Fortigate Interface settings

few articles you can read up about it :

https://community.fortinet.com/t5/Support-Forum/why-do-we-assign-secondary-IP-address/m-p/219857

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Set-a-secondary-IP-on-a-FortiGate-interface/ta-p/226046

Hi @Umesh 

Thank you for the question.

Benefits of using Secondary WAN IP:

It depends on the destination network. e.g. if anyone is trying to reach the subnet the same as the primary IP, then it will use the primary IP and if it wants to reach the subnet as a secondary IP, then it will use the secondary IP. If the destination is part of the secondary IP or the gateway is a secondary IP subnet, then it will use a secondary IP, or else it will use a primary IP.

It is possible to consider secondary IP as a separate virtual interface, functionality will be the same as a separate virtual interface. The only difference is, that both traffic, will use the same physical interface.

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Set-a-secondary-IP-on-a-FortiGate-interface/ta-p/226046

https://community.fortinet.com/t5/Support-Forum/why-do-we-assign-secondary-IP-address/m-p/219857

Best regards,

Erlin