Holdon, i don't even see the need for that; if you traceroute out, the packets will cross the exchange irregardless of whether you're announcing it to your customers, and a message of TTL exceeded will be generated from the exchange's address.. The important question is, should they be globally UNIQUE for troubleshooting purposes? I think so.
When not advertising exchange point address space, the conditions with respect to traceroute are much the same as that of RFC 1918 address space; you can traceroute *through* it and get reports of TTL exceeded from source addresses that you cannot reach as a destination. No big deal to some, major diagnosis headache to others. I'd argue in favor of reducing diagnosis headaches ... and I *hate* it when I can't traceroute to points in the interior of providers who use RFC 1918 addresses internally. :-) If the shared L2 media of an exchange point fabric did *not* have a shared L3 address prefix as Alan Hannan noted earlier, where (as Alan said) one party of a bi-lat coughed up a /30, then the reachability of said prefix could be left to the parties to the bi-lat. Modern equipment should be able to handle the quantity of secondary/alias addresses needed to deal with number of /30s required to uniquely number each bi-lat, no? Stephen