Fortigate as a Radius Client

Hi,

If there is an external firewall between the Radius server(which is outside my network) and my Fortigate as the radius client, then I need to have a rule on the external firewall to allow RADIUS traffic from my Fortigate firewall. so the source address on the rule should be teh address of RADIUS client which is my Fortigate. My question is what address do I have to use ? would it be the outside interface of my Fortigate?