the communication is between Management Center (NetSight) and Fortigate.
The communicaiton is
NOT bewteen AccessControllEngine (NAC-GW) and Fortigate.
Configure IP address of Management Center as your radius server on the Fortigate = that means the Fortigate will understand the shared secret and will accept radius accounting from the Management Center.
Configure Extreme Connect (OneFabric Connect) module to talk to your fortigate.
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client connects to the network access switch/AP, AccessControllEngine (NAC-GW) wil process it. when the IP resolution is done, the Management Center (NetSight) sends radius accounting to the Fortigate with appropriate radius attributes. finaly the fortigate knows IP-profile-username
good luck
Regards
Zdeněk Pala