Believe me, this will occur. It will probably start with "Well, we've got this connection to you and this connection to ISP B, and, you guys peer, so, can you pass our ULA prefixes along to each other?"
Talk to the other ISP, work out pricing, and sell an IP over IP solution, MPLS solution or some such. Look at this as an opportunity, instead of a
problem, and there's money to be made without leaking the prefixes into the backbone. Embrace progress and conceive of creative solutions to customer needs.
In today's network, is there anyone left who uses 1500 byte MTUs in their core? Surely even the people with Ethernet switches in their PoPs are now using jumbo frames. In yesterday's Internet, encapsulation was a problem because of the fragmentation required, but it should be a routine thing in today's Internet where fragmentation is avoided. If ULAs, i.e. site local addresses, need to be visible at two disjoint locations in the network, we have the technical means to do this without using up global routing table slots. The same thing applies to geographical addresses which may sometimes need to be made visible in another region. We *CAN* do things at the edges of the network without burdening the core. --Michael Dillon