Which protocol propagates multicast join requests across the SD-WAN fabric?

IGMP (Internet Group Management Protocol) is the correct answer because it is specifically designed for hosts to inform local routers of their desire to join a multicast group. In an SD-WAN context, this protocol is used to propagate those join requests from the end devices into the SD-WAN fabric, allowing the fabric to then build the necessary multicast forwarding paths. Auto-RP is a Cisco proprietary protocol used to discover and advertise multicast group-to-RP mappings, not for hosts to join groups. ARP (Address Resolution Protocol) resolves IP addresses to MAC addresses within a local network segment, unrelated to multicast group membership. OMP (Overlay Management Protocol) is Cisco SD-WAN's routing protocol for advertising routes, TLOCs, and service routes within the overlay, but it doesn't handle host-to-router multicast join requests directly.