Although the default IP address of the physical interface is used to communicate with the back end nodes, you can change this behaviour by creating a Traffic IP (TIP) and then using custom static routing to map that TIP address to the specified host. When using a virtual appliance (VA), the network configuration should be carried out under the System > Networking tab.
Custom static routing can be accomplished on Linux by using the following command:
route add -host <node IP> dev <TIP interface ID eg. eth0:2>
So this may look like:
Traffic Manager IP - 192.168.98.50 (eth0)
Traffic IP - 192.168.98.52 on eth0:2
Backend Webserver IP - 192.168.98.51
By default, the 192.168.98.50 address is used to communicate with the web server, but if you add a custom static route as follows, then the TIP is used for communication with backend.
# route add -host 192.168.98.51 dev eth0:2
If you have a small number of TIPs to communicate to a pool of back-end servers, this would allow you to manually load balance across these back-end servers by adding customer routes via their alias. Ex. node1 via TIP1, node2 via TIP2, etc...
NOTE: This solution may not be ideal for a large number of services and TIPS and would not protect against failover (unless there is a cluster for redundancy) or if the system is rebooted.