Hello Patrick
In the Configuration section of the lesson, you can see the process that is followed by the router and switch with the debugging that has been enabled.
Specifically, as soon as multicast routing is enabled on the router, it sends out an IGMP packet. The switch receives that packet and realizes it is a router on that port. Thus, it will set the IGMP snooping querier as the IP address of the router. You can see all of this in the lesson like so:
SW1#
IGMPSN: router: Received IGMP pak on Vlan 1, port Gi0/1
IGMPSN: router: Is not a router port on Vlan 1, port Gi0/1
IGMPSN: router: Is not a router port on Vlan 1, port Gi0/1
IGMPSN: router: Created router port on Vlan 1, port Gi0/1
IGMPSN: router: Learning port: Gi0/1 as rport on Vlan 1
The received IGMP packet on Gi0/1 indicates to the switch that the router is on that port. Take a look at the resulting querier that has been set up on the switch:
SW1#show ip igmp snooping querier
Vlan IP Address IGMP Version Port
-------------------------------------------------------------
1 192.168.1.254 v2 Gi0/1
So at this point, the switch knows the IP address of the multicast router (querier) as well as the port to which it is connected.
So in the send only source scenario, the switch is aware of the querier on the network, and can choose to send such multicast traffic only to the router.
I hope this has been helpful!
Laz