3Com 4200G 12-Port Configuration Manual page 457

A multicast packet from Source arrives to VLAN-interface 1 of Switch C, and the corresponding
forwarding entry does not exist in the multicast forwarding table of Switch C. Switch C performs an
RPF check, and finds in its unicast routing table that the outgoing interface to 192.168.0.0/24 is
VLAN-interface 2. This means that the interface on which the packet actually arrived is not the RPF
interface. The RPF check fails and the packet is discarded.
A multicast packet from Source arrives to VLAN-interface 2 of Switch C, and the corresponding
forwarding entry does not exist in the multicast forwarding table of Switch C. The router performs
an RPF check, and finds in its unicast routing table that the outgoing interface to 192.168.0.0/24 is
the interface on which the packet actually arrived. The RPF check succeeds and the packet is
forwarded.
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