Guido Appenzeller, who co-founded Big Switch with Kyle Forster and was its CEO up till last fall, has left the software-defined networking company, according to several sources.

Last November, Appenzeller stepped back from the CEO slot, which went to Douglas Murray, a former Juniper Networks exec, but stayed on as chief technology officer. Now he's left that gig as well, sources said.

The company's PR rep did not respond to several phone calls or emails requesting comment but Appenzeller (pictured above, in the middle) is no longer on the Big Switch management page although he remains listed as a board member.

Big Switch, founded in 2010, is one of a handful of companies that pioneered software-defined networking, technology that allows customers to add new features and functions to their network infrastructure without buying expensive, often proprietary hardware.

It secured about $45 million in venture funding including a $25 million Series B round led by Redpoint Ventures and Goldman Sachs two years ago with a $6.5 million follow-on from Intel Capital last year.

The whole SDN category basked in the reflected glow of VMware's $1.2 billion buyout of Nicira in 2012. But since then things have cooled down for the SDN startups as legacy tech players got into the game. Most notably, Cisco (s csco) pulled Insieme, its own SDN spinout, in house.

Big Switch "was betting that it was worth as much as Nicira and that it would get real traction and neither has happened," said one source close to Appenzeller. "Plus all the big big players have stepped in."