[rbridge] Multicast options

[Ghanwani, Anoop]

> My suspicion is that for layer 2 multicasts other than 
> IP-derived ones, 
> we have no choice
> but to do a), even though I think 802 is working on some IGMP-like 
> thing, since we
> can't assume all endnodes will have implemented it.

IEEE 802.1D has had for a while now a protocol called 
GMRP which limits the propagation of multicasts to segments 
of the spanning tree where there are registered participants.
The protocol hasn't seen much deployment, since IGMP-snooping
seems to work well for most applications that are out there.

Anoop