Which switch type is discovered first in the Cisco ACI fabric discovery process?

Spine switches are discovered after the leaf switches have registered with the APIC and the APIC starts mapping the interconnections.

Distribution is a traditional network role and not a specific switch type in the ACI fabric model (which has leaf and spine).

The Cisco ACI fabric discovery process begins with APIC controllers discovering the leaf switches to which they are directly connected. These leaf switches then learn about the spine switches they connect to, and eventually, the entire fabric topology is built by the APIC. This initial leaf-to-APIC connection is fundamental for the APIC to start fabric inventory.

Access is a traditional network role and not a specific switch type in the ACI fabric model, nor is it the first type discovered.