 In 627, Emperor Taizong promoted Cui Dunli to a higher post within the legislative bureau.It was said that particularly because Cui admired the great Han Dynasty diplomat Su Wu, he was frequently sent as a diplomat to Eastern Tujue.At a later point, Cui became the deputy minister of defense, a post that he still held as of 642, when he was sent to Eastern Tujue's successor state Xueyantuo to negotiate a marriage of Emperor Taizong's daughter Princess Xinxing with Xueyantuo's Zhenzhu Khan Yi'nan, in order to gain the release of the general Qibi Heli (契苾何力), who had been held captive by Xueyantuo.