May one violate a rabbinic prohibition (such as carrying a knife in a Karmilis on Shabbos) in order to perform a Bris Milah? (A Karmilis is a domain which a person can't carry stuff into or from a private domain on shabbat)
Rambam Sefer Ahava Hilchos Mila 2:6 says one may not carry the Mila knife on Shabbos even in a area where it is a D'Rabanan since it is possible to bring before Shabbos.
Rabbi Eliezer Melamud maintains that in extenuating circumstances the first choice would be to get a non Jew to carry it for you. If that is not an option a Jew may carry it with a Shinui in a Karmelis.