The group of settings required to meet the requirements for workload migration where servers will physically terminate at the Cisco ACI, but their gateway must stay in the existing network, is:

L2 Unknown Unicast: Flood

L3 Unknown Multicast Flooding: Flood

Multi Destination Flooding: Flood in BD

ARP Flooding: Enable5

These settings ensure that the bridge domain is configured to handle unknown unicast and multicast traffic appropriately, and ARP flooding is enabled to allow for the resolution of IP addresses when the gateway is outside the fabric5.

https://www.cisco.com/c/en/us/td/docs/switches/datac enter/aci/apic/sw/migration_guides/migrating_existing_networks_to_aci.html