Which RFC?

Anyone know *which* RFC says a packets should be routed using the
most specific route in a routing table, not (for instance) the first
route in the table that matches, or, for instance, using a less specific
route that has a better metric? This is so basic I hardly
know where to find it, but I have a (for the time being anonymous)
systems vendor who really prefers the way their kernel does it ....
Oddly enough it doesn't work well with subnetting.

Alex Bligh
Xara Networks