The correct answer includes the statement that VLANs provide broadcast isolation between different VLANs, because a VLAN creates a separate Layer 2 broadcast domain, ensuring that broadcast traffic from one VLAN never reaches hosts in another VLAN on the same switch. This isolation is a core function of VLANs on Juniper EX switches, where each VLAN is mapped to a unique logical interface and broadcast traffic is confined to that VLAN’s members. On the JNCIA-Junos exam, this concept tests your understanding of how VLANs segment traffic at Layer 2, often appearing alongside questions about trunk ports and VLAN tagging. A common trap is confusing VLAN isolation with security—VLANs do not inherently provide security, only broadcast containment. Remember the mnemonic: “VLANs block broadcasts, not bad actors.”
JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 statements are true about VLANs on Juniper EX switches? (Select three.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A single VLAN can extend across multiple switches via trunk ports
Option B is correct because VLANs are designed to span multiple switches by using trunk ports that carry traffic for multiple VLANs. On Juniper EX switches, trunk ports (configured with the 'trunk' port mode) allow VLAN-tagged frames to traverse between switches, enabling a single VLAN to extend across a network. This is fundamental to VLAN operation, as it allows devices in the same VLAN to communicate regardless of which switch they are connected to.
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.
✗
Hosts in different VLANs can communicate without any additional configuration
Why it's wrong here
Inter-VLAN communication requires a Layer 3 device.
✓
A single VLAN can extend across multiple switches via trunk ports
Why this is correct
Trunk ports carry multiple VLANs between switches.
Related concept
Read the scenario before looking for a memorised answer.
✓
Each VLAN must have a unique VLAN ID within a single switch
Read the scenario before looking for a memorised answer.
✗
VLANs are automatically created when you assign an interface to an untagged VLAN
Why it's wrong here
VLANs must be created explicitly before or after interface assignment.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume VLANs are automatically created when an interface is assigned to them (as in some Cisco IOS versions), but Juniper EX switches require explicit VLAN definition in the configuration, and they also mistakenly think inter-VLAN communication is possible without a router due to the 'native VLAN' concept.
Detailed technical explanation
How to think about this question
VLANs operate by tagging Ethernet frames with a 12-bit VLAN ID (per IEEE 802.1Q), allowing switches to segregate broadcast domains. On Juniper EX switches, the 'vlan' statement under the 'interfaces' hierarchy associates an interface with a VLAN, but the VLAN itself must be defined in the 'vlans' stanza with a unique VLAN ID. A real-world scenario where this matters is in large campus networks where a single VLAN (e.g., VLAN 10 for the accounting department) must span multiple access switches, requiring trunk links between them to carry the tagged traffic.
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 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.
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: A single VLAN can extend across multiple switches via trunk ports — Option B is correct because VLANs are designed to span multiple switches by using trunk ports that carry traffic for multiple VLANs. On Juniper EX switches, trunk ports (configured with the 'trunk' port mode) allow VLAN-tagged frames to traverse between switches, enabling a single VLAN to extend across a network. This is fundamental to VLAN operation, as it allows devices in the same VLAN to communicate regardless of which switch they are connected to.
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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