VMware Inc. executive Martin Casado, who’s currently in charge of its NSX software-defined networking business unit, is to leave the company to become a general partner at well known venture capital firm Andreessen Horowitz.

VMware’s NSX business unit was created shortly after the networking giant acquired Casado’s former startup Nicira back in 2012 for what seemed like an outrageous $1.26 billion at the time. Since taking charge, Casado has helped grow NSX into a business that’s on track to do $600 million in annual revenues. Incidentally, Andreessen was one of Nicira’s main backers before it sold itself to VMware, having led its $26 million Series C funding round in 2011.

Not only was Nicira was one of Andreessen Horowitz’s earliest bets, but it also ranks as one of its most successful, the venture capital firm’s co-founder Marc Andreessen said in a blog post announcing Casado’s move. In his post, Andreessen relates his first meeting with Casado back in 2009 when he was still a “newly minted Stanford Computer Science Ph.D., vigorously pitching us on a networking future where software would run the show.”

“He was clearly speaking our language,” Andreessen admitted.

Casado’s departure from VMware comes at an unusual time for the virtualization giant. The company has big plans for NSX, with Casado himself recently discussing how networking virtualization technology could potentially be applied to network security. Still, he’s chosen not to be a part of those plans.

Why? Well, that’s not immediately clear, though some sources speculate it may have something to do with Dell Inc.’s impending $67 billion takeover of VMware’s parent company EMC Corp. Then again, it could just be that Casado was bored and fancied a new challenge. Whatever the reason, VMware will have a tough task in looking for a replacement.

Casado previously appeared on SiliconANGLE’s TV show theCUBE during VMworld 2012 where he held an in-depth discussion of his then vision of the ‘fully virtualized data center of the future’, a vision that is today, fast becoming a reality.