Layer 3 Multicast
In the ACI fabric, most unicast and multicast routing operate together on the same border leaf switches, with the multicast protocol operating over the unicast routing protocols.
In this architecture, only the border leaf switches run the full Protocol Independent Multicast (PIM) protocol. Non-border leaf switches run PIM in a passive mode on the interfaces. They do not peer with any other PIM routers. The border leaf switches peer with other PIM routers connected to them over L3 Outs and also with each other.
The following figure shows the border leaf (BL) switches (BL1 and BL2) connecting to routers (R1 and R2) in the multicast cloud. Each virtual routing and forwarding (VRF) in the fabric that requires multicast routing will peer separately with external multicast routers.