Which mode allows FortiGate to function in layer 2 network operations?

The mode that allows FortiGate to function in layer 2 network operations is referred to as Transparent mode. In this mode, FortiGate acts as a transparent bridge, meaning it can operate without changing IP addresses on the network. It can pass traffic directly between its interfaces without acting as a router, effectively allowing it to intercept and inspect packets at Layer 2.

Transparent mode is useful for deployment in environments where it’s important to maintain the existing network architecture without modifying IP addresses or topology. This makes it ideal for inline security deployments where the device can still enforce security policies, monitor traffic, and protect against threats without introducing additional routing complexities.

In contrast, the other modes primarily work at Layer 3. Router mode, for instance, involves configuring the FortiGate as a traditional router, which includes routing and managing IP addresses. NAT mode uses techniques for translating addresses between private and public addresses, while Bridge mode typically refers to connecting multiple devices at Layer 2 but does not apply the same security features as FortiGate's Transparent mode. Thus, Transparent mode is the distinct choice for operating at Layer 2 within a FortiGate device.