An Cisco ACI leaf switch learns the source IP address of a packet that enters...

[: Cisco APIC Bridge Domain Configuration Guide, "Unicast Routing Enablement" and Cisco ACI Endpoint Learning White Paper., B. L3 Unknown Multicast Flooding - Flood: , This setting controls how unknown multicast traffic is handled at Layer 3 (e.g., flooding or dropping). It is unrelated to learning the source IP address of a unicast packet entering the front panel port., Reference: Cisco ACI Multicast Configuration Guide., C. ARP Flooding: , ARP Flooding allows ARP requests to be flooded within the bridge domain, which helps resolve IP-to-MAC mappings for silent hosts or external devices. However, it is not the primary setting for learning the source IP of an active packet; it is a supplementary mechanism., Reference: Cisco APIC Bridge Domain Settings, "ARP Flooding Configuration.", D. Unknown Unicast - Hardware Proxy: , The "Hardware Proxy" setting optimizes unicast traffic by using a proxy to respond to ARP requests, reducing flooding. While it aids in endpoint management, it is not the setting that directly enables learning the source IP address from incoming packets., Reference: Cisco ACI Routing Configuration Guide, "Hardware Proxy.", Final Answer Justification, A is correct because enabling "Unicast Routing" in the bridge domain allows the leaf switch to learn the source IP address of a packet entering the front panel port by associating it with the source MAC address and VLAN. This is the foundational setting for Layer 3 endpoint learning in ACI., Primary Cisco References: , Cisco APIC Layer 3 Networking Configuration Guide, "Bridge Domain Unicast Routing.", Cisco ACI Endpoint Group Configuration Guide, "IP Address Learning.", ]