Multicast - generating a routing table entry

In a multicast environment, IGMP messages and general multicast traffic will generate entries within the multicast routing table. Each one however will generate them in different ways.


A router receives an IGMP - membership report from a host. The router will indeed generate a [*,G] entry and the outgoing interfaces will be all PIM-enabled interfaces plus the interface on which the report was received.

Scenario 2

A device sends multicast traffic into the network. The multicast router receives multicast traffic from this source, and the outgoing interface list is different from the previous one. It only includes PIM-enabled interfaces, without the one on which the router received the multicast traffic. And this makes sense because the router only needs to forward multicast traffic from the source to other interested hosts. Since the source is already on the incoming interface, there is no need to include that interface in the outgoing interface list.

So the outgoing interface list in the [*,G] entry is decided based on whether the router receives an IGMP Membership Report or actual multicast traffic. The former includes all PIM-enabled interfaces and the interested host’s interface, while the latter only includes the PIM-enabled interfaces.