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.