In an Active/Standby BIG-IP design, application availability during failover depends on both units having equivalent data-plane connectivity for the networks that carry application traffic. Specifically:
VLANs are bound to specific interfaces (and optionally VLAN tags).
Floating self IPs / traffic groups move to the new Active device during failover.
For traffic to continue flowing after failover, the new Active device must have the same VLANs available on the correct interfaces that connect to the upstream/downstream networks.
What the symptom tells you:
Traffic works when Device A is Active
Traffic fails when Device B becomes Active
Failback immediately restores traffic
This pattern strongly indicates the Standby unit does not have the VLAN connected the same way (wrong physical interface assignment), so when it becomes Active, it owns the floating addresses but cannot actually pass traffic on the correct network segment.
Why Interface mismatch is the best match:
If the Active unit is already working, its interface mapping is correct.
The fix is to make the Standby unit’s VLAN/interface assignment match the Active unit.
That corresponds to changing the Standby device interface to 1.1.
Why the Tag options are less likely here (given the choices and the exhibit intent):
Tag issues can also break failover traffic, but the question/options are clearly driving toward the classic HA requirement: consistent VLAN-to-interface mapping on both devices so the data plane remains functional after the traffic group moves.
Conclusion: To avoid an outage on the next failover, the BIG-IP Administrator must ensure the Standby device uses the same interface (1.1) for the relevant VLAN(s) that carry the application traffic, so when it becomes Active it can forward/receive traffic normally.
