In chapter 15 we read about the Brit bein HaBetarim, the covenant God made with Avraham in which He appeared in a night vision and told Avraham about the future nation that would descend from him, the exile and redemption. Avraham is shown an overview of the history of the future nation and the Land they would inherit.
In chapter 17, we read about the Brit Mila. In contrast, Avraham received this covenant during the day while he was awake and he was instructed to carry out an actual act. Brit Mila is the covenant that promotes the family, which was first performed by Avraham and, with the birth of Yitzchak, would become the sign designating the entry of every newborn boy into the family of Am Yisrael.
Circumcision was common amongst ancient nations and in Egypt mass ceremonies for adolescents would take place. Thus when Avraham circumcised all the males in his household, including the 13-year-old Yishmael, it was not a novelty for Hagar, the Egyptian slave. Avraham thought that he would only have descendants from Yishmael and had given up hope of having a child from Sara.
Then God promised Avraham that: "Indeed, your wife Sarah will bear you a son, and you shall name him Isaac, and I will establish My covenant with him as an everlasting covenant for his seed after him." (17:19). God assigns them new names – Avram became Avraham and Sarai became Sara and the future Am Yisrael would descend from this new family. Yitzchak's brit would be different to the mass circumcision ceremony of Yishmael. It would be done when he was eight days old within the family setting. Ever since then, brit mila has been the sign of entry into the family of the Jewish people.