What: Optical network equipment maker Infinera's (NASDAQ:INFN) stock fell 10% during the month of September, according to S&P Capital IQ data. But long-term shareholders don't have much to complain about: The stock remains well ahead of the market -- up 40% in 2015 and close to a double over the past 52 weeks.

INFN PE Ratio (TTM) Chart

INFN P/E Ratio (TTM) data by YCharts

So what: Infinera didn't announce any major news itself last month, but the stock was still hit with negative sentiment from other directions. First, an analyst at Goldman Sachs downgraded shares to a sell rating while lowering his 12-month price target to $19 per share from a prior $24-per-share estimate. While the company should boost revenue at a double-digit pace through at least 2017, the analyst said, gross margin will likely shrink as Infinera broadens out its networking product portfolio. The fundamentally lower profitability will earn it a less generous price/earnings valuation for the stock going forward, he argued .

Adding to the selling pressure a few days later was a surprisingly weak quarterly report by fellow network equipment giant Finisar, which missed profit and sales estimates while forecasting a challenging nine-month period ahead for the industry. Price competition on networking components is rising, that company warned, which is putting pressure on its profitability. Stocks across the industry sold off in the trading day following FNSR's announcement.

Now what: Infinera investors shouldn't read too much into those pricing warnings, since they may not translate into weakness for Infinera's business. Yet Finisar management also said in a conference call that it has noticed some large clients opting to delay major network upgrades. If that's a sign of slowing demand growth across the industry, then Infineria wouldn't be immune to that gravitational pull.

In the meantime, shareholders can look forward to Infinera's third-quarter earnings announcement that's due out on Oct. 27. Wall Street is looking for the tech maker to post a scorching 31% jump in sales while improving its earnings to $0.17 per share from the prior year's $0.11 per share. Three months ago, CEO Tom Fallon said the company was in a "particularly favorable position" in the industry that should power rapid sales and profit growth. We'll soon learn if that positioning paid off into the third quarter.