Is this really an issue? So long as they're not advertising the space I
see no issue with routing traffic through a 10. network as transit. If
you have no reason to reach their router directly (and after Cisco's last
exploit, I'd think no one would want anyone to reach their router directly
), what's the harm done?
RFC1918 merely states that it shouldn't be routed on the global internet,
not that it can't be used for transit space.
RFC1918:
Because private addresses have no global meaning, routing information
about private networks shall not be propagated on inter-enterprise
links, and packets with private source or destination addresses