What is the meaning of "v'Avar mi'Sham"?
Malbim: From there the border turned to the south; Luz was [north of the border,] in Yosef.
Here it says that that Beis Kel is Luz, like Yakov called it (Bereishis 28:19). Above (16:2), it says that [Yosef's] border went from Beis Kel to Luz!
Rashi: That is a different Beis Kel, near Ai. Luz was in Yosef 1 , and not in Binyamin, for the border was south of it. We find below (verse 22) that [the other] Beis Kel was in Binyamin.
Which mountain is discussed?
Bi'ur ha'Gra: It is Har Beis Choron.