We have numerous branches Wan'd via bgp with a 10.x.0.0/16 scheme. A consultant to a stub like portion of our network wants to inject a 10.x.0.0/12 "supernet" how he put's it scheme. If for instance then the question is put to me whether a 10.120.0.0/16 network and a 10.120.0.0/12 network are advertised, will there be any chance of overlapping addresses? I should know this...my inclination is no, but our senior network tech has some reservations.
Not quite. 10.130.0.0/12 does not exist as it has an inconsistent mask. The prefix would be 10.128.0.0/12. You are right that you should not permit anything in the range 10.128.0.0/16 to 10.143.0.0/16 to be injected from anywhere else as all those addresses would be covered by the 10.128.0.0/12 injection.