From: "Norman W. Finn" <nfinn@cisco.com>
Subject: Re: VLANs and IP multicast
To: osnova@ix.netcom.com (milan momirov)
Date: Thu, 9 May 1996 20:37:24 -0700 (PDT)
Cc: p8021@hepnrc.hep.net


milan momirov writes:
> 
> Is there an understanding as to how to process IP multicast packets 
> that have to go to multiple VLANs, but onto a single physical segment?  
> Presumably the router isn't expected to output onto the physical 
> segment per-VLAN copies of each IP multicast packet.  Would each IP 
> multicast space be dynamically assigned its own VLAN id?

We've been discussing this problem in the ATM Forum MPOA group a lot,
lately, including today.  Same goal, in my opinion: in the presence of
lots of VLANs, distribute a multiast packet destined for some or all of
those VLANs in such a way that the packet is (ideally) never sent more
than once over any given wire.  That includes cases where the source is
on a "non-trunk" LAN switch port or a router on the backbone.  Oh yes,
and make sure that the packet on every non-trunk LAN switch port has the
right MAC source address so that it looks like it came from the
originator or the right router, as appropriate.  And there may be
more than one router.

A number of solutions have been proposed.  None of them claim to meet
all those goals, yet.  If we discover The Answer, it should be applicable
to 802.1 VLANs, and vice-versa.

-- Norm