By Rex Crum SAN FRANCISCO (MarketWatch) -- Tech stocks fell across the board Thursday, one of the sector's biggest retreats in weeks, as a disappointing revenue forecast from Cisco Systems Inc. put sellers firmly in control. Cisco (CSCO) shares, part of the Dow Jones Industrial Average, fell by $4, or more than 16%, to $20.51 as investors reacted to the networking equipment giant's second-quarter forecast. Late Wednesday, Cisco said it expects sales for the three months ending in January to rise just 3% to 5% from a year ago -- well short of the 13% growth that analysts surveyed by FactSet Research had been anticipating for year-over-year revenue. Cisco's facing a "challenging economic environment" that includes weakness in the cable market and in government spending in the U.S., Chief Executive John Chambers said. With Cisco in the lead, nearly every other major tech stock also fell into the red. Electronics contract manufacturer Jabil Circuit Inc. (JBL), which makes equipment for Cisco and other tech giants, fell $1.17 to stand at $13.95, down close to 8%. Shares of Xlinx Inc. (XLNX) and Altera Corp. (ALTR), chip-component suppliers with close ties to Cisco, both fell as much as 5%. And companies with big ties to corporate spending were also hit, as Dell Inc. (DELL) shares gave up 5% and Intel Corp. (INTC) fell as much as 2%. Losses also came from Apple Inc. (AAPL), Microsoft Corp. (MSFT), Oracle Corp. (ORCL), Hewlett-Packard Co. (HPQ) and Texas Instruments Inc. (TXN). The Nasdaq Composite Index (RIXF) took back some of its initial Cisco-led losses, but it was lately down by 37 points, or 1.4%, at 2,541. The Philadelphia Semiconductor Index (SOX) retreated more than 2%. Meanwhile, the battle for Netflix Inc.'s content-delivery business heated up, with Level 3 Communications Inc. (LVLT) saying it had reached a multi-year deal to "serve as the primary content delivery network provider" for Netflix (NFLX). Level 3's shares rallied 16 cents, or more than 15%, to $1.19. Akamai Technologies Inc. (AKAM), seen as losing out on some of its Netflix business due to the Level 3 deal, saw its shares fall $1.71, or more than 3%, to $50.17.