JN0-106 Networking Fundamentals Practice Question
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?
⚠ Common exam trap
Watch out — 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.
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
✓
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.
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
A switch never forwards a frame out the same port it arrived on. Doing so would send the frame back to the source device, which already delivered the frame and would interpret the echo as an error or loop. Moreover, the MAC address table specifically identifies the egress port for the destination, so sending it to the source port cannot deliver it to the intended recipient.
- ✗
It floods the frame to all ports except the receiving port
Why it's wrong here
Flooding to all ports except the receiving port is reserved for unknown unicast frames (where the destination MAC is not in the MAC table), as well as broadcast and multicast frames. Because the switch already knows the destination MAC from its MAC address table, it can make a targeted forwarding decision. Unnecessary flooding wastes bandwidth on every other segment and defeats the efficiency of a switched network.
- ✗
It drops the frame
Why it's wrong here
A known unicast frame is not dropped; the MAC address table contains an entry that maps the destination MAC to a specific port. Dropping would only occur under exceptional conditions, such as a MAC table lookup failure with flooding disabled, spanning-tree blocking, or frame corruption. Here, the correct action is to forward the frame based on the learned address, not to discard it.
- ✓
It forwards the frame only out of the port associated with that MAC address
Why this is correct
When a switch receives a unicast frame with a destination MAC address already in its MAC address table, it forwards the frame only out of the port associated with that MAC address. This is the fundamental switching behavior: the MAC table records source MAC addresses and their corresponding ingress ports, and later lookups for known destinations are delivered to that single port. This preserves bandwidth, reduces unnecessary traffic, and ensures the frame reaches only the intended station.
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 |
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JA
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.