MSDP enables advertisement of multicast source information between different PIM-SM domains. This function of MSDP in SPB-PIM GW topologies is to advertise multicast source information between SPB domains and PIM domains. MSDP routers in a PIM-SM or SPB domain have a peering relationship with MSDP peers in another domain. The peering relationship is a TCP connection in which the control information is exchanged. The TCP connection between peers uses the underlying unicast routing system.