Cisco Systems Inc. agreed to buy Starent Networks Corp. The deal is valued at $2.9 billion, its second multibillion- dollar acquisition in less than two weeks. Cisco will pay $35 a share in cash and assume outstanding equity awards, according to a statement today. The per-share price is 21 percent more than Starent’s closing price yesterday. Cisco, based in San Jose, California, expects the transaction to add to earnings by fiscal 2012.