VLANS

Hello @ahmadking22 ,

You can do this with VDOMs. If you create separate VDOM for each building, you can use the same subnet for all buildings. 

If I get it right, you want to use a single subnet/VLAN for all the APs in the network for all the buildings. To span the same VLAN on multiple ports of the FGT a hardware switch can be configured. It allows to use the same L2 network through different FGT ports.

Hi Ahmad

If I understand your requirement, you need the same SSID on all APs.

Single SSID = Single Interface = Single range

Configure DHCP servers for each VLAN associated with the APs in each building.

Enable the shared-subnet feature for each DHCP server to allow IP address allocation from the same range.

Set up DHCP relay agents for each building to forward DHCP requests to the FortiGate.

https://docs.fortinet.com/document/fortigate/7.4.0/administration-guide/486838/dhcp-shared-subnet-new

https://docs.fortinet.com/document/fortigate/7.4.3/administration-guide/486838/dhcp-shared-subnet

https://docs.fortinet.com/document/fortigate/7.4.1/administration-guide/783526/dhcp-servers-and-relays

Interestingly this many interpretations and suggestions. Yet another one.
I interpreted as [VLAN11+13+15] with one DHCP range.
How about combining all three VLAN interfaces into one software-switch? This can have only one interface/GW IP and subnet, therefore needs only one DHCP range.

Toshi

maybe the most easiest way would be to create a dhcp relay on every vlan that relays to one specific dhcp server that has a pool covering all three vlan so they can get ip from the same nets.