Hi @Leftz
There are many things which can be told about how traffic forwarding is happening in ACI. The best source to learn in depth about this is, as @ecsnnsls mention, by watching the ciscolive presentation "Mastering ACI Forwarding Behavior". You can search for it on ciscolive.com at ondemand videos, or simply look by yourself over the ppt presentation. Before jumping to the it, I would recommend you get familiar with the ACI basics (what is a BD, EPG etc).
Meanwhile, here is a very brief description on how ACI forward traffic, taken from one of the ACI whitepapers:
Cisco ACI forwards traffic by using Virtual Extensible LAN (VXLAN) encapsulation. The way that packets are sent to the VXLAN Tunnel Endpoint (VTEP) at which the destination MAC or IP address is located depends on the bridge domain settings. Cisco ACI can forward traffic based on either the destination MAC address of the packet prior to VXLAN encapsulation or the destination IP address of the packet prior to VXLAN encapsulation.
In Cisco ACI, routed traffic is traffic whose destination MAC address is the router MAC address: that is, the subnet MAC address in the bridge domain. Layer 2, or bridged, traffic is traffic whose destination MAC address is not the router MAC address.
Layer-2 traffic forwarding can be based on the MAC address–to–VTEP mapping learned as a result of flooding along the multicast tree of each bridge domain, or it can be based on the endpoint database that discovers endpoints. The first forwarding mechanism is the classic VXLAN forwarding approach. It is enabled by setting the bridge domain to perform unknown unicast flooding. With the second mechanism, the Layer 2 forwarding of unknown unicast frames is based on the endpoint database, and you need to enable the hardware-proxy option.
Routing traffic is always based on the lookup of the IP address–to–VTEP mapping information. The endpoint IP address is learned through the leaf switch. The leaf switch discovers the endpoint IP address from the ARP requests of the endpoint or from the data-plane traffic from an endpoint that is sending traffic to the destination MAC address of the router.
Take care,
Sergiu
