- A
The LACP system priority is set too low on the new switch, causing it to lose LACP negotiations
Why wrong: LACP priority affects which side controls aggregation, but not RSTP behavior.
- B
The LAG is not recognized as a single logical link by RSTP, causing RSTP to see two separate links and create a loop
RSTP should treat a LAG as one link, but if the LAG is not configured correctly (e.g., missing 'lacp' or 'aggregate' statements), RSTP can see separate links and cause a loop, leading to intermittent blocking.
- C
The uplink interfaces are configured as alternate ports instead of root ports
Why wrong: Port roles are determined by RSTP based on lowest path cost, not configurable as such.
- D
The new switch has a different root bridge priority, causing it to become the root and disrupting topology
Why wrong: If it becomes root, it would not cause intermittent blocking; it would stabilize.
Quick Answer
The answer is that the LAG is not recognized as a single logical link by RSTP, causing RSTP to see two separate links and create a loop. This is correct because Rapid Spanning Tree Protocol (802.1w) operates on a per-interface basis; unless the two uplink interfaces are bundled into a single aggregated Ethernet (ae) interface on Junos, RSTP treats each physical port as an independent link. When the new EX2300 switch connects to two core switches via these separate links, RSTP detects a physical loop and places one interface into a blocking state to break it. However, during link flaps or configuration changes, the port transitions between blocking and forwarding, causing the sporadic packet loss described. On the JNCIA-Junos exam, this scenario tests your understanding that LACP LAGs must be configured as ae interfaces for spanning-tree protocols to treat them as a single logical path—a common trap is assuming LACP alone prevents loops. Remember the memory tip: "LACP bundles links, but only an ae interface hides them from RSTP."
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 enterprise network uses Juniper EX4300 switches in a collapsed core design with RSTP (802.1w) as the Layer 2 loop prevention protocol. You add a new EX2300 switch to an access closet and connect it to two different core switches using two uplink interfaces configured in an LACP LAG. After connecting the new switch, you notice intermittent connectivity issues across the entire network, with some devices reporting temporary packet loss. The issue occurs sporadically, especially during configuration changes or when links flap. You suspect the problem is related to RSTP. Upon investigation, you see that the new switch's uplink interfaces are both in the forwarding state, but occasionally one of them transitions to blocking and then back to forwarding. What is the most likely cause of the intermittent issues?
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 LAG is not recognized as a single logical link by RSTP, causing RSTP to see two separate links and create a loop
B is correct because RSTP (802.1w) treats each physical interface in a LAG as a separate link unless the LAG is explicitly configured as a logical interface (ae interface) on Junos. When the two uplinks are not bundled into a single ae interface, RSTP sees two separate links between the new switch and the core, creating a loop. This causes RSTP to block one of the links to break the loop, and when the link flaps or configuration changes occur, the port transitions between blocking and forwarding, leading to intermittent packet loss across the network.
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 LACP system priority is set too low on the new switch, causing it to lose LACP negotiations
- ✓
The LAG is not recognized as a single logical link by RSTP, causing RSTP to see two separate links and create a loop
Why this is correct
RSTP should treat a LAG as one link, but if the LAG is not configured correctly (e.g., missing 'lacp' or 'aggregate' statements), RSTP can see separate links and cause a loop, leading to intermittent blocking.
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 uplink interfaces are configured as alternate ports instead of root ports
Why it's wrong here
Port roles are determined by RSTP based on lowest path cost, not configurable as such.
- ✗
The new switch has a different root bridge priority, causing it to become the root and disrupting topology
Why it's wrong here
If it becomes root, it would not cause intermittent blocking; it would stabilize.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume LACP automatically bundles interfaces into a single logical link for all Layer 2 protocols, but RSTP operates at the port level and requires the ae interface to treat the LAG as one link; without it, RSTP sees multiple links and blocks one, causing intermittent connectivity.
Detailed technical explanation
How to think about this question
In Junos, a LAG must be configured as a logical interface (ae0) with member interfaces under it; RSTP then treats the ae interface as a single link. Without this, each physical interface runs RSTP independently, and the bridge protocol data units (BPDUs) exchanged on each link cause RSTP to detect a loop and block one port. This is a common misconfiguration in collapsed core designs where LAGs are used for redundancy but the logical interface abstraction is overlooked, leading to flapping and temporary loops during topology changes.
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.
- →
Junos OS Fundamentals — study guide chapter
Learn the concepts, then practise the questions
- →
Junos OS Fundamentals practice questions
Targeted practice on this topic area only
- →
All JNCIA-JUNOS questions
514 questions across all exam domains
- →
Juniper Networks Certified Associate Junos JNCIA-Junos study guide
Full concept coverage aligned to exam objectives
- →
JNCIA-JUNOS practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related JNCIA-JUNOS practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
User Interfaces practice questions
Practise JNCIA-JUNOS questions linked to User Interfaces.
Junos Configuration Basics practice questions
Practise JNCIA-JUNOS questions linked to Junos Configuration Basics.
Operational Monitoring and Maintenance practice questions
Practise JNCIA-JUNOS questions linked to Operational Monitoring and Maintenance.
Routing Fundamentals practice questions
Practise JNCIA-JUNOS questions linked to Routing Fundamentals.
Networking Fundamentals practice questions
Practise JNCIA-JUNOS questions linked to Networking Fundamentals.
Junos OS Fundamentals practice questions
Practise JNCIA-JUNOS questions linked to Junos OS Fundamentals.
JNCIA-JUNOS fundamentals practice questions
Practise JNCIA-JUNOS questions linked to JNCIA-JUNOS fundamentals.
JNCIA-JUNOS scenario practice questions
Practise JNCIA-JUNOS questions linked to JNCIA-JUNOS scenario.
JNCIA-JUNOS troubleshooting practice questions
Practise JNCIA-JUNOS questions linked to JNCIA-JUNOS troubleshooting.
Practice this exam
Start a free JNCIA-JUNOS practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
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 LAG is not recognized as a single logical link by RSTP, causing RSTP to see two separate links and create a loop — B is correct because RSTP (802.1w) treats each physical interface in a LAG as a separate link unless the LAG is explicitly configured as a logical interface (ae interface) on Junos. When the two uplinks are not bundled into a single ae interface, RSTP sees two separate links between the new switch and the core, creating a loop. This causes RSTP to block one of the links to break the loop, and when the link flaps or configuration changes occur, the port transitions between blocking and forwarding, leading to intermittent packet loss across the network.
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
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.