When a FortiSwitch is connected to a FortiGate via a FortiLink interface, the default discovery process requires the FortiSwitch to obtain an IP address from the FortiGate. The FortiGate's FortiLink interface is pre-configured to act as a DHCP server for this purpose. If the FortiSwitch does not receive an IP address, it cannot communicate with the FortiGate for management, and consequently, will not appear in the FortiGate's list of managed devices. Therefore, verifying that the FortiGate's DHCP server has successfully assigned an IP lease to the FortiSwitch is a fundamental and logical first troubleshooting step.

A. FortiLink management traffic between the FortiGate and FortiSwitch is handled implicitly by the system and does not require explicit security policies for discovery.

B. Manually assigning a static IP is an alternative configuration method, not a primary troubleshooting step for the default, DHCP-based discovery mechanism.

D. A FortiSwitch does not require internet access to be discovered or managed by a FortiGate over the local FortiLink connection.

1. FortiSwitch Manager 7.6.0 Administration Guide, Page 13, "FortiLink mode": "When you connect a FortiSwitch unit to a FortiGate FortiLink port, the FortiSwitch unit is automatically discovered. The FortiGate device acts as a DHCP server and assigns an IP address to the FortiSwitch unit." This establishes the DHCP mechanism as central to discovery.

2. FortiOS 7.6.0 Administration Guide, Page 1201, "Configuring FortiLink": "The FortiLink interface has a DHCP server that is enabled by default to assign IP addresses to the managed FortiSwitches." This confirms the default DHCP server configuration on the FortiGate side.

3. Fortinet Certified Professional - Network Security Study Guide for FortiOS 7.6, Page 289, "FortiLink Troubleshooting": "If a FortiSwitch device doesn't appear on the Managed FortiSwitch page, you should start by checking the physical connections... Then, you should check if the FortiSwitch device received an IP address from the FortiGate DHCP server on the FortiLink interface." This explicitly identifies checking the DHCP lease as a primary troubleshooting step after physical checks.