NimbostratusThe answer depends on if you are referring to the IP address the BIG-IP will use for health monitoring the pool members, or if you have Source Network Address Translation (SNAT) enabled and are refering to the client traffic.
In the context of health monitors, BIG-IP should be using the SelfIP address that has the lowest cost in the route table to reach the pool member.
When using SNAT, the IP address(es) the BIG-IP uses for the client traffic will depend on the settings of the Virtual Server (VS). IF SNAT is set on a virtual server to automap, the BIG-IP will pick the IP address the same way it does for the health monitors.
If SNAT is set on a VS to a SNAT Pool, the BIG-IP will use one or more of the IP addresses in that SNAT pool. This can get more complicated because a pool can be used by multiple virtual servers. In that case, it will depend on which VS is handling a specific client connection.
When using SNAT, you can see all the IP addresses the BIG-IP is actively using to talk to a specific pool member by using the TMSH command "show /sys connection ss-server-addr ".
