Cisco Systems Inc's quarterly revenue and outlook missed Wall Street expectations and Chief Executive John Chambers cited unusual uncertainty in the economy, sending its shares plunging 8 percent.

The results from the world's biggest network equipment maker disappointed investors who had thought that growing Internet traffic would have spurred stronger sales of routers and switches, even amid concerns about the economic recovery.

We are seeing a large number of mixed signals in both the market and from our customers' expectations, and we think the words 'unusual uncertainty' are an accurate description of what is occurring, Chambers said on a conference call.

The Federal Reserve's comments yesterday that the pace and output of the recovery has slowed in recent months, and that the recovery is likely to be more modest in the near term than had been anticipated just a few months ago, are comments that most of our large customers that I have talked with recently would agree with.

Cisco forecast its current-quarter revenue would grow 18 percent to 20 percent from a year earlier, while the average analyst estimate had been for 21 percent growth to $10.95 billion.

Revenue in the quarter ended July 31 rose 27 percent from a year earlier to $10.8 billion, Cisco said on Wednesday. That was also below the average analyst forecast of $10.9 billion, according to Thomson Reuters I/B/E/S. (For a graphic on Cisco earnings, click

Cisco is one of the technology sector's prime bellwethers due to its broad, global operations. Since Cisco's latest results are for the full month of July, instead of June for many of its peers, they are also seen as an early indicator of industry trends.

In addition to selling network equipment, the company has been expanding into new areas such as data center servers, and acquiring companies like Norwegian videoconference company Tandberg, to bolster its product line and maintain double-digit sales growth.

While Cisco's earnings per share came in at a penny above expectations, investors have been used to a bigger beat from the company, of 3 cents to 5 cents in recent quarters.

Cisco's gross margin fell to 64.1 percent from 65.2 percent in the previous quarter, showing a slight erosion in profitability as it enters more competitive markets like consumer electronics and data center servers.

Cisco also faces tough competition from Juniper Networks Inc , Alcatel-Lucent SA and Huawei Technologies Co Ltd .

It said net profit rose to $1.9 billion, or 33 cents a share, from $1.1 billion, or 19 cents a share, a year earlier. Excluding special items, its profit rose to 43 cents a share, a penny above Wall Street's forecast.

The stock fell 8 percent to $21.82 in extended trading. Before the results, Cisco shares had fallen around 6 percent so far this year, due to worries that slower growth in Europe and China could hurt a nascent recovery in technology spending.

(Reporting by Ritsuko Ando; Editing by Richard Chang)