JN0-106 Networking Fundamentals Practice Question
Which THREE statements are true about VLANs on Juniper EX switches? (Select three.)
⚠ Common exam trap
Many exam-takers 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.
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
✓
A single VLAN can extend across multiple switches via trunk ports
Option B is correct because a VLAN is a Layer 2 broadcast domain that can span multiple Juniper EX switches when the inter-switch links are configured as trunk ports carrying 802.1Q tags, allowing the same VLAN ID to be present on each switch. Option C is correct because within a single switch the VLAN ID (1–4094) must be unique; you cannot create two distinct VLANs with the same ID on one EX device, as the ID is the local identifier for that broadcast domain. Option D is correct because VLANs segment a switch into separate broadcast domains, so broadcast, unknown unicast, and multicast traffic from one VLAN is not flooded into another VLAN, providing broadcast isolation. Option A is not correct because hosts in different VLANs are in separate Layer 3 subnets and require a router or Layer 3 interface (e.g., an IRB/RVI) to communicate; no communication occurs by default. Option E is not correct because on Juniper EX switches, assigning an interface to a VLAN requires the VLAN to already exist (or be created with 'set vlans <name> vlan-id <id>'); it is not automatically created by the interface assignment.
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
VLANs segment into separate broadcast domains; without routing, hosts on different VLANs cannot exchange frames at Layer 2. A Layer 3 device such as a router or an EX Series switch with an IRB interface must be configured to forward traffic between VLANs. Thus, this statement is false as it ignores the necessity of Layer 3 routing.
- ✓
A single VLAN can extend across multiple switches via trunk ports
Why this is correct
On Junos, trunk ports are configured with interface-mode trunk and a list of allowed VLANs; they tag frames with 802.1Q so the same VLAN identifier is recognized on each switch. This allows end hosts in the same VLAN on different switches to communicate as if they were on the same LAN segment. The VLAN is a single logical broadcast domain that spans the trunk.
- ✓
Each VLAN must have a unique VLAN ID within a single switch
Why this is correct
Within a given switch, each VLAN ID maps to exactly one broadcast domain and one set of forwarding decisions; duplicate VLAN IDs would create ambiguity in frame classification and filtering. On Junos, VLANs are identified by a VID in the range 1-4094, and the ID must be unique within that switch. Therefore, you cannot configure two separate VLANs with the same VID on the same switch.
- ✓
VLANs provide broadcast isolation between different VLANs
Why this is correct
Broadcast frames, unknown unicast, and multicast frames are confined to the VLAN in which they originate; they are not forwarded across VLAN boundaries at Layer 2. This isolation reduces unnecessary traffic and improves security by segmenting a network into smaller broadcast domains. To cross VLAN boundaries, traffic must be routed, which is a deliberate configuration choice.
- ✗
VLANs are automatically created when you assign an interface to an untagged VLAN
Why it's wrong here
On Junos, you cannot assign an interface to a nonexistent VLAN; the VLAN (or bridge domain) must first be defined in configuration, then the interface can be bound to it. Even if you assign an interface to an access (untagged) port, the VLAN entry must exist as a configuration element. Unlike some platforms that auto-create VLANs, Junos requires explicit configuration of the VLAN so that its ID, name, and associated routing or filtering options are clear.
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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.