A short and sweet problem/resolution. If you are looking to enable subnet overlapping on a Fortigate so that you can give multiple interfaces an IP in the same subnet, this is the post for you.
NOTE: This feature can only be enabled in the Fortigate’s CLI. To enable the overlapping feature, enter the following commands:
config system settings set allow-subnet-overlap [enable/disable] end
What is subnet overlapping?
Subnet overlapping is disabled by default in fortiOS and for good reason; if you misuse subnet overlapping it can cause massive routing issues for your clients and their traffic. Subnet overlapping lets you apply IPs from the same subnet (e.g 192.160.1.X/24) to multiple interfaces that are not in the same virtual/physical switch.
When trying to set an overlapping IP to an interface without enabling overlapping, the FortiGate will give the following error messages, CLI or GUI respectively:
'Subnets overlap between 'port2' and the primary IP of 'port2' object set operator error, -54 discard the setting'
'IP address is in same subnet as the others.'
When might I use subnet overlapping?
One of my most common reasons for using subnet overlapping on a Fortigate is to give a HA interface a management IP on the same subnet as the shared (floating) management interface for your Fortigate. This makes it easy to access the web interface or SSH to the CLI of a HA slave if you need to do some troubleshooting.
Supported FortiOS versions
All of the following branches support subnet overlapping, but all have it disabled by default:
FortiGate v4.0 MR2
FortiGate v4.0 MR3
Fortinet KB for Subnet Overlapping – https://kb.fortinet.com/kb/documentLink.do?externalID=FD30014
Other Fortigate Tips and How-To posts
Double NAT port forwarding with a Fortigate – https://exitcode0.net/double-nat-port-forwarding-with-a-fortigate/