Fortigate E101 DHCP problem

[darkmoon]

Hello everyone!

i have fortigate E101 which im having 2 lans both are hardware-switches with two different subnets

(as seen in the picture below)

each of them connected to their own switches on the building (they're not looping) and for some reason computers connected to the switch belong to 172.16.31 getting DHCP offers from 10.10.10 as well when they are discovering 

how do i fix this problem so fortigate will not allow end points connected to 1 lan request IP addresses belonged to another?

https://i.ibb.co/R0Zc6pM/...ate-In-Manage-0118.png

[darkmoon]

https://i.ibb.co/R0Zc6pM/...ate-In-Manage-0118.png

[Toshi_Esumi]

Likely an L2 loop, or two, somewhere between those two switches or the device connected to one switch is connected another switch indirectly, or something like that. Since FGT doesn't participate in spanning-tree protocols, you need to either trouble shoot on the switches, which could detect the loop or, if that's not an option due to the capability of the switches, disconnect everything from one switch then recover one connection at a time and check the DCHP result then add another if the symptom doesn't occur exhaustively.