Doubts about Monitored interface in HA Active Pasive

[Fortimaster1]

Hi all.

I have Fortigate 600E with 7.4.9 FortIOS. I have an HA cluster active pasive:

Unit 1 has 100 Priority.

Unit 2 has 50 Priority.

I have too, override disable.

If I put down a monitored interface in Unit 1, cluster converges and Unit 2 becomes active. But, if I turn UP again disconnected interface in Unit 1, Unit 1 becomes active again.

Why  doesn't unit2 stay Active, if I have configured the cluster with HA Override disable? If it is not properly configured, what should I do?

Thanks

[KevinGue]

Read this: https://community.fortinet.com/t5/FortiGate/Technical-Tip-FortiGate-HA-Primary-unit-selection-proces...

Specifically pay attention to the part about the HA uptime and check this on your units after a failover. If it falls in the default 300 seconds the uptime won't be considered.

[Toshi_Esumi]

Most likely the unit1's S/N is higher than unit2 when uptime difference was <5min. If you check "get sys ha status" it would tell you why they swapped like this.

Primary selected using:
<2026/01/09 00:12:04> vcluster-1: FG1K0FTBxxxxxxxxx is selected as the primary because its serialno is larger than peer member FG1K0FTByyyyyyyyy.

Toshi