To understand which device becomes active, we must look at how the BIG-IP system handles HA Order and Auto Fallback within a traffic group.

HA Order Mechanism: When a traffic group is configured with an "HA Order" list, the system prefers to host the traffic group on the highest-ranking available device in that list (1 being the highest).

The Impact of "Forced to Standby": BIGIP-D is the first choice in the order, but it has been "Forced to Standby." This state is persistent and manual; until an administrator releases the "Force to Standby" state, the device is ineligible to host the traffic group, effectively removing it from the top of the preference list.

Auto Fallback: When "Auto Fallback" is enabled, the traffic group will automatically migrate back to a higher-priority device in the HA order as soon as that device becomes available and is in a healthy "Standby" state.

The Scenario Logic: 1. BIGIP-D is ineligible (Forced Offline/Standby).

2. BIGIP-B is the next highest device in the HA Order (Rank 2).

3. While BIGIP-B was rebooting, the traffic group would have failed over to BIGIP-C (Rank 3).

4. Once BIGIP-B finishes booting and joins the cluster in a "Standby" state, the Auto Fallback setting triggers.

5. Because BIGIP-B is higher in the HA Order than the current active device (BIGIP-C) and the only device above it (BIGIP-D) is ineligible, the traffic group fails back to BIGIP-B.