Multicast ( *, G) question

From: Sheahan, John (John.Sheahan@priceline.com)
Date: Mon Jun 13 2005 - 11:36:24 GMT-3

When you first turn up multicast with the command "ip multicast-routing"
and add the appropriate PIM statements on the interfaces, you usually
get a (*,G) for a certain multicast group destination. For instance, (*,
224.0.1.24).

I see that this will occur even when you have no source traffic to the
destination (in this case, 224.0.1.24).

When the source starts up, you will then see (10.1.1.1, 224.0.1.24) in
the mroute table.

My question is, what causes the router to know about and put in the
(*,224.0.1.24) entry when there is no source? Is this caused by a
join-request for that group? How does the router initially know about
this multicast destination and why does it not note the source?

thanks

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