Multicast across BGP domains

Hello

Sometimes we may have multicast sources in one BGP domain and receivers in another BGP domain. In this case we need to exchange information across domains about:

1. routes to get to the source of the multicast traffic
2. sources of multicast traffic

To do 1), we need BGP to share multicast information, not just unicast.

To do 2), we need MSDP, a special TCP application that will inform domain B that domain A has a multicast source.

1. To activate sharing RPF information, that is how to get to a source of multicast traffic, we need to activate a neighbor for a given address family, e.g.router bgp 100
   bgp log-neighbor-changes
   neighbor IBGP peer-group
   neighbor IBGP remote-as 100
   neighbor IBGP update-source Loopback0
   neighbor 150.1.3.3 peer-group IBGP
   neighbor 150.1.6.6 peer-group IBGP
   neighbor 150.1.9.9 peer-group IBGP
   !
   address-family ipv4
   neighbor IBGP route-reflector-client
   neighbor 150.1.3.3 activate
   neighbor 150.1.6.6 activate
   neighbor 150.1.9.9 activate
   exit-address-family
   !
   address-family ipv4 multicast
   neighbor IBGP route-reflector-client
   neighbor 150.1.3.3 activate
   neighbor 150.1.6.6 activate
   neighbor 150.1.9.9 activate
   exit-address-family
2. We need to establish an MSDP peering on an RP in domain A with the RP in domain B, e.g.
   ip msdp peer 150.1.8.8 connect-source Loopback0 remote-as 200
   ip msdp peer 150.1.5.5 connect-source Loopback0 remote-as 200

Now we have everything we need to get multicast traffic from domain A to domain B. If a host in domain B joins a multicast stream, this info will go to its RP. In domain A, a source starts streaming. This goes to the RP in domain A. Domain A RP informs Domain B RP that it has a streaming source.