JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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
MAC address table:
VLAN MAC address Type Age Interfaces
default 00:00:5e:00:01:01 Static - ge-0/0/0.0
vlan10 00:0c:29:2a:3b:4c Dynamic 20 ge-0/0/1.0
vlan10 00:0c:29:2a:3b:4d Dynamic 10 ge-0/0/2.0
vlan20 00:0c:29:2a:3b:4e Dynamic 5 ge-0/0/3.0
Refer to the exhibit. A frame with destination MAC 00:0c:29:2a:3b:4d arrives on interface ge-0/0/0. What action will the switch take?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Forward the frame out of ge-0/0/2 only
The switch learns MAC addresses and their associated VLANs from incoming frames. Since the destination MAC 00:0c:29:2a:3b:4d is already in the MAC address table and mapped to interface ge-0/0/2 within the same VLAN, the switch forwards the frame only out of ge-0/0/2. This is the fundamental behavior of transparent bridging: unicast frames are forwarded only to the port where the destination MAC was last seen.
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.
✗
Flood the frame to all ports except ge-0/0/0
Why it's wrong here
Flooding occurs for unknown unicast, not known.
✓
Forward the frame out of ge-0/0/2 only
Why this is correct
The MAC table shows the destination MAC on interface ge-0/0/2.
Related concept
Read the scenario before looking for a memorised answer.
✗
Drop the frame because the VLAN does not match
Why it's wrong here
The frame belongs to vlan10, which is correct.
✗
Forward the frame out of both ge-0/0/1 and ge-0/0/2
Why it's wrong here
The switch forwards to a single port unless it is multicast or broadcast.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a switch always floods unknown unicast frames, but the question explicitly provides a known destination MAC, so the correct action is unicast forwarding, not flooding.
Detailed technical explanation
How to think about this question
The MAC address table (also called the forwarding database or FDB) is populated dynamically by examining the source MAC of incoming frames. Each entry includes the MAC address, VLAN ID, and outgoing interface. When a frame arrives, the switch performs a lookup on the destination MAC and VLAN; if a match is found, it forwards the frame only to the associated port. This process is defined by IEEE 802.1D for transparent bridging. In a real-world scenario, if the MAC table entry ages out (default 300 seconds on Junos), the switch would revert to flooding the frame to all ports in the VLAN.
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: Forward the frame out of ge-0/0/2 only — The switch learns MAC addresses and their associated VLANs from incoming frames. Since the destination MAC 00:0c:29:2a:3b:4d is already in the MAC address table and mapped to interface ge-0/0/2 within the same VLAN, the switch forwards the frame only out of ge-0/0/2. This is the fundamental behavior of transparent bridging: unicast frames are forwarded only to the port where the destination MAC was last seen.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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