Rambam to Hilchot Melachim 10:8 holds that the sons of Avraham's wife, Ketura, which are the seed of Avraham that came after Yishmael and Yitzchak are obligated in Brit Mila (circumcision) And since in our days the descendants of Yishmael and the sons of Ketura have intermarried (losing their distinct identity) all of this mixed group will be obligated in Brit Mila on day 8 of a baby's life.רמב"ם הלכות מלכים פרק י
הלכה ח
אמרו חכמים שבני קטורה שהם זרעו של אברהם שבא אחר ישמעאל ויצחק חייבין במילה, והואיל ונתערבו היום בני ישמעאל בבני קטורה יתחייבו הכל במילה בשמיני
Rashi disagrees. He holds that only the first generation of the sons of Ketura had the Brit Mila obligation. Torah Temima explains that Rashi holds that these people are not considered on a religious level, the seed of Avraham and the first generation from Ketura were only obligated because all that were born to Avraham's household needed a Brit Mila.