Exam 300-620 topic 1 question 243 discussion

Mapping the MAC address of the external default gateway to the bridge domain ensures that ACI can properly forward traffic destined for the gateway. This step is necessary because ACI must associate the default gateway's MAC address with its external location via L2Out. Set L2 Unknown Unicast Flood: While this could be helpful in some cases, ARP Flooding is the primary requirement for external gateway communication.

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/aci/apic/sw/migration_guides/migrating_existing_networks_to_aci.html#_Toc133408118 Enable ARP flooding: ARP requests originated from devices connected to the Cisco ACI fabric should be able to reach the default gateway or other endpoints part of the same IP subnet and still connected to the brownfield network. Since those entities are unknown to the Cisco ACI fabric, it is required to flood ARP requests across the Cisco ACI fabric and toward the brownfield network. Enable Unknown Unicast flooding: similar considerations valid for ARP traffic apply also to Layer 2 unknown traffic (unicast and multicast), so it is required to ensure flooding is enabled in this phase for those traffic types.