Two years ago I worked with Cisco Systems, Inc. on a contract basis helping build the program for the company’s annual CIO Summit. The event brought together about 100 CIOs from some of the world’s largest companies for two days of strategizing and feedback with Cisco executives, including CEO John Chambers.

I’ve attended a lot of vendor-sponsored events over the years, but never one quite like this. Typically, the CEO of the host company parachutes in for a speech and maybe a meal and then quickly exits, followed by an entourage of assistants and security personnel.

Not Chambers. The CEO of one of the world’s largest tech companies spent every minute of those two days in the ballroom and break rooms talking one-on-one with the CIOs. Not once did I see him with a cell phone to his ear nor disappear into a closed-door meeting.

At one point during a break I was having difficulty retrieving an item from the hors d’oeuvre table while balancing a notebook and a soft drink. A hand reached out from behind me and a soft voice with a trace of a southern accent said, “Here, let me help you with that.” I turned around to see that my assistant was John Chambers. Later that day, he shared his private cell phone number with the entire audience and invited the CIOs to call him at any time, day or night.

From my own experience, as well as those related to me by colleagues who know him, those stories are vintage Chambers. An unassuming Midwesterner who achieved success due to smarts and hard work, he has guided Cisco for two decades through a remarkable period of growth in a market that has demanded nearly constant correction. Now he is stepping down after 20 years at the helm, handing the reins to Chuck Robbins, a veteran sales executive who is considered a bit of a surprise choice as a successor. Chambers will be a tough act to follow.

Chambers has arguably navigated through a greater number of fundamental business challenges than any other high-tech executive with his kind of tenure. It seems as if market forces were constantly threatening to do Cisco in.

In the 1990s, Cisco fed the insatiable demand for growth by becoming better than anyone at assimilating acquisitions. A list of its nearly 200 purchases is here. When you consider that most acquisitions don’t work out very well, its success is remarkable. Cisco briefly became the world’s most valuable company before the Internet bubble burst. It then endured painful downsizing during the post-bubble years but never took its eye off the ball, continuing to invest in custom ASIC development in the belief that managing network growth would be a significant long-term challenge for enterprise customers. It was on the mark with that strategy.

In 2009 it launched a data center server initiative called the Unified Computing System and quickly became a major player in a market that few people thought it could even enter.

Two years ago CIOs told Cisco in no uncertain terms that it needed to up its security game. It has responded with acquisitions of companies like ThreatGRID, Inc., Neohapsis, Inc. and Sourcefire, Inc. while making significant investments in growing its own security-focused technology workforce. Chambers told CIOs last year that Cisco intends to be the number one security vendor, and history has shown that the company rarely fails when going after big goals like that.

There are been missteps along the way as well. Cisco made an ill-advised foray into consumer networking products and even digital cameras several years ago and had to take some painful charges to get out. Chambers himself has been a big advocate of telepresence, but Cisco’s products are widely considered to be too expensive to make that concept mainstream. The company has also been a big advocate of workforce collaboration, but its products have never measured up. It also endures continual complaints about its prices.

But the bottom line is that the company has grown and prospered in a business that has claimed scores of its competitors. Its newest challenge may be software-defined-networking, which threatens to turn a traditionally high-margin market into a commodity. Under Chambers, Cisco has had a knack for seeing around corners and targeting growth markets before others see them. In this respect, its vision of what it calls the Internet of Everything may be the next big idea.

John Chambers won’t be around to find out, at least not in an operational role. That task will fall to Chuck Robbins, a veteran sales executive who was a bit of a surprise choice as Chambers’ successor. Like Steve Ballmer succeeding Bill Gates in 2000, he succeeds a legendary figure during a time of tumultuous change. I hope he has the good sense to take advantage of whatever wisdom his predecessor can offer.