A BIG-IP is configured with a pool member located on a different subnet that is...

SNAT Automapis a feature that automatically selects a self-IP address to use as the source address for translated packets. The selection logic follows a strict hierarchy to ensure that traffic is routable back to the BIG-IP:

Egress VLAN Priority:The BIG-IP first looks at the VLAN through which the traffic is exiting toward the pool member (the egress VLAN).

Floating Self-IP Preference:If the egress VLAN has afloating self-IP address, the BIG-IP will always prefer it for SNAT Automap. This is critical for High Availability (HA) because, during a failover, the floating IP moves to the new active device, allowing existing connections to be maintained or correctly timed out.

Non-Floating Fallback:If no floating self-IP is available on the egress VLAN, the system will use a floating self-IP from a different VLAN. If no floating IPs exist at all, it will then fall back to the non-floating self-IP.

Key Data Plane Concept:

The management IP is never used for data plane traffic. In this scenario, since the administrator has configured a floating self-IP, that specific address becomes the source for all SNAT Automap traffic leaving that VLAN to ensure symmetric routing during HA events.