I am considering what happens to multicast routing in frame-relay hub-and-spoke topologies.
As I understand it, there are two problems to be overcome when doing multicast in hub-and-spoke. One is that the multicast router will not forward a multicast out the same interface it came in, which makes it difficult to multicast from spoke to spoke. The other is (I think) specific to dense mode, and occurs when the source is behind the hub. It is that a prune from one spoke will cut off the feed to another spoke as it is on the same interface at the hub.
Now, I know that nbma-mode at the hub can solve both problems by treating each PVC as a seperate point-to-point link (a bit like point-to-multipoint in OSPF). But what I have never understood clearly is why nbma-mode is allowed with sparse-mode, but it is not allowed with dense-mode. Surely it would be an ideal solution to all dense-mode's problems with hub-and-spoke.
And, BTW, what about nbma-mode and spare-dense-mode? Does nbma-mode suddenly become feasible as soon as the RP is found?
I would be grateful if someone could talk me through the reasoning behind this.