Sri Aurobindo translates Mundaka Upanishad, Chapter 3, Section 2, Verses 10 and 11: “This is That declared by the Rig-veda. Doers of works, versed in the Veda, men absorbed in the Brahman, who putting their faith in the sole-seer offer themselves to him sacrifice, — to them one should speak this Brahman-Knowledge, men by whom the Vow of the Head has been done according to the rite. This is That, the Truth of things which the seer Angiras spoke of old. This none learns who has not performed the Vow of the Head. Salutation to the seers supreme. Salutation to the seers supreme!”
There is a certain amount of obscurity around the language of these final 2 verses of the Mundaka Upanishad. Noted commentators have different interpretations of the meaning of the “Vow of the Head”. There are specific sacrifices that are referenced here as well that may have both an outer and an esoteric meaning. The sense seems to be that this teaching is only for individuals who have made a commitment to the path of realisation and who have carried out their duties, studied the Vedas and dedicated their lives to this path of Knowledge.
Some commentators believe that the references here imply that this is only for those who take up the path of renunciation, sannyasa. However, the larger sense of the Upanishad, with the teaching called for and being related to a noted householder, and with references about doing works in the world and the destiny of the lineage of those who accomplish these realisations, is that renunciation of works is not a pre-requisite. In fact, once the knowledge by identity has taken place, the seeker is identified with the manifestation of the universe and has no need to withdrawn; rather, he can carry out the duties for which he has been created.
In an omnipresent reality, neither absorption and distraction in worldly affairs, nor renunciation of all action are the complete understanding. The Upanishad implies that the seeker should renounce egoistic desire and identify with the Supreme Brahman in both the Immutable and Mutable poises. This aligns with the later teachings of the Bhagavad Gita. While this Upanishad has been used as a substantial underpinning for the path of renunciation of the world in pursuance of the path of Knowledge, the intention appears to have been more nuanced.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210