Quick Answer
The answer is the source MAC address. A switch performs MAC learning by examining the source MAC address field of every incoming Ethernet frame; when a frame arrives on a specific port, the switch records that source MAC address and associates it with that port in its MAC address table. This process allows the switch to build a dynamic mapping of which MAC addresses are reachable via which ports, so future frames destined for that MAC can be forwarded only to the correct port rather than being flooded. On the JNCIA-Junos exam, this concept tests your understanding of Layer 2 switching fundamentals, often appearing in questions about how a switch populates its forwarding table or handles unknown unicast frames. A common trap is confusing the source MAC address with the destination MAC address—remember that the destination MAC is used for forwarding decisions, while the source MAC is used for learning. A helpful memory tip: "Source for learning, destination for sending."
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.
Which field in an Ethernet frame is used by a switch to learn which source MAC address belongs to which port?
Ethertype field
Why wrong: Ethertype identifies the upper-layer protocol.
Destination MAC address
Why wrong: The destination MAC is used to look up the outgoing port.
VLAN ID field
Why wrong: VLAN ID determines the broadcast domain, not the MAC-to-port mapping.
Source MAC address
The switch learns the source MAC and associates it with the incoming port.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Source MAC address
A switch learns MAC address-to-port mappings by examining the source MAC address field of incoming Ethernet frames. When a frame arrives on a port, the switch records the source MAC address and associates it with that port in its MAC address table, enabling future frames destined for that MAC to be forwarded only to the correct port.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Ethertype field
Why it's wrong here
Ethertype identifies the upper-layer protocol.
Destination MAC address
Why it's wrong here
The destination MAC is used to look up the outgoing port.
VLAN ID field
Why it's wrong here
VLAN ID determines the broadcast domain, not the MAC-to-port mapping.
Source MAC address
Why this is correct
The switch learns the source MAC and associates it with the incoming port.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often confuse the role of the destination MAC address (used for forwarding decisions) with the source MAC address (used for learning), especially when they recall that switches forward frames based on destination MAC, but forget that learning is done from the source MAC.
Detailed technical explanation
Under the hood, a switch performs MAC learning by inspecting the source MAC address of every received frame and updating its MAC address table (also called CAM table) with the port and VLAN information. This process is defined in IEEE 802.1D, and the table is used for subsequent forwarding decisions; if a source MAC address is already learned on a different port, the switch updates the entry (a process known as MAC move) to reflect the new location, which is critical in networks with redundant links or when devices are moved between ports.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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FAQ
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: Source MAC address — A switch learns MAC address-to-port mappings by examining the source MAC address field of incoming Ethernet frames. When a frame arrives on a port, the switch records the source MAC address and associates it with that port in its MAC address table, enabling future frames destined for that MAC to be forwarded only to the correct port.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This JNCIA-JUNOS practice question is part of Courseiva's free Juniper Networks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the JNCIA-JUNOS exam.
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