The answer is that the switch forwards the frame only out of the port associated with that destination MAC address. This is correct because the switch performs a lookup in its MAC address table, and when it finds a matching entry for a known unicast address, it uses the associated port as the sole egress interface—a process called unicast forwarding or filtering. On the JNCIA-Junos exam, this concept tests your understanding of how a switch avoids unnecessary flooding by using its learned MAC table; a common trap is confusing this with broadcast or unknown unicast handling, where the frame is flooded out all ports except the ingress. A reliable memory tip is to think of the MAC table as a precise map: if the destination address is “known,” the switch takes the direct route, not the floodplain.
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.
A switch receives a unicast frame with a destination MAC address that is present in its MAC address table. How does the switch process the frame?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It forwards the frame only out of the port associated with that MAC address
When a switch receives a unicast frame and the destination MAC address is already in its MAC address table, it performs a lookup and forwards the frame only out of the specific port associated with that MAC address. This is the fundamental switching behavior known as 'unicast forwarding' or 'filtering,' which avoids unnecessary flooding and preserves bandwidth.
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.
✗
It sends the frame back to the source port
Why it's wrong here
The switch does not forward frames back to the source port to prevent loops.
✗
It floods the frame to all ports except the receiving port
Why it's wrong here
Flooding is for unknown unicast, broadcast, or multicast frames.
✗
It drops the frame
Why it's wrong here
Known unicast frames are forwarded, not dropped.
✓
It forwards the frame only out of the port associated with that MAC address
Why this is correct
The switch uses the MAC table to forward the frame only to the correct port.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the behavior for an unknown unicast (which is flooded) with a known unicast (which is forwarded only to the specific port), leading them to incorrectly select option B.
Detailed technical explanation
How to think about this question
The switch maintains a MAC address table (also called CAM table) that maps MAC addresses to specific ports, learned by examining the source MAC address of incoming frames. When a known unicast frame arrives, the switch performs a hardware-based lookup in the CAM table and forwards the frame only to the egress port associated with that MAC address, a process called 'store-and-forward' or 'cut-through' depending on the switch architecture. In Juniper EX series switches, this behavior is controlled by the Layer 2 forwarding engine and can be verified with the 'show ethernet-switching table' command.
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.
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: It forwards the frame only out of the port associated with that MAC address — When a switch receives a unicast frame and the destination MAC address is already in its MAC address table, it performs a lookup and forwards the frame only out of the specific port associated with that MAC address. This is the fundamental switching behavior known as 'unicast forwarding' or 'filtering,' which avoids unnecessary flooding and preserves bandwidth.
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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Question Discussion
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