An engineer is extending EPG connectivity to an external network. The external network houses the Layer 3 gateway and other end hosts. Which ACI bridge domain configuration should be used?

When extending an Endpoint Group (EPG) to an external network that contains the Layer 3 gateway, the ACI fabric acts as a Layer 2 transit switch. To ensure that ARP requests successfully reach the external gateway and that unknown MAC addresses are properly resolved, ARP Flooding must be enabled. Additionally, L2 Unknown Unicast must be set to Flood so that traffic destined for unlearned external MAC addresses is flooded out the Layer 2 extension rather than being dropped by the spine proxy.

A. Setting L2 Unknown Unicast to Hardware Proxy causes the fabric to drop traffic for unlearned MAC addresses, breaking connectivity to unknown external hosts.
C. Hardware Proxy drops unknown unicast, and disabled ARP flooding prevents ARP requests from reaching the external gateway, completely breaking Layer 3 resolution.
D. Disabling ARP flooding prevents broadcast ARP requests from reaching the external gateway, which breaks Layer 3 address resolution for the endpoints.

1. Cisco ACI Layer 2 Networking Configuration Guide, Release 5.2(x), Chapter: "Layer 2 Unknown Unicast", Section: "Flooding L2 Unknown Unicast".

2. Cisco APIC Layer 2 Networking Configuration Guide, Release 5.2(x), Chapter: "Extending the EPG Out of the ACI Fabric".