When Layer 3 routed traffic is destined to a Cisco ACI fabric, which mechanism does ACI use to detect silent hosts?

Gratuitous ARP is sent by a host to announce its own presence or address change, not a mechanism ACI actively uses to discover other silent hosts.

ACI uses ARP gleaning to learn the MAC addresses of silent hosts for Layer 3 routed traffic by inspecting ARP replies received for its own ARP requests or by learning from other ARP exchanges in the Bridge Domain. When ACI's hardware proxy receives a packet for an unknown destination MAC, it generates an ARP request and then gleans the host's MAC from the response.

Proxy ARP involves a device responding to ARP requests on behalf of another device, which is different from ACI's mechanism to detect and learn silent host MAC addresses.

Inverse ARP is a protocol used to discover IP addresses from known MAC addresses, typically in technologies like Frame Relay, and is not applicable for silent host detection in ACI.