RFC 4541 IGMP and MLD Snooping Switches Considerations May 2006
2.1.2. Data Forwarding Rules
1) Packets with a destination IP address outside 224.0.0.X which are
not IGMP should be forwarded according to group-based port
membership tables and must also be forwarded on router ports.
This is the main IGMP snooping functionality for the data path.
One approach that an implementation could take would be to
maintain separate membership and multicast router tables in
software and then "merge" these tables into a forwarding cache.
2) Packets with a destination IP (DIP) address in the 224.0.0.X range
which are not IGMP must be forwarded on all ports.
This recommendation is based on the fact that many host systems do
not send Join IP multicast addresses in this range before sending
or listening to IP multicast packets. Furthermore, since the
224.0.0.X address range is defined as link-local (not to be
routed), it seems unnecessary to keep the state for each address
in this range. Additionally, some routers operate in the
224.0.0.X address range without issuing IGMP Joins, and these
applications would break if the switch were to prune them due to
not having seen a Join Group message from the router.