I'd expect lots of Russian targets outside of Russia proper to receive some ballistically-delivered nastygrams if Russia attacked a NATO member. They'd all have to be treated as hostile. Forces in Syria, in Belarus, ships outside Russian waters, et c., would all be at grave risk. Not a great move if they've got a large percentage of their forces outside their own borders (i.e. in Ukraine) when that goes down.

I agree that NATO would be very reluctant to so much as fire an artillery shell over the Russian border, but Russia might well forfeit a great deal of personnel and equipment in such a move, anyway. Having troops & equipment abroad is a major liability if you pick a fight with a country (or coalition) that badly outclasses you.