Juniper Networks (JNPR) warned on Monday it expects to post fourth-quarter earnings below Wall Street’s expectations as the telecom equipment maker suffers from weaker-than-expected router demand from U.S. service providers.

The gloomy news from Sunnyvale, Calif.-based Juniper sent the company’s stock tumbling more than 6% in after-hours trading, threatening to wipe out all of the day’s big gains.

Juniper said it expects to earn between 26 cents and 28 cents a share on a non-GAAP basis, compared with projections to earn between 32 cents and 36 cents a share previously. Analysts had been calling for more robust EPS of 34 cents.

Likewise, the company is now projecting revenue of $1.11 billion to $1.12 billion, which would miss the Street’s view of $1.19 billion.

Juniper blamed the results on “weaker-than-expected router demand from service providers” and pointed specifically to U.S. service providers.

"2011 was a record year of revenue for Juniper, even though our fourth quarter revenue was weaker than expected due to service provider demand," Juniper CEO Kevin Johnson said in a statement. “We remain committed to our strategy of innovation in high-performance networking as we continue to manage costs and drive excellence in execution."

Juniper is scheduled to release its full fourth-quarter results on January 26.

Shares of Juniper dropped as much as 6% in extended trading but they were recently off just 3.81% to $20.77. During regular trading on Monday, Juniper soared 5.38% to $21.53.

The news comes as aluminum maker Alcoa (AA) kicked off earnings season after Monday’s closing bell by posting an in-line quarterly loss due to slumping metals prices.

Follow Matt Egan on Twitter @MattMEgan5