When connecting to a routable address on the network, the host operating system typically chooses the physical network interface that will be used to facilitate the connection (for example, wired Ethernet versus WiFi). The choice can be made on the basis of any number of variables—detected connection speed, latency, user preference, etc.—but one element the operating system knows for certain is whether a given network interface can reach the requested address. Interfaces that cannot reach the destination are quickly disqualified when making this choice.

Multicast poses a unique challenge here: Any multicast-range IP address is joinable from any network interface on the host computer (this is not strictly true as some network interfaces do not support IGMP multicast, but we’ll ignore that for this discussion). In other words, any network interface can 'reach' any requested multicast group address. Unfortunately, because each network typically has its own multicast universe, joining 239.1.1.1 from the WiFi network will not necessarily yield the same results as joining 239.1.1.1 from the wired Ethernet interface. Radio multicast traffic made available on the wired Ethernet network by a radio gateway will likely not be 'seen' from the WiFi interface.

Complicating this matter further, the same multicast group address may be used for different purposes on different networks. 239.1.1.1 on the WiFi network might yield radio traffic from the 'Alpha' channel and 239.1.1.1 on the Ethernet network might be used for 'Bravo' channel. Which channel’s traffic did you intend to reflect when you asked the software to join 239.1.1.1?

It is incumbent on the system administrator to understand these details and choose the correct interface for their environment. Most reports of reflected multicast 'not working' are caused by a network interface misconfiguration.