Why TCP Sessions Drop After FortiGate HA Failover and How to Fix
A company is deploying a FortiGate HA cluster in active-passive mode across two data centers. The network team reports that after a failover, some existing TCP sessions are dropped. Which configuration change should be applied to maintain session persistence during failover?
Quick Answer
The answer is to enable session synchronization between cluster members. In an active-passive FortiGate HA deployment, the passive unit maintains no awareness of existing TCP sessions unless session synchronization is explicitly configured via `config system ha session-sync`. Without this, after a failover the new active unit has no session state information, causing established TCP sessions to be dropped as if the connection never existed. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your understanding of HA session persistence versus device-level failover—a common trap is assuming that HA alone preserves sessions, when in fact session synchronization is a separate, required setting. Remember the key distinction: HA handles device redundancy, but session synchronization handles stateful session continuity. A useful memory tip is “sync or sink”—without session sync, your TCP sessions sink after failover.
⚠ Common exam trap
Many exam-takers confuse gratuitous ARP (which handles Layer 2 updates) with session persistence, assuming that updating MAC tables is sufficient to maintain TCP sessions, but session state must be replicated at Layer 4.
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
✓
Enable session synchronization between cluster members
In an active-passive HA cluster, session synchronization (config.sys ha session-sync) ensures that TCP session state information is replicated from the active unit to the passive unit. Without this, the passive unit has no knowledge of existing sessions after a failover, causing them to be dropped. Enabling session synchronization allows the new active unit to continue forwarding traffic for established sessions seamlessly.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Enable session synchronization between cluster members
Why this is correct
Session sync shares session state between primary and secondary, maintaining sessions during failover.
- ✗
Disable NAT inspection on the firewall policy
Why it's wrong here
Disabling NAT inspection is unrelated to session persistence.
- ✗
Configure gratuitous ARP on the virtual IP
Why it's wrong here
Gratuitous ARP updates MAC tables but does not preserve sessions.
- ✗
Increase the heartbeat interval to 2 seconds
Why it's wrong here
Increasing heartbeat interval may delay failover detection, not preserve sessions.
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Same concept, more angles
2 more ways this is tested on NSE4
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. An administrator configures two FortiGate units in an active-passive HA cluster. After a failover, some existing TCP sessions are dropped. What is the most likely reason for this behavior?
medium- ✓ A.Session synchronization is not enabled
- B.The failover time is too slow
- C.The FortiGate units are running different firmware versions
- D.The HA cluster is using a virtual MAC address
Why A: In an active-passive HA cluster, session synchronization must be explicitly enabled to replicate TCP session state from the primary unit to the backup unit. Without session sync, the backup unit has no knowledge of existing TCP sessions after a failover, causing those sessions to be dropped because the new primary cannot match incoming packets to any session table entry.
Variation 2. An administrator configures an HA cluster of two FortiGates in active-passive mode. The cluster is synchronized, but after a failover, some existing TCP sessions are dropped. What is the most likely cause?
hard- A.The heartbeat interface is configured as a dedicated management interface
- ✓ B.Session synchronization (session-pickup) is disabled
- C.The cluster is operating in NAT mode
- D.The cluster is using a virtual MAC address for the HA interface
Why B: Session synchronization (session-pickup) is required for active-passive HA clusters to replicate TCP session state from the primary FortiGate to the secondary. When disabled, the backup unit has no knowledge of existing sessions after a failover, causing those sessions to be dropped because the new primary cannot match incoming packets to any session table entry.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.