Despite Babelsberg's promotion, Frahn opted to stay in the fourth tier, signing for big-spending RB Leipzig along with strike partner Stefan Kutschke. He is the captain of the Red Bulls, and memorably scored a hat-trick to eliminate VfL Wolfsburg in the first round of the 2011–12 DFB-Pokal. He finished the 2011–12 season as Regionalliga Nord top scorer for a second time (with 26 goals), but the club missed out on promotion, finishing in second place behind Hallescher FC. In the 2012–13 season, RB Leipzig entered the re-formed Regionalliga Nordost, and won the division unbeaten, with Frahn again finishing as top scorer with 20 goals. They beat Sportfreunde Lotte in a playoff to secure promotion to the 3. Liga, although Frahn missed both legs of the tie. He scored the club's first ever 3. Liga goal, in a 1–0 win over Hallescher FC on the opening day of the 2013–14 season. His sixth goal of the season came ten seconds into a home-match against VfB Stuttgart II: seven Leipzig players stormed the Stuttgart half immediately after kick-off; the ball was played back then a long-ball was hit forward to Matthias Morys, who crossed for Frahn to score. RB Leipzig went on to win the match 3–1. Frahn ended the season as the league's secondtop scorer with 19 goals, behind SV Darmstadt 98's Dominik Stroh-Engel, as the Red Bulls were promoted to the 2. Bundesliga as runners-up.