NSE4 High Availability and Diagnostics Practice Question
A FortiGate administrator notices that after upgrading the firmware, the HA cluster fails to form. Both units show the correct HA configuration. What is the most likely cause?
⚠ Common exam trap
NSE4 often tests the misconception that HA configuration parameters like priority or mode are the primary cause of cluster formation failure, when in fact firmware version mismatch is a critical and common cause after upgrades.
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 firmware versions are different on the two units
FortiGate HA requires that all cluster members run the same firmware version. After an upgrade, if one unit is upgraded and the other is not, the HA cluster will not form because the HA protocol version and heartbeat packet format may differ. The units will show correct HA configuration but will not establish a heartbeat due to version mismatch. This is a common issue during firmware upgrades in an HA cluster.
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 HA heartbeat interfaces are not connected
Why it's wrong here
Unplugged or failed heartbeat interfaces would prevent the two units from exchanging cluster status and would normally generate an explicit 'heartbeat lost' or 'uninitialized' HA error on the dashboard, not a silent refusal to form. Since the administrator verified the HA configuration, including the heartbeat link, is properly set, a physical heartbeat problem can be ruled out as the cause. Moreover, the post-upgrade timing of the fault points directly to a version mismatch rather than a Layer 1 connectivity issue.
- ✗
The HA mode is set to active-active on one unit and active-passive on the other
Why it's wrong here
FortiGate HA requires both units to use the same HA mode (active-active or active-passive) because the mode determines how session failover and load balancing are negotiated. If one unit were set to active-active and the other to active-passive, the cluster would fail to synchronize and the HA status would show a mode mismatch or negotiation error. However, the admin confirmed the configuration is correct, and the failure began immediately after upgrading only one unit, so a mode mismatch is unlikely to be the root cause.
- ✓
The firmware versions are different on the two units
Why this is correct
The most common cause of an HA cluster refusing to form after a firmware upgrade is that FortiGate HA does not support mixed FortiOS versions across cluster members. When one unit runs a newer build than the other, the cluster cannot synchronize because the internal protocol/data structures differ, and the HA status will report a firmware version mismatch. Since the problem appeared right after the upgrade, verifying that both units are on the same upgraded firmware build is the correct first step.
- ✗
The HA priority values are identical
Why it's wrong here
Identical HA priority values do not break cluster formation; FortiGate treats priority as the primary tie-breaker only when a new primary must be elected. When two units have the same priority, the cluster simply falls back to comparing serial numbers, with the lower serial number winning the primary role. This is a normal and supported situation, so it cannot explain the units failing to form a cluster after the firmware upgrade.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Fortinet exam blueprint
This NSE4 practice question is part of Courseiva's free Fortinet 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 NSE4 exam.