Hence increased overhead. The tunnel header is unicast-addressed to the next hop. This seems the easiest solution to implement. The Ethernet modules of Xcast nodes does not have to be modified at all. One disadvantage is multicast bandwidth utilization with this scheme is inferior to that of the Host Group Model multicast, since the link sees Ethernet frame as many as the number of the next hops.
Ethernet is already too widespread to make some modification over it.
Since only 4 bytes are available, it is not possible to include any Ethernet address address in this field. Instead, this field can be used as an indication bitmap posterior to link-layer level signaling that shares information who are the Xcast node in that Ethernet segment. Note that this still requires another kind of signaling between nodes and modifications of IEEE standards are required. Since the Ethernet frame is destined to a unicast address, the frame will be delivered to the intended destination without erroneous frame congestion. One advantage of this scheme is that it requires no extra signaling or state management.