Quick Answer
The answer is the `request system software rollback` command, which is the correct method to revert Junos OS to the previous version because it triggers a reboot and loads the prior software set from the `/altroot` partition, effectively undoing the upgrade while preserving the existing configuration. This command is essential for a software rollback when an engineer suspects a recent upgrade caused a compatibility issue, as it safely restores the last known working environment without requiring manual reinstallation. On the JNCIA-Junos exam, this question tests your understanding of Junos software lifecycle management and the distinction between a rollback (which reverts the OS) and a configuration rollback (which reverts settings). A common trap is confusing `rollback` with `revert` or attempting to use `request system software add` with a previous package; remember that the Junos rollback command is a single, atomic operation that handles the entire OS switch. Memory tip: think of the word “rollback” as a single action—like rolling a boulder back down a hill—where the system does all the heavy lifting for you.
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.
An engineer suspects that a recent software upgrade caused a compatibility issue. Which command should be used to revert the Junos OS to the previous version?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
rollback 0
Why wrong: This rolls back configuration, not the OS image.
request system software rollback
This command triggers a rollback to the previous software partition.
request system reboot
Why wrong: This reboots the device but does not revert the software version.
request system software delete jinstall-*.tgz
Why wrong: This deletes the installed package but does not activate the previous version.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
request system software rollback
The 'request system software rollback' command is the correct method to revert the Junos OS to the previously installed version. This command triggers a reboot and loads the previous software set from the /altroot partition, effectively undoing the upgrade while preserving the configuration.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
rollback 0
Why it's wrong here
This rolls back configuration, not the OS image.
request system software rollback
Why this is correct
This command triggers a rollback to the previous software partition.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
request system reboot
Why it's wrong here
This reboots the device but does not revert the software version.
request system software delete jinstall-*.tgz
Why it's wrong here
This deletes the installed package but does not activate the previous version.
Common exam traps
The trap here is confusing 'rollback' in the context of configuration management (rollback 0) with software version rollback, leading candidates to mistakenly choose option A.
Detailed technical explanation
Junos stores the active and backup software images in separate partitions (/altroot for backup, /root for active). The 'request system software rollback' command swaps these partitions and reboots, allowing a clean fallback without manual file manipulation. This is distinct from configuration rollback, which only affects the active configuration file (juniper.conf.gz) and not the OS kernel or user-space binaries.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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
Junos OS Fundamentals — This question tests Junos OS Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: request system software rollback — The 'request system software rollback' command is the correct method to revert the Junos OS to the previously installed version. This command triggers a reboot and loads the previous software set from the /altroot partition, effectively undoing the upgrade while preserving the configuration.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Read the scenario before looking for a memorised answer.
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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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