Assignor estoppel is officially dead at the Patent Trial and Appeal Board (PTAB).

That's the equitable doctrine that forbids an inventor who sells his or her patent from then turning around and attacking the patent's validity. In a decision Friday from the U.S. Court of Appeals for the Federal Circuit, the inventor was former Cisco Systems Inc. technical adviser David Cheriton, who then went on to co-found networking rival Arista Networks Inc.

Chief Judge Sharon Prost focused on language in Section 311(a) of the America Invents Act that says any person who's not the owner of a patent can bring a validity challenge known as an IPR. This plain language “unambiguously dictates that assignor estoppel has no place in IPR proceedings,” Prost wrote in Arista Networks v. Cisco Systems.

The upshot is that Arista was within its rights to challenge the validity of Cisco's 7,340,597 patent on network security.

Cisco argued that it doesn't make sense for the International Trade Commission to recognize assignor estoppel and the Patent Trial and Appeal Board not to. Prost said it does, because the statute governing the ITC explicitly provides for equitable defenses, while the America Invents Act does not. “Such a discrepancy between forums—one that follows from the language of the respective statutes—is consistent with the overarching goals of the IPR process that extend beyond the particular parties in a given patent dispute,” Prost wrote. That includes the public interest in keeping patent monopolies within their legitimate scope, she noted.

Judges Alvin Schall and Raymond Chen concurred in Prost's decision.

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It's a win for Tensegrity Law Group's Matthew Powers, who argued the assignor estoppel issue to the Federal Circuit in February. Fish & Richardson partner Lauren Degnan argued claim construction issues for Arista. Kirkland & Ellis partner John O'Quinn argued for Cisco.

It's not clear how much impact, if any, the decision will have on the parties. The Federal Circuit remanded the case to the PTAB for reconsideration under a new claim construction. Given that Cisco and Arista reached a global settlement of their IP issues last summer, it's possible they may not litigate the case further.

Assignor estoppel has been part of American common law for more than 100 years. It's been under attack in federal court as well, though it is still recognized there. Critics say the doctrine is outmoded in a world where employers routinely compel employees to assign all of their IP to the company.

Assignor estoppel restrains employees from taking their know-how to a competitor or starting their own company, the argument goes, because the former employer can then assert patents against them without fear of reprisal.

Last year a chip company backed by 25 law professors asked the U.S. Supreme Court to rethink the doctrine, and the justices asked for the solicitor general's views before the case settled. Two Federal Circuit judges went on record last year saying they'd be open to reconsidering the doctrine if the right case came along.