Quick Answer
The answer is the PAN-OS version mismatch between the HA peers. This is the most likely cause because Palo Alto Networks requires both firewalls in an HA pair to run the same major PAN-OS version to synchronize configuration and session state; the active firewall at 10.0 and the passive at 10.1 are incompatible, so the passive firewall fails to synchronize. On the PCNSA exam, this scenario tests your understanding of HA synchronization requirements and the critical rule that version parity must be maintained before and after any upgrade—a common trap is assuming a passive unit can be upgraded independently without breaking HA. Remember the memory tip: “Same major, sync the data; mismatch major, HA goes under.”
This PCNSA practice question tests your understanding of device management and services. 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 company has a PA-5250 firewall in an active/passive HA pair. During a maintenance window, the administrator upgrades the passive firewall from PAN-OS 10.0 to 10.1. After the upgrade, the passive firewall fails to synchronize with the active firewall. The active firewall remains at 10.0. 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 or misconfigured
Why wrong: HA2 link failure would prevent sync, but version mismatch is more likely.
The HA keepalive timer is misconfigured
Why wrong: Keepalive failure leads to split-brain, not sync failure.
The passive firewall has preemption enabled
Why wrong: Preemption affects failback, not synchronization.
The PAN-OS versions are different between the HA peers
HA peers must run the same PAN-OS version for sync.
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 PAN-OS versions are different between the HA peers
PAN-OS requires both HA peers to run the same major version to synchronize configuration and state. The active firewall at PAN-OS 10.0 and the passive at 10.1 are incompatible, preventing HA synchronization. Even though the passive firewall was upgraded, the active firewall remains on the older version, breaking the HA session.
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 or misconfigured
Why it's wrong here
HA2 link failure would prevent sync, but version mismatch is more likely.
The HA keepalive timer is misconfigured
Why it's wrong here
Keepalive failure leads to split-brain, not sync failure.
The passive firewall has preemption enabled
Why it's wrong here
Preemption affects failback, not synchronization.
The PAN-OS versions are different between the HA peers
Why this is correct
HA peers must run the same PAN-OS version for sync.
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 may focus on connectivity or timer issues (options A or B) rather than recognizing that PAN-OS enforces strict version matching for HA synchronization, even if the passive firewall is upgraded correctly.
Detailed technical explanation
Palo Alto Networks HA synchronization uses the HA2 link for configuration and session state replication, but the PAN-OS version must match exactly (same major and minor version) for the HA pair to form. The HA keepalive timer (default 2 seconds) and preemption are unrelated to version compatibility; version mismatch is a hard requirement enforced by the HA state machine. In real-world scenarios, upgrading one peer without the other will always break HA until both are on the same version.
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.
Device Management and Services — study guide chapter
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FAQ
Device Management and Services — This question tests Device Management and Services — Read the scenario before looking for a memorised answer..
The correct answer is: The PAN-OS versions are different between the HA peers — PAN-OS requires both HA peers to run the same major version to synchronize configuration and state. The active firewall at PAN-OS 10.0 and the passive at 10.1 are incompatible, preventing HA synchronization. Even though the passive firewall was upgraded, the active firewall remains on the older version, breaking the HA session.
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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Last reviewed: Jun 11, 2026
This PCNSA practice question is part of Courseiva's free Palo Alto 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 PCNSA exam.
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