The sports network said in a story posted on its Web site that $40 million would be guaranteed with the higher amount possible if McNabb reaches incentive levels. McNabb, who turns 34 Nov. 25, came to the Redskins this season after 11 years with Philadelphia.
The Washington Post reported McNabb's agent had confirmed the extension had been reached.
"We've been working on this for a while now, and we're obviously glad to get it done," Fletcher Smith told the newspaper. "Donovan's very happy about the situation because this is where he wanted to be all along."
The Sports Network reported a Redskins spokesman would not confirm or deny the deal and said the team had no announcement planned.
The Redskins and Eagles play Monday night.
So far this season, McNabb has thrown for 1,971 yards with seven touchdowns and eight interceptions in eight games. His passer rating was just 76.0, having completed just 57.4 percent of his throws. He also has been sacked 22 times.
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