JN0-106 Networking Fundamentals Practice Question
Which field in an Ethernet frame is used by a switch to learn which source MAC address belongs to which port?
⚠ Common exam trap
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.
Answer choices
Why each option matters
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.
Answer analysis
Option-by-option breakdown
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
The Ethertype field indicates the protocol carried in the payload (e.g., IPv4 or IPv6). During MAC learning, a switch examines only the source MAC address and the ingress port; it does not parse the Ethertype or any higher-layer headers. The Ethertype is used by the data plane for demultiplexing the frame to the appropriate upper-layer protocol stack, not for building the forwarding database. Thus, while important for protocol identification, it plays no role in learning MAC-to-port associations.
- ✗
Destination MAC address
Why it's wrong here
Destination MAC address is used for forwarding decisions: the switch looks up this address in its MAC address table to determine the outgoing port (or flood if unknown). MAC learning is the process of associating the source MAC address with the port on which the frame arrives, not the destination. When a frame arrives, the switch records the source MAC and ingress port, then uses the destination MAC for the forwarding lookup. The destination MAC is never used as the key for learning an entry; it is the target of the lookup.
- ✗
VLAN ID field
Why it's wrong here
VLAN ID determines which broadcast domain the frame belongs to, and it is used alongside the MAC address table to isolate traffic between VLANs. The MAC address table entries are per-VLAN, but the learning process itself is driven by the source MAC address combined with the ingress port; the VLAN ID is a qualifier for the entry, not the learned attribute. A switch does not learn the VLAN ID from the frame; it assigns the port's configured VLAN ID to the frame. The VLAN ID influences where the MAC entry is stored, but it is not the field used to learn a new MAC-to-port mapping.
- ✓
Source MAC address
Why this is correct
The switch learns that the source MAC address is reachable out of the port on which the frame arrived, creating or updating a MAC address table entry. This is the fundamental operation of transparent bridging: the switch records the association between the source MAC address, ingress port, and VLAN ID. Once learned, the entry is used to forward future frames destined to that MAC address only to the correct port, rather than flooding. Thus, the source MAC address is the field that triggers and drives MAC learning in an Ethernet switch.
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 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 JN0-106 exam.