When visiting the virtual server service, the client will create a connection with FortiADC. On the connection, source IP address is client address, and destination address is virtual server address. After FortiADC receives the request from the client, FortiADC will load balance the request back to the real servers. This time, FortiADC will create a connection with the real server; on this connection, source IP address is FortiADC’s out interface IP address, and destination address is the real server address.
The connection between the client and virtual server is considered frontend.
The connection between FortiADC and real server is considered backend.