Avaya president and CEO Lou D'Ambrosio will step down after six years in the post for medical reasons and will be replaced on an interim basis by Charles Giancarlo, formerly of Cisco Systems.

Avaya officials did not explain D'Ambrosio's health problem, but said he would stay on in an advisory role.

Avaya was acquired a year ago by private equity firms TPG Capital and Silver Lake Partners. Giancarlo left Cisco where he was chief development officer late last year to join Silver Lake as a partner. He was long considered to be a successor to John Chambers, Cisco's CEO, but Chambers extended his tenure there.

Avaya provides communications gear and services as well as applications for unified communications that bring computing and telephone functions together.

D'Ambrosio was considered an aggressive CEO willing to take on Cisco and Microsoft, both of which have an intense interest in building unified communications products, said analyst Zeus Kerravala at Yankee Group.

Kerravala said Avaya's future rests on becoming an "overlay" of unified communications applications for both Cisco and Microsoft. "Maybe Avaya should even be the middleware that bridges the two," he said.

Avaya has hired executive search firm Heidrick & Struggles to find D'Ambrosio's successor, and Giancarlo will remain full time in the CEO position until one is hired, Avaya said.