The pragmatic way of interconnecting isolated multicast realms or islands is to use unicast tunnels (GRE or IP-IP) constituting the MBONE. With the advent of multiprotocol features for BGP, it is now possible to let BGP deal with separate sets of unicast and multicast network layer reachability information (NLRI). This is also the evolution we see today?fewer tunnels, more and more MBGP/MSDP-enabled border or peering routers, and MBGP/MSDP and PIM-SM replacing DVMRP for interautonomous system multicast routing, as sketched in Figure 14-4.
Figure 14-4. Modern MBGP Architecture
It is the general consensus that this task has to be carried out with a sparse-mode protocol. Zebra/Quagga supports MBGP hooks. I consider BGP peering points and exchanges the best place to exchange interdomain multicast signaling information. Eventually, multicast route server and looking glasses will become as popular as their unicast counterparts.