The answer is to upgrade the backup Routing Engine first, perform a graceful switchover, and then upgrade the former primary. This procedure minimizes downtime because it ensures that the active Routing Engine continues processing traffic and maintaining routing protocol adjacencies—such as BGP and OSPF—while the backup is being upgraded. After the graceful switchover, the newly upgraded RE takes over seamlessly, and the old primary can be rebooted without any service interruption. On the JNCIA-Junos exam, this question tests your understanding of high-availability features and the Junos dual routing engine upgrade procedure, often appearing as a scenario where you must choose the sequence that avoids a full outage. A common trap is to upgrade the primary first, which would drop all active sessions. Memory tip: “Backup first, switch, then the rest.”
JNCIA-JUNOS Junos OS Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of junos os fundamentals. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
A network engineer needs to upgrade Junos on an MX router with dual Routing Engines (RE0 and RE1). What is the recommended procedure to minimize downtime?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Upgrade the backup RE, perform a graceful switchover (RESTART ROUTING), then upgrade the former primary
Option C is correct because it minimizes downtime by first upgrading the backup RE (RE1), then performing a graceful Routing Engine switchover (using the 'request chassis routing-engine master switch' command or equivalent), which causes the upgraded backup to become the new primary without disrupting traffic. After the switchover, the former primary (now the backup) can be upgraded and rebooted, ensuring that at least one RE is always processing traffic and routing protocols like BGP and OSPF maintain their adjacencies.
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.
✗
Upgrade the primary RE first, then the backup
Why it's wrong here
Upgrading primary first causes traffic interruption during reboot, then backup upgrade is redundant.
✗
Reboot both REs at the same time after copying the image
Why it's wrong here
Rebooting both simultaneously causes complete outage; no redundancy.
✓
Upgrade the backup RE, perform a graceful switchover (RESTART ROUTING), then upgrade the former primary
Why this is correct
This is the standard upgrade procedure for dual RE systems, minimizing traffic impact.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Upgrade both REs simultaneously using the same image
Why it's wrong here
Simultaneous upgrade can cause both REs to reboot at the same time, leading to total loss of control plane.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume upgrading the primary first is safer or more logical, but Junos requires upgrading the backup first and performing a graceful switchover to maintain control plane continuity and avoid traffic loss.
Detailed technical explanation
How to think about this question
The graceful Routing Engine switchover (GRES) feature, combined with nonstop active routing (NSR) or graceful restart, allows the backup RE to take over control plane functions without resetting interfaces or tearing down BGP/OSPF sessions. During the upgrade, the 'request system software add' command installs the image on the backup RE, and after switchover, the new primary inherits the forwarding table and routing state, ensuring zero packet loss for established flows. In real-world deployments, this procedure is critical for MX routers in service provider edge roles where even seconds of downtime can violate SLAs.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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 — 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: Upgrade the backup RE, perform a graceful switchover (RESTART ROUTING), then upgrade the former primary — Option C is correct because it minimizes downtime by first upgrading the backup RE (RE1), then performing a graceful Routing Engine switchover (using the 'request chassis routing-engine master switch' command or equivalent), which causes the upgraded backup to become the new primary without disrupting traffic. After the switchover, the former primary (now the backup) can be upgraded and rebooted, ensuring that at least one RE is always processing traffic and routing protocols like BGP and OSPF maintain their adjacencies.
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: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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