Multicast only works if the 2 domains are interconnected with links that run PIM. Also, multicast traffic must pass RPF check in order to be forwarded. When performing the RPF check, a router that uses PIM will look into the unicast routing table for the route that points to the destination of the multicast traffic. The unicast routing table may be populated with routes from all routing protocols, including BGP.
However, what if you need the inter-domain multicast traffic to use another link, different than unicast traffic? Well, you could use static mroutes, but these are local to each router. Anoher option is to use MP-BGP’s address-family ipv4 multicast
What this does, is to exchange unicast routes between routers, but these routes will not be used for unicast routing, instead they will be used for RPF checks when routing multicast traffic.
Actually, you can have the same routes that are advertised by unicast ipv4 BGP advertised in multicast ipv4 BGP. But since they are used for different functions, there’s no problem. Using this method you can apply different routing policies for multicast and unicast traffic.
To enable the exchange of routes used for multicast routing, you must activate neighbors in the address-family ipv4 multicast. This address-family runs completely independent of the ipv4 unicast address-family, so you will need to configure it appropriately (route-reflectors, advertised networks, redistribution, and so on).