about IP. So this is tricky. Actually within same VLAN, IP subnet must match for Layer 3, but Layer 2 should work. However if hosts are on different subnets, they need a router. But the question says 'cannot reach' - likely they try to ping. If on different subnets, they need default gateway. So the most likely cause is they are on different subnets and no gateway. Alternatively, the switch might have port security. But given typical JNCIA, this is plausible.
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.
✗
An IRB interface for VLAN10 is not configured.
Why it's wrong here
IRB is for Layer 3 routing, not needed for intra-VLAN.
✗
The interface ge-0/0/2 is configured as a trunk port.
Why it's wrong here
Exhibit shows ge-0/0/2 is in VLAN10 in the VLAN list, implying it's an access port? Actually the VLAN list shows ports, but not its mode. Could be trunk. But not stated.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume VLAN membership alone guarantees IP connectivity, overlooking the fundamental requirement that hosts must share the same IP subnet for direct Layer 2 communication.
Trap categories for this question
Command / output trap
The VLAN is shown as active.
Detailed technical explanation
How to think about this question
In a switched network, VLANs segment the broadcast domain at Layer 2, but IP communication still requires hosts to be on the same subnet to send frames directly via ARP. If hosts are on different subnets, they will send packets to their configured default gateway, which may not exist or may not be reachable, resulting in no connectivity. This is a common misconfiguration when VLANs are used to separate subnets but hosts are accidentally placed in the same VLAN with mismatched IP addresses.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
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.
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The hosts are configured with IP addresses on different subnets. — Option B is correct because for two hosts in the same VLAN to communicate at Layer 2, they must be in the same IP subnet. If the hosts are configured with IP addresses on different subnets, they will attempt to route traffic through a default gateway rather than sending ARP requests directly, causing communication failure even though they are in the same broadcast domain.
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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