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We are having a BGP configuration problem which looks trivial but is not working the way we expect. I'm going to throw myself on the mercy of the court and ask for help.

There are three routers A, B and C. These are in ASa, ASb and ASc, respectively. B imports 10.1.64/19 from A and 10.1/16 from C. What we were expecting was for B's route to the /19 to be via A, just because this is the more specific route. However, the only route (both in the BGP and routing tables on B) is for the entire /16 via C.

Even if it isn't the best route, you should see the table of choices (in a Crisco) if you do 'sho ip bgp x.y.z.0'. You'll only see the most specific choices that match with that - you could also do a 'sho ip bgp' and watch (presumably 10/8 should be near the top) for 10.1.64.0's range to show up and see what choices are presented.

How about: show ip bgp 10.1.0.0 255.255.0.0 longer-prefixes