Site to Site, IPsec VPN used when you allow communicating your two different Site A and Site B. Fortigate device is located at both site as a gateway device and private network are behind the Fortigate device. Through the Fortigate device Site to Site IPsec VPN will created by using the VPN feature.
I will explained in short and simple text with step.
Configuration at Site A
Step1. Login in Fortigate device on the Site a FortiGate,
Go to VPN > IPsec > Wizard and select Site to Site – FortiGate > Click to Next button.
NAT Configuration – it is not require because the private IP addresses on both sides do not overlap there is no need for NAT. both side subnet addressing is different.
Step2. Set the Site B FortiGates IP as the Remote gateway(Remote FortiGate ILL interface IP address).
Gateway IP address – Select the outgoing interface which is ILL interface.
Pre-Shared key– Enter pre-shared key (pre-shared key should be same at both end)
Step3. Policy & Routing – Select the local interface where traffic came from.
Local Subnet – Assigned local subnet
Remote Subnet – Assigned remote subnet (Site B network subnet needs to assigned)
Step4. Verify the summary of VPN configuration
The VPN has been set up summary of created objects.
Step5. You have to perform same step 1,2 & 3 in Site B firewall.
Check VPN tunnel status from Go to VPN > Monitor > IPsec Monitor to verify the status of the VPN tunnel.
Check the traffic is fall in logs. firewall policy was created for LAN to VPN tunnel interface once Tunnel is created. If you required to allowed any specific service or source to create new policy as created.