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.
A network engineer is troubleshooting connectivity between two hosts on the same VLAN connected to different Juniper EX switches. The MAC address table on each switch shows the correct MAC addresses for both hosts, but ping fails. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VLAN is not allowed on the trunk between the switches
If both hosts are on the same VLAN and their MAC addresses are correctly learned on each switch, the issue is likely that the VLAN is not allowed on the trunk link connecting the switches. Without the VLAN being permitted on the trunk, frames from that VLAN will be dropped at the trunk interface, preventing Layer 2 communication between the hosts even though the MAC tables are correct.
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 VLAN is not allowed on the trunk between the switches
Why this is correct
If the VLAN is not allowed on the trunk, frames are dropped, even though MAC addresses are learned on access ports.
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 hosts are configured with IP addresses in different subnets
Why it's wrong here
Hosts on the same VLAN should be in the same IP subnet; if not, they cannot communicate at Layer 3.
✗
The ARP cache on the hosts is stale
Why it's wrong here
A stale ARP cache would cause a unicast frame to be sent to the wrong MAC, but the MAC table entries would not both be present.
✗
Spanning Tree Protocol is blocking a port
Why it's wrong here
STP blocking would prevent the port from forwarding, but the MAC table would not show that port.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume MAC table correctness implies full Layer 2 connectivity, overlooking the trunkVLAN filtering mechanism that can drop frames even when MAC addresses are learned.
Trap categories for this question
Command / output trap
STP blocking would prevent the port from forwarding, but the MAC table would not show that port.
Detailed technical explanation
How to think about this question
On Juniper EX switches, trunk ports are configured with the 'set interfaces ge-0/0/X unit 0 family ethernet-switching port-mode trunk' command, and VLAN membership is controlled via 'set vlans VLAN-NAME vlan-id X' and 'set interfaces ge-0/0/X unit 0 family ethernet-switching vlan members VLAN-NAME'. If a VLAN is not listed as a member on the trunk, frames tagged with that VLAN ID will be discarded at the trunk interface, effectively isolating the VLAN between switches. This is a common misconfiguration when adding a new VLAN to an existing trunk without updating the allowed VLAN list.
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 VLAN is not allowed on the trunk between the switches — If both hosts are on the same VLAN and their MAC addresses are correctly learned on each switch, the issue is likely that the VLAN is not allowed on the trunk link connecting the switches. Without the VLAN being permitted on the trunk, frames from that VLAN will be dropped at the trunk interface, preventing Layer 2 communication between the hosts even though the MAC tables are correct.
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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Question Discussion
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