The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
35. And because the kshattriyahood (of Gânasruti) is understood from the inferential mark (supplied by his being mentioned) later on with Kaitraratha (who was a kshattriya himself).
Gânasruti cannot have been a Sûdra by birth for that reason also that his being a kshattriya is understood from an inferential sign, viz. his being mentioned together (in one chapter) with the kshattriya Kaitraratha Abhipratârin. For, later on, i.e. in the passage complementary to the samvarga-vidyâ, a kshattriya Kaitrarathi Abhipratârin is glorified, 'Once while Saunaka Kâpeya and Abhipratârin Kâkshaseni were being waited on at their meal a religious student begged of them' (Kh. Up. IV, 3, 5). That this Abhipratârin was a Kaitrarathi (i.e. a descendant of Kitraratha) we have to infer from his connexion with a Kâpeya. For we know (from Sruti) about the connexion of Kitraratha himself with the Kâpeyas ('the Kâpeyas made Kitraratha perform that sacrifice;' Tândya. Br. XX, 12, 5), and as a rule sacrificers of one and the same family employ officiating priests of one and the same family. Moreover, as we understand from Scripture ('from him a Kaitrarathi descended who was a prince 2') that he (Kaitraratha) was a prince, we must
understand him to have been a kshattriya. The fact now of Gânasruti being praised in the same vidyâ with the kshattriya Abhipratârin intimates that the former also was a kshattriya. For as a rule equals are mentioned together with equals. That Gânasruti was a kshattriya we moreover conclude from his sending his door-keeper and from other similar signs of power (mentioned in the text).--Hence the Sûdras are not qualified (for the knowledge of Brahman).
226:2 I translate this passage as I find it in all MSS. of Sankara consulted by me (noting, however, that some MSS. read kaitrarathinâmaikah). Ânanda Giri expressly explains tasmâd by kaitrarathâd ity arthah.--The text of the Tândya Br. runs: tasmâk kaitrarathînâm ekah kshatrapatir gâyate, and the commentary explains: tasmât kâranâd adyâpi kitravamsotpannânâm madhye eka eva râgâ kshatrapatir p. 227 balâdhipatir bhavati.--Grammar does not authorise the form kaitraratha used in the Sûtra.