Activist investment firm Elliott Management Corp. boosted its offer to buy Riverbed Technology Inc. by 11 percent to $21 a share, or $3.36 billion, after the network-equipment maker rejected an earlier bid as too low.

New York-based Elliott and its affiliates own about 10.5 percent of Riverbed, making it one of the company’s largest shareholders. The investor sent a letter to Riverbed’s board today, saying the new bid represents “undeniable premium value” above the company’s share price before any offers.

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