Techniques for utilizing Software-Defined Networking (SDN) controllers and network border leaf nodes of respective cloud computing networks to configure a data transmission route for a multicast group. Each border leaf node may maintain a respective external sources database, including a number of records indicating associations between a multicast data source, one or more respective border leaf nodes disposed in the same network as the multicast data source, and network capability information. A border leaf node, disposed in the same network as a multicast data source, may broadcast a local source discovery message to all border leaf nodes in remote networks to which it is communicatively coupled. A border leaf node may also communicate network capability information associated with one or more remote networks to a local SDN controller. The SDN controller may utilize the network capability information to configure a data transmission route to one or more destination nodes.