Hi David,
What exactly do you have in mind? To my best knowledge, BGP does not exactly carry multicast prefixes, i.e. prefixes from within the 224.0.0.0/4 range. What it does, with respect to multicast routing, is carrying the prefixes of multicast sources as a separate address family, allowing the topology for unicast and multicast routing to be independent. For these multicast source IP prefixes, normal rules apply, i.e. AD=20 for eBGP-learned prefixes, AD=200 for iBGP-learned prefixes.
Does this perhaps answer your question?
Best regards,
Peter
