In Cisco ACI, which result occurs to the traffic when communication between the data plane and the Cisco APIC controller is lost?

While ACI leaf switches do retain locally cached policy entries, this answer describes the behavior for a management-plane-only APIC outage rather than a full data plane connectivity loss to the controller.

ACI fabric switches do not buffer or queue data traffic pending restoration of APIC connectivity - there is no such hold-and-forward buffering mechanism in the ACI data plane for this scenario.

Data in transit is not simply dropped and discarded as lost - the fabric's response is to stop forwarding rather than to selectively lose only in-flight packets.

In Cisco ACI, the APIC controller is responsible for programming and synchronizing the policy model that governs how leaf and spine switches make forwarding decisions at the data plane. When the data plane loses connectivity to the APIC, the fabric cannot maintain consistent or updated forwarding state, and ACI's architecture requires controller-driven convergence to resume correct forwarding operation. Traffic forwarding halts until the fabric reconnects to the APIC and re-converges on a consistent policy and endpoint database.