Quick Answer
The answer is a PAN-OS version mismatch between the two firewalls. In an Active/Passive HA pair, the passive firewall enters a non-functional state when it runs a different software version than the active unit, because PAN-OS requires identical versions for configuration synchronization to function. The passive firewall cannot interpret or apply the active firewall’s configuration if the code bases differ, which directly causes the HA passive non-functional version mismatch scenario. On the PCNSE exam, this question tests your understanding of HA prerequisites and common failure states—a frequent trap is assuming network connectivity or licensing issues are the cause, when the version mismatch is the most likely culprit. Remember the memory tip: “Passive goes passive-aggressive when versions are aggressive,” meaning the passive firewall refuses to sync unless both versions match exactly.
This PCNSE practice question tests your understanding of manage, monitor and operate. 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.
A firewall is deployed in an Active/Passive HA pair. The administrator notices that the passive firewall is not synchronizing configuration changes. The 'show high-availability state' command shows the passive firewall in a 'non-functional' state. What is the most likely cause?
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.
The HA2 link is down but HA1 is up
Why wrong: HA2 down would cause session sync issues, not non-functional state.
The session sync is disabled
Why wrong: Session sync is separate from config sync.
The passive firewall has link monitoring enabled
Why wrong: Link monitoring does not cause non-functional state.
The passive firewall is running a different PAN-OS version
Version mismatch causes non-functional state.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The passive firewall is running a different PAN-OS version
The passive firewall showing a 'non-functional' state in an Active/Passive HA pair most likely indicates a version mismatch. PAN-OS requires both firewalls in an HA pair to run the exact same software version for configuration synchronization to work. If the passive firewall is running a different PAN-OS version, it cannot properly interpret or apply the configuration from the active firewall, causing it to enter a non-functional state.
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.
The HA2 link is down but HA1 is up
Why it's wrong here
HA2 down would cause session sync issues, not non-functional state.
The session sync is disabled
Why it's wrong here
Session sync is separate from config sync.
The passive firewall has link monitoring enabled
Why it's wrong here
Link monitoring does not cause non-functional state.
The passive firewall is running a different PAN-OS version
Why this is correct
Version mismatch causes non-functional state.
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.
Common exam traps
The trap here is that candidates often confuse 'non-functional' with connectivity issues (like a down HA link) or session sync settings, but the key is that configuration sync requires identical PAN-OS versions, and a mismatch manifests as a 'non-functional' state on the passive firewall.
Detailed technical explanation
In PAN-OS HA, configuration synchronization uses the HA2 link (or HA3 in some setups) to transfer XML configuration files between peers. A version mismatch is detected during the HA handshake process, where both firewalls exchange their PAN-OS version information; if they differ, the passive firewall refuses to accept the configuration and enters a 'non-functional' state. This is a safety mechanism to prevent corruption or misapplication of configuration objects that may have changed between versions.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the PCNSE exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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
Manage, Monitor and Operate — This question tests Manage, Monitor and Operate — Read the scenario before looking for a memorised answer..
The correct answer is: The passive firewall is running a different PAN-OS version — The passive firewall showing a 'non-functional' state in an Active/Passive HA pair most likely indicates a version mismatch. PAN-OS requires both firewalls in an HA pair to run the exact same software version for configuration synchronization to work. If the passive firewall is running a different PAN-OS version, it cannot properly interpret or apply the configuration from the active firewall, causing it to enter a non-functional state.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. The firewall is active in an HA pair, but the peer is non-functional. The HA2 link is down. What is the most likely cause of the peer being non-functional?
hardWhy C: Option C is correct because when an HA pair detects a version mismatch between peers, the firewall with the higher PAN-OS version will not form an active/passive HA state and will remain non-functional (or in a 'non-functional' state) to prevent configuration or session incompatibilities. The HA2 link being down is a separate issue that affects session synchronization but does not cause the peer to be completely non-functional; the peer can still operate with reduced HA capabilities. A version mismatch is a critical condition that prevents HA peering entirely, leading to one peer appearing non-functional.
Last reviewed: Jun 11, 2026
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