Dave O'Shea supposedly said:

I've noted several providers, including a couple of better-known ones,
using RFC1597 addresses internally. While not a Really Optimal Solution, it
does work, and if you find yourself with only a couple of class C's to work
with.. I'd probably rather preserve them for my customers, and go with
whatever I had to internally, as long as packets still got from A to B.

I would like to contidict your statement of "not a Really Optimal Solution"
and state that it is a really good solution. The value of a uniques
address is its routability. For purely transit links what does it matter?
Using an RFC 1918 net for your internal network is a good move for
providers to provide transit. It provides great flexibilty in your
numbering options and as long as you don't leak your IGRP routes your AS
the only thing it effects is traceroutes.

Much more interesting is when you have to connect to several different
customers, many of whom have chosen 10/8, 172.16/16, or 192.168.1/24 as
their core network addresses.

It shouldn't make a bit of difference since the routes should not be
visable outside the individual AS.

---> Phil

On top of that, it makes the InterNIC (or insert your favorite address
space allocation body here) very happy to know that you are trying to
stretch your existing address space as far as possible when going to
ask for more.