According to the FortiOS 7.6 Security Fabric documentation and Study Guide, several conditions must be met for a downstream FortiGate to successfully join a Security Fabric.

First, the Upstream FortiGate IP/FQDN configured on the downstream device must point to the IP address of the interface on the upstream device that is listening for fabric connections. In the provided logical topology, the Fabric Root (HQ-NGFW-1) uses port4 with the IP 10.0.11.254 to connect to the internal segmentation firewalls (ISFWs). Since HQ-ISFW-2 is in the same subnet as HQ-ISFW, it is physically and logically connected to the network segment serviced by port4. Therefore, the current configuration of 10.0.13.254 (which is port6, likely the WAN side) is incorrect, and it must be set to 10.0.11.254 (Statement A).

Second, once the downstream device successfully reaches the upstream device, it enters a Pending state. For security purposes, FortiOS does not allow devices to join the fabric automatically; the administrator of the upstream device (in this case, HQ-ISFW or the root) must manually authorize the new device (Statement C) in the Fabric Management console. Until this authorization is granted, the status will remain "Pending" and no fabric data will be synchronized. Statements B and D are incorrect as SAML settings do not block the initial fabric join, and the management IP should be the local device's IP, not the upstream's IP.