Apparently, Sankara cites the Rig Vedic Purusha Sukta while commenting about the caste system advocated in the Bhagavad Gita.
But since the Vedas are regarded as apara vidya in comparison to the Upanishads, do any of Isha, Kena, Katha, Prashna, Mundaka, Mandukya, Tattiriya, Aitareya, Chhandogya and Brihadaranyaka Upanishads mention or advocate the caste system?
By "unequivocally" I mean ONLY advocate the traditional caste hierarchy (by birth) Brahmin > Kshatriya > Vaishya > Sudra and not allow for exceptions.