BIG-IP VM Connectivity

Actually, i have BIG-IP with two interfaces, external: 172.16.10.100 connect to my pc:172.16.10.1 and internal interface: 10.10.10.100 connect to two servers: 10.10.10.1 & 10.10.10.2, when choice Source Address Translation: None; on virtual server, and trying to browse vServer ip: 172.16.10.100 traffic not forward to to back-end server; but when i switch Source Address Translation to Auto Map browse the traffic forward to back-end server; so i think the issue is when i use None in Source Address Translation the traffic come as 172.16.10.1 but back-end server can't reach this subnet.