With respect to IP Multicast having PIM sparse mode and IGMP snooping in place consider the scenario having 2 x routers connected to LAN switch. The multicast receivers are also connected to same LAN Switch.
The connectivity diagram is shown herewith.
Below are few points regarding protocols :-
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PIM Sparse Mode is running on R1/R2 and rest of network
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IGMP Snooping is running on LAN Switch
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Hosts are running IGMP V2
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R1 is the IGMP Querier and PIM DR
Once the Host H1 initiates the desire to join multicast group say 239.10.10.10 it sends IGMP membership report which is sent by LAN Switch to R1 and R2. R1 being PIM DR sends out PIM Join towards the rest of network as we know the story.
Once R1 starts to receive multicast traffic , it forwards the traffic towards the LAN switch as it is its Outgoing Interface List. The LAN Switch , running IGMP Snooping will forward multicast traffic to H1 but not to H2 following IGMP Snooping functionality.
However , the switch also forwards the same multicast traffic to R2 which is unwanted traffic that R2 has not asked for. The reason the switch forwards this traffic is that from IGMP snooping perspective its a router port i.e. an interface that either receives IGMP queries or PIM hellos is classified as router port.
It seems to be expected behavior as per the RFC 4541 that says :-
We can remove Interface running on R2 towards LAN switch from PIM Sparse mode. The Switch will list the port as Host Port instead of router port and the problem will be fixed but this is not fault tolerant. In case R1 goes down , multicast will not work as R2 is not running PIM sparse mode on interface facing switch.
Is there a way to stop LAN switch not sending multicast traffic towards R2? or this is a expected behavior and nothing can be done about this.
Thanks !