Technical Tip: Configuring IPsec VPN with NAT on FortiGate to Handle Overlapping Subnets between SITE-B and SITE-A

[Raghu_Kumar]

Description	This article describes how to configure an IPsec VPN between two FortiGate devices where traffic coming from SITE-B which should be NATed. The traffic from SITE-B must be NATed because SITE-B and SITE-C use the same subnet, and it is desired to avoid conflicts when connecting to a server at SITE-A.
Scope	FortiGate.
Solution	To configure the IPsec VPN between SITE-B and SITE-A, where the traffic from SITE-B is NATed, follow these steps: Create the IPsec VPN Tunnel on SITE-B and SITE-A: Configure the VPN tunnel on both FortiGate devices (SITE-A and SITE-B) as done for any site-to-site VPN connection. Ensure that the tunnel interfaces are correctly set up and that the phase 1 and phase 2 configurations match between both sites. Configure the IPv4 Policy on SITE-B: Navigate to Policy & Objects -> Firewall Policy on SITE-B. Create a new IPv4 policy for the traffic that should be sent through the VPN tunnel to SITE-A. In the policy, enable NAT and select an appropriate IP Pool. This will NAT the traffic coming from SITE-B to an IP address or range that does not conflict with SITE-C. Ensure that the IP Pool is correctly configured with an IP address or range that is unique and does not overlap with any subnets used by SITE-C. Create the IP Pool on SITE-B: Navigate to Policy & Objects -> IP Pools. Create a new IP Pool with the IP address range that should be used for NATing the traffic from SITE-B. Assign this IP Pool to the IPv4 policy created in the previous step. Configure the IPv4 Policy on SITE-A: On SITE-A, ensure that there is a corresponding IPv4 policy to accept traffic from the NATed IP addresses (from SITE-B) and route it to the server located at SITE-A. Verify the Configuration: Test the VPN connection to ensure that traffic from SITE-B is correctly NATed and that users at SITE-B can access the server at SITE-A without any issues. Monitor the traffic logs to ensure that there are no conflicts with SITE-C’s subnet. Diagram Example: Scenario: SITE-A Subnet: 192.168.10.0/24 (Server IP: 192.168.10.100). SITE-B Subnet: 192.168.20.0/24 (conflicts with SITE-C). NAT IP Pool on SITE-B: 172.16.20.0/24. VPN Tunnel: Secure connection between SITE-A and SITE-B.   Traffic Flow: From SITE-B: Traffic from the subnet 192.168.20.0/24 is NATed to 172.16.20.0/24 when it passes through FortiGate-B. Through VPN Tunnel: The NATed traffic (172.16.20.0/24) passes securely through the IPsec VPN tunnel. To SITE-A: Traffic reaches SITE-A's server (192.168.10.100) without conflicts.