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The deal includes $18 million guaranteed over the first two years, with a player option after 2015, according to NFL Media Insider Ian Rapoport.
NFL Media's Albert Breer also reported that the weighty contract includes a no-trade clause that runs through the first three years of the deal. And Mack can void any time before the start of the 2016 league year, which allows him to wait until after the franchise tag period is over.
"We have talked about keeping our own players and this is a positive for us," general manager Ray Farmer said in a statement. "Alex is a quality person and player that truly brings to life what playing like a Brown means. The ending is positive for everyone. Keeping our young, good nucleus of players is vital for clubs and specifically the Browns, and therefore this is a good step."
Mack ranked as the NFL's fourth-best center last season, per Pro Football Focus, grading out as a strong pass-blocker and road-grader. He's known as a gritty, intelligent lineman and also one of the league's most durable, having not missed a single snap since 2009.
It's a mountain of money for a center, but the still-under-construction Browns aren't a squad that can afford to watch talent walk out the door.