Multicast at broadcast exchanges

[Stephen Griffin <stephen.griffin () rcn com>]

Hmm, I was thinking about the topic of multicast at broadcast exchanges, and
had a weird thought.

Presently, the various participants may have divergent peering relationships
and routing preferences. Such that RPF choices for the same (s,g) can
go back to different places.

If I remember correctly, all PIM routers on the same broadcast domain
will elect a designated router, to keep from sending the same traffic
for the same group multiple times.

If network A is preferring network B (via localpref, for example) and
network C is preferring network D, such that A rpf's to B for prefix E
and C rpf's to D for prefix E, how would this be resolved?

What if A doesn't peer with D at all, and C doesn't peer with B?

Wouldn't one of B and D send the traffic, such that the other isn't
sending any, regardless of the policies of any of the networks
involved?

What if the elected DR isn't one of B or D?

The only "workaround" I could see, would be if the multicast traffic
were unicasted across the exchange (much like how things are at the
atm peering points). However, this nulls out the benefit of having a
multicast broadcast exchange.