Brocade Communications System, dominant in an obscure corner of the data storage market, wants a piece of a bigger pie: Cisco Systems’ cash cow business of networking equipment that shuttles Internet traffic. Brocade said Monday it has agreed to pay $3 billion to acquire one of Cisco’s much-smaller competitors, Foundry Networks, to try and make that happen. The proposed acquisition would meld two companies with presence deep in the data center and pose a direct challenge to Cisco, which at $29 billion in annual sales is the world’s leader in Internet networking equipment.