Microsoft Corp. (Nasdaq: MSFT) executive Kevin Johnson has been named CEO of Juniper Networks Inc. (NYSE: JNPR), taking the place of Scott Kriens as the company infuses its upper ranks with new blood.

Kriens isn't leaving Juniper, the company he's run for 12 years. He'll stick around as chairman after Johnson, 47, arrives in September to take the CEO job. (See Juniper Appoints CEO.)

News of the switch broke late yesterday. (See Report: Juniper Finds... a CEO?) Separately, Johnson apparently sent a goodbye email to his 14,000 employees at Microsoft without specifying where he was headed.

Some observers have wondered if Juniper needs a new CEO for its next phase of life. Juniper ascended quickly on the strength of its technology, but one theory has been that the company needs more than that to combat rival Cisco Systems Inc. (Nasdaq: CSCO), particularly in the enterprise. (See Feature Story: Juniper's Enterprise Vision and Scott Kriens, CEO, Juniper Networks.)

Johnson is certainly experienced at running large, high-stakes operations. As president of the Platforms and Services division, he ran strategy for Windows and for Microsoft's online services, a position that put him in battle with Google (Nasdaq: GOOG). He was also responsible for Microsoft's attempts to acquire Yahoo Inc. (Nasdaq: YHOO).

Johnson's appointment continues a quick string of executive hires at Juniper. Just this month, the company brought in Luis Avila-Marco from Cisco to be its chief strategist, eyeing mergers and acquisitions. It's also hired John Morris, a biometrics industry executive, to run worldwide field operations. (See Juniper Nabs Cisco Strategist .)

More details about the CEO transition could emerge this afternoon, when Juniper is scheduled to announce quarterly earnings.

Juniper stock was down 56 cents (2.5%) at $22.33 in midday trading.

— Craig Matsumoto, West Coast Editor, Light Reading