A Ceph 2.X client, like the one that ships on an OSP10 image, should work with a Ceph 3.0 Server. For example Red Hat has tested and verified that OSP12, whose image ships with Ceph2.x, can talk to an external Ceph 3 server. For that same reasoning, one would expect that OSP10 should also be able to talk to an external Ceph 3 server. That said, it is not a tested combination. This is a request to test that combination.