Quick Answer
The answer is the Source MAC address, Destination MAC address, and EtherType (or Length) field. These three fields form the core of an Ethernet frame header as defined by the IEEE 802.3 standard, where the Destination MAC identifies the intended recipient, the Source MAC identifies the sender, and the EtherType indicates the upper-layer protocol (such as IPv4 or ARP) encapsulated in the frame. On the JNCIA-Junos exam, this question tests your understanding of Layer 2 framing fundamentals, often appearing in the "Ethernet Switching" domain. A common trap is confusing the Preamble or Frame Check Sequence (FCS) as part of the header—they are not; the header ends after the EtherType. To remember, think "D-S-E" for Destination, Source, and EtherType—the three essential fields that switches use to forward frames and identify protocols.
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 THREE fields are part of an Ethernet frame header? (Select three.)
Destination MAC address
DA is a mandatory header field.
VLAN tag (802.1Q)
Why wrong: VLAN tag is optional and not always present.
EtherType field
EtherType indicates the protocol type and is mandatory.
Frame Check Sequence (FCS)
Why wrong: FCS is part of the trailer, not the header.
Source MAC address
SA is a mandatory header field.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Destination MAC address
The Ethernet frame header is defined by the IEEE 802.3 standard and consists of the Destination MAC address (6 bytes), Source MAC address (6 bytes), and the EtherType field (2 bytes) or Length field. The Destination MAC address is the first field in the header and identifies the intended recipient of the frame on the local network segment. Without it, switches and hosts would not know which device should process the frame.
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.
Destination MAC address
Why this is correct
DA is a mandatory header field.
Related concept
Read the scenario before looking for a memorised answer.
VLAN tag (802.1Q)
Why it's wrong here
VLAN tag is optional and not always present.
EtherType field
Why this is correct
EtherType indicates the protocol type and is mandatory.
Related concept
Read the scenario before looking for a memorised answer.
Frame Check Sequence (FCS)
Why it's wrong here
FCS is part of the trailer, not the header.
Source MAC address
Why this is correct
SA is a mandatory header field.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often confuse the optional 802.1Q VLAN tag or the FCS trailer as part of the Ethernet frame header, when in fact the header strictly contains only the Destination MAC, Source MAC, and EtherType/Length fields.
Detailed technical explanation
The Ethernet frame header fields are fixed in order: Destination MAC (6 bytes), Source MAC (6 bytes), and then either EtherType (for DIX Ethernet II, value > 1536) or Length (for IEEE 802.3, value ≤ 1500). The EtherType field identifies the upper-layer protocol (e.g., 0x0800 for IPv4, 0x86DD for IPv6). In a real-world scenario, a switch uses the Destination MAC address to make forwarding decisions via its MAC address table, and the Source MAC address to learn which port a device is connected to.
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: Destination MAC address — The Ethernet frame header is defined by the IEEE 802.3 standard and consists of the Destination MAC address (6 bytes), Source MAC address (6 bytes), and the EtherType field (2 bytes) or Length field. The Destination MAC address is the first field in the header and identifies the intended recipient of the frame on the local network segment. Without it, switches and hosts would not know which device should process the frame.
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
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