FINALLY, kai randi paaiškinimą žaliems, tai iš karto ir aišku pasidaro.

Su išoriniu WAN interface pagauni UPSTREAM, ir DOWNSTREAMINI į savo 
vidinius interface'sus.

"IGMP proxy is very simple in the sense that it acts as IGMP router for 
downstream interfaces and as a host on upstream interfaces. It listens 
for IGMP reports coming on downstream interfaces from hosts interested 
in listing to multicast traffic. It consolidates report information 
coming from different hosts across different downstream interfaces and 
send consolidatedreport to upstream routers.

In Office edge routers, the interfaces connected to inside machines are 
typically downstream interfaces and WAN interfaces connected to ISP 
routers are upstream interfaces. In case of DSLAM, WAN interfaces 
towards its customers are downstream interfaces and upstream interface 
is connected towards Internet.

Multicast stream providers get unique Multicast IP address for each type 
of stream from IANA. This IP address is advertised. Any machines willing 
to receive this stream send IGMP membership reports to routers 
indicating their willingness to receive packets coming on a particular 
multicast IP address. IGMP protocol is used to send membership reports.

IGMP proxy devices receives multicast stream from upstream interfaces, 
it knows the downstream interfaces interested in this traffic based on 
membership reports it received before. It duplicates the traffic and 
send it to these downstream interfaces. It provides great advantage on 
cost savings as only one copy of stream is sent on WAN link. Gateway 
implementing IGMP proxy duplicates on multiple downstream interfaces."