[T]he cognomen, indicator par excellence of the stable, geographically localized, diachronically perpetuated lineage, is an emblem of patrilineal prestige. The poem's invidious perception of the cognomen is reflected in the frequent repetition of the toponym associated with the Cid's foes: "Carrión." Used 135 times (versus 22 examples for Vivar), the name occurs in most instances as a component of the cognominal formula "iffantes de Carrión" [...] with the cognomen "de Carrión" appearing far more frequently than the patronym González[…]
Source: wiktionary