When multicast traffic is sent over unicast tunnels, ingress replication is done for each remote VTEP node. IP multicast fabric enables IP fabric to distribute BUM (Broadcast, Unknown Unicast, and Multicast Overlay) traffic using multicast VxLAN tunnels established over underlay fabric links.
Multicast Vxlan tunnels use Protocol Independent Multicast - Source Specific Multicast (PIM-SSM) and Multicast Distribution Tree (MDT) to deliver traffic effectively while minimizing packet replication in the fabric.
When multicast fabric is configured, a default MDT is created using PIM-SSM protocol running on fabric links and all the EVPN domain (VLANs and BDs) traffic is routed using the default tree.
The following figures show Clos topology for VxLAN unicast and multicast tunnels.
