Cisco CEO John Chambers took the helm of the San Jose, Calif.-based networking company in January 1995 and presided over one of the great runs in the industry, pushing the company onto the Fortune 500 list (#332, in 1997) and past a market capitalization of $100 billion (1998) to become, for a brief moment in 2000, the most valuable company in the world (at about $550 billion). Then the market’s bottom fell out in the dot-com bust.

In 2012, Chambers said that he expected to retire in two to four years, naming development and sales president Robert Lloyd, field operations SVP Chuck Robbins, and services SVP Edzard Overbeek as possible successors. For this reason, an unusual amount of anxiety and breathless anticipation accompanied Cisco’s most recent earnings report. The date, August 14, was just nine days before Chambers’ 65th birthday. Could investors expect succession news with the financial results?

It came and passed with little leadership news, of course—another debunked retirement rumor in a long line of them that stretches back well over a decade. Chambers’ retirement has been “imminent” for years, yet the test of time has shown each and every rumor to be greatly exaggerated. This one was no exception, despite news of thousands of layoffs.

“John has consistently said that the next time he talks about succession is when he announces succession,” a company spokesman tells Fortune. “This has not changed.”

Two things have changed, however. First, Chambers is now 65, the symbolic age that at many companies would mandate retirement. Second, the research firm Gordon Haskett issued a report stating that the firm still expects Chambers to announce his retirement soon, calling Cisco “a company operating without a named successor.”

The average tenure of a CEO is roughly eight years. Chambers has been in the role for almost 20. What has allowed him to buck that trend—and how much longer can he last?

‘Networking is a brutal business’

“John Chambers has had a remarkable ride by any measure,” says Charles King, principal analyst with Pund-IT.

That ride began back in 1995, when Chambers, then 46, took Cisco’s top job. At the time, the rise of the Internet had put a lusty wind in Cisco’s sails, and the company’s routers and switches soon came to be viewed as key vertebrae in the backbone of an increasingly connected world. Since March 2000, when Cisco (CSCO) briefly edged out Microsoft as the world’s most valuable company, there have been plenty of rough patches: several rounds of layoffs, failed acquisitions (remember the Flip camcorder?), and lackluster stock performance.

Cisco under Chambers has also been under considerable pressure in recent years from emerging technology trends that challenge its core businesses. One example? Software-defined networking, an approach that is considered to be cheaper and more flexible than the traditional networking equipment upon which Cisco built its name.

Cisco does have an answer for that, which it calls Insieme. (“I see SDN as something we’ll embrace and get the benefits of,” Chambers said in the company’s most recent earnings call.) Still, there has been “some public grumbling about the company being late to that game and with less innovative products than some others,” King points out, even as Cisco remains the leader in its core networking markets and performs well in newer areas like x86 servers.

“Networking as a whole is a brutal business because, unlike computing, applications haven’t changed that much—switching is switching and routing is routing—while semiconductor progress has commoditized costs and put downward pressure on prices,” says Peter Christy, a research director for networking with 451 Research. “Chambers has driven many attempts to broaden Cisco’s portfolio and grab more wallet share. Some, like server computing or IP telephony, have succeeded brilliantly. Others, like consumer efforts—e.g. the Flip camera—not so.”

Those failed bets are fodder for critics calling for Chambers to retire. Supporters, meanwhile, say he has shown resilience as well as the confidence to make bets that may not work out.

Christian Renaud, a senior analyst also with 451 Research, worked under Chambers at Cisco for more than a decade in the late 1990s and early 2000s. He credits Chambers’ tenure for a long-view perspective that short-term CEOs often miss. “He steered the company from just over a thousand people when I started working for him to over 65,000 when I left,” Renaud recalls. “Under his leadership, Cisco has successfully weathered large market transitions, and not only survived but thrived.”

Other former colleagues tell a similar tale. “Having known him for more than two decades, I am amazed at his nonstop energy and decisive courage in navigating Cisco through both its highs and lows,” says ex-Cisco exec Jayshree Ullal, who is now chief executive of Arista Networks, which competes with Cisco.

It is an inherent part of the CEO role that you get credit for the good times and blame for the bad. Take Cisco’s layoffs, for example—Chambers is equally responsible for those as he is for the company’s growth, Renaud says. “I know how much the layoffs in 2001 emotionally impacted John, as I suspect the layoffs in 2009 did as well,” he says. “This can’t be easy for him, but it is part of the cycle of every business.”

Chambers spent many years in sales during the early part of his career. It’s possible that background may be part of what has helped him endure. “Great salesmen run on a special kind of real-world optimism. You know—head in the clouds, feet on the ground,” says John Waters, editor-at-large for Application Development Trends and author of John Chambers and the Cisco Way. “Add an almost devout focus on the customer, and you’ve got a combo of personal traits that surely helped to keep him in the big chair.”

‘I expect some investors are wondering’

Chambers’ two-decade tenure is relatively unusual in the technology industry, but it’s far from unprecedented. Of those chief executives active in the role today, Activision Blizzard’s Robert Kotick, Microchip Technology’s Steve Sanghi, and Nvidia’s Jen-Hsun Huang exceed Chambers’ tenure. Concur’s Steven Singh and Amazon’s Jeff Bezos aren’t far behind.

“IT CEOs typically call the shots on their career paths until they lose the faith of their customers or shareholders,” King says. That usually happens in one of three ways: Either the CEO misses a key evolutionary shift in the industry, wastes money on unsuccessful products, or fails to deliver the returns desired by large institutional investors, he says.

In Chambers’ case, the last reason is the most pressing. “Much like Microsoft under [Steve] Ballmer, Cisco shares have for the past decade been mired in a fairly narrow trading range—between $15 and $30—and the stock has mostly failed to catch fire in major market advances, including the one currently under way,” King says. “Since the company offers only relatively small dividends—currently $0.19 per share, compared to $0.28 per share for Microsoft and $1.10 per share for IBM—I expect some Cisco investors are wondering how the company might fare under a different leader.”

Cisco has plenty of top talent in its executive suite, says Robert Bradford, president and CEO of the Center for Simplified Strategic Planning. “If the company were in a more difficult strategic situation, they might look outside,” he says. “Given their performance, I wouldn’t.”

Chambers has done all the right things to ensure a smooth succession, says Scott Saslow, founder and CEO of the Institute of Executive Development. The process so far seems “near perfect,” he says: Chambers has pre-announced his intentions years in advance; he has mentioned specific executives who are internal candidates for the job; he would leave the company on very solid financial footing. “It is rare to see all three of these conditions,” Saslow says.

Most boards of directors facing a transition aren’t as lucky as Cisco’s will be, Saslow adds. “The vast majority don’t think they have enough ready successors for the CEO position,” Saslow says. “Given that Chambers has stated that Cisco has several, they appear to be way ahead of the market.”

Cisco’s retirement rumors once worried Renaud, the 451 Research analyst. That’s no longer so. “John has done a great job in recent years of building a succession plan and strong bench in preparation for an inevitable transition,” Renaud says. “That wasn’t always the case, and I’d say his bench now is the strongest I’ve ever seen it. If he was going to retire, the company would be left in good shape.”