Adding a Fortigate firewall to a network without routing all traffic through it involves setting it up in a mode that allows monitoring and controlling traffic selectively. One effective way to achieve this is by configuring the Fortigate in "Transparent Mode" or using it for specific functions like web filtering, intrusion detection, or VPN services. Here’s a detailed guide on how to do it:-

1. Planning and Preparation
Network Assessment:- Understand your current network topology and identify where the Fortigate will be placed.
Objectives:- Determine the primary purpose of adding the Fortigate (e.g., monitoring, filtering, VPN, etc.).
2. Setting Up Fortigate in Transparent Mode
In Transparent Mode, the Fortigate operates at Layer 2 (like a bridge) and does not require reconfiguring the IP scheme of your network.

1. Initial Configuration:-

Connect to the Fortigate via console or web interface.
Configure basic settings like management IP, admin credentials, etc.
2. Change to Transparent Mode:-

Navigate to System > Settings.
Set the Operation Mode to Transparent.
3. Configure Management IP:-

Assign a management IP to the Fortigate that matches your network’s subnet.
Ensure the IP does not conflict with other devices on the network.
4. Bridge Network Interfaces:-

Bridge the internal and external interfaces so the Fortigate can pass traffic between them.
Example: Bridge port1 and port2 if those are the interfaces connected to your network.
5. Policy and Security Profiles:-

Set up security policies to inspect and filter traffic as needed.
Apply necessary security profiles (e.g., antivirus, web filtering, IPS) to these policies.
3. Selective Traffic Routing
If the goal is to route only specific types of traffic through the Fortigate, you can achieve this by setting up policy routes or static routes.

1. Define Policy Routes:-

Go to Network > Policy Routes.
Create rules to direct specific traffic (based on IP, protocol, port, etc.) through the Fortigate.
2. Static Routes:-

Navigate to Network > Static Routes.
Add static routes for specific subnets or destinations that need to go through the Fortigate.