What's so hard to understand??? In the US you create an aggregate route
for the US address block and you accept /48s for US destinations. In
Europe, you create an aggregate for the European address block and accept
/48s for European destinations. So when you have a packet in the US that
needs to go to a European destination, you don't have a /48 route so the
packet is routed to Europe by means of the aggregate. In Europe, the
packet is routed further as per the /48 that is in the routing tables of
European routers. Rinse, repeat.
Do you think it reasonable to have (at least) two DFZs--one using PA
addresses and the other using PI addresses? Presumably, there would be a
large number of interconnects between the DFZs. Even if there are known
problems in multihoming with PA addresses, I think there is too large a
base to get rid of them.