- A
The new switch is running a different version of Junos that is not compatible with the fabric
Why wrong: Version compatibility is critical, but the scenario does not indicate a version mismatch.
- B
The new switch does not have the same virtual-chassis ID configured as the existing fabric
The virtual chassis ID must be the same across all members; otherwise, they cannot form a VCF.
- C
The new switch is only connected to one spine node, and VCF requires full mesh connections to all spines
Why wrong: A leaf only needs a connection to at least one spine to be discovered, though full mesh is recommended.
- D
The new switch has 'set virtual-chassis no-split-detection' enabled, preventing it from joining
Why wrong: No-split-detection is for split prevention, not for joining.
Quick Answer
The answer is that the new switch does not have the same virtual-chassis ID configured as the existing fabric. In a Juniper Virtual Chassis Fabric (VCF), all members must share an identical virtual-chassis ID to be recognized as part of the same logical switch; without this match, the new switch will remain in a standalone state even if it is physically cabled correctly and assigned a member ID. This scenario tests your understanding of VCF prerequisites on the JNCIA-Junos exam, where a common trap is assuming that physical connectivity alone is sufficient for a new switch to join. The key insight is that the virtual-chassis ID acts as a fabric-wide identifier, and a mismatch prevents the new leaf from merging into the operational topology. Remember the memory tip: “ID must match to catch the fabric patch”—if the virtual-chassis ID doesn’t align, the switch stays solo.
JNCIA-JUNOS Junos OS Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of junos os fundamentals. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Your data center uses Juniper QFX5110 switches as leaf nodes in a Virtual Chassis Fabric (VCF) topology with QFX5100 as spine nodes. You are adding a new QFX5110 leaf node to the fabric. The new switch is physically connected to all spine nodes as per the cabling guidelines. However, after powering on the new switch, it does not automatically join the fabric. The existing fabric shows the new switch's member ID as 8, but it remains in 'standalone' state. The VCF has been operational for several months without issues. You check the configuration on the existing VC and notice that the 'virtual-chassis' configuration does not include the new member. What is the most likely reason the new switch does not join?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The new switch does not have the same virtual-chassis ID configured as the existing fabric
In a Virtual Chassis Fabric (VCF), all member switches must share the same virtual-chassis ID to be recognized as part of the same fabric. The existing fabric has a configured virtual-chassis ID, but the new QFX5110, by default, has a different or no virtual-chassis ID set. Without a matching virtual-chassis ID, the new switch cannot join the fabric and remains in 'standalone' state, even though it is physically connected and assigned a member ID.
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.
- ✗
The new switch is running a different version of Junos that is not compatible with the fabric
Why it's wrong here
Version compatibility is critical, but the scenario does not indicate a version mismatch.
- ✓
The new switch does not have the same virtual-chassis ID configured as the existing fabric
Why this is correct
The virtual chassis ID must be the same across all members; otherwise, they cannot form a VCF.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The new switch is only connected to one spine node, and VCF requires full mesh connections to all spines
Why it's wrong here
A leaf only needs a connection to at least one spine to be discovered, though full mesh is recommended.
- ✗
The new switch has 'set virtual-chassis no-split-detection' enabled, preventing it from joining
Why it's wrong here
No-split-detection is for split prevention, not for joining.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume a switch with a member ID assigned has successfully joined the fabric, but in VCF, member ID assignment happens early in the discovery process and does not guarantee full integration—the switch must also have a matching virtual-chassis ID to move out of 'standalone' state.
Trap categories for this question
Scenario analysis trap
Version compatibility is critical, but the scenario does not indicate a version mismatch.
Detailed technical explanation
How to think about this question
The virtual-chassis ID is a 16-bit identifier that must be identical across all members of a VCF to ensure consistent fabric topology and prevent accidental merging of separate fabrics. When a new switch is powered on and connected, it receives a member ID from the existing fabric via the VCF control protocol (based on IS-IS), but if its virtual-chassis ID does not match, it will not transition to 'present' or 'active' state. This is a common misconfiguration when adding a new switch that was previously used in a different fabric or has a default virtual-chassis ID of 0.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Junos OS Fundamentals — This question tests Junos OS Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The new switch does not have the same virtual-chassis ID configured as the existing fabric — In a Virtual Chassis Fabric (VCF), all member switches must share the same virtual-chassis ID to be recognized as part of the same fabric. The existing fabric has a configured virtual-chassis ID, but the new QFX5110, by default, has a different or no virtual-chassis ID set. Without a matching virtual-chassis ID, the new switch cannot join the fabric and remains in 'standalone' state, even though it is physically connected and assigned a member ID.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jun 24, 2026
This JNCIA-JUNOS 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 JNCIA-JUNOS exam.
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