Paul Singer's Elliott Management Corp. had been pressing Riverbed to sell itself due to a slowdown in its main wide-area network optimization business and its struggle to integrate Opnet, a maker of software to manage traffic on networks that it bought in 2012 for about $1 billion.

The offer of $21 per share in cash is a premium of 12 percent to Riverbed's closing price on Friday. Riverbed's stock was trading at $20.41 in early trading on Monday.

Riverbed, whose products boost data speeds on networks, said in October it would explore strategic alternatives.

The company earlier rejected Elliott's offer of $21 per share as well as its initial offer of $19.

Both offers had included a "go shop" provision, allowing Riverbed's board to solicit competing bids.

The private equity firm, which held 9.65 percent of Riverbed as of Nov. 14, said it supported the deal with Ontario Teachers and Thoma Bravo.

Frank Quattrone's Qatalyst Partners and Goldman Sachs & Co. advised Riverbed.

Up to Friday's close, Riverbed's stock had risen 24 percent since Elliott disclosed a stake in the company in November last year.