The border itself is not. Within the framework of a single union there are no restrictions on the movement of citizens.

If a young man of draft age (18 - 27 years) has already served in the army, his internal passport should have a note on the registration for military registration. If a young man has not yet been drafted by the military registration and enlistment office and does not have the necessary mark, he can not be detained on the border without good reasons. Usually, such problems do not arise when crossing the borders of Russia and Belarus.

It is recommended that you take a certificate and a military ID card with you. Moreover, a border service officer can detain a traveler as a potential evasive, only having a copy of the commission's decision on recruiting a young man to the army.