A copy of this description, with all servers marked Unknown.

def reset(self):
        """A copy of this description, with all servers marked Unknown."""
        if self._topology_type == TOPOLOGY_TYPE.ReplicaSetWithPrimary:
            topology_type = TOPOLOGY_TYPE.ReplicaSetNoPrimary
        else:
            topology_type = self._topology_type

        # The default ServerDescription's type is Unknown.
        sds = dict((address, ServerDescription(address))
                   for address in self._server_descriptions)

        return TopologyDescription(
            topology_type,
            sds,
            self._replica_set_name,
            self._max_set_version,
            self._max_election_id,
            self._topology_settings)