3Com 8800 Configuration Manual page 283

3Com Switch 8800 Configuration Guide Chapter 23 IP Multicast Overview
uses the source address of a received multicast packet to query the unicast routing
table or the independent multicast routing table to determine that the receiving interface
is on the shortest path from the receiving station to the source. If a source tree is used,
the source address is the address of the source host sending the multicast packet. If a
shared tree is used, the source address is the RP address of the shared tree. A
multicast packet arriving at the router will be forwarded according to the multicast
forwarding entry if it passes the RPF check, or else, it will be discarded.
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