Cisco Systems has boosted its stock buyback effort by $10 billion, a sign the Internet networking supplier still believes its shares are undervalued. The move, announced Friday, is a way for the San Jose-based company to increase the value of remaining investors" stakes, counter the effect of employees exercising stock options and protect its financial image. Cisco"s decision highlights the dilemma many companies face in trying to reduce the dilution caused by generous employee stock option plans. The company has bought back 2.3 billion shares for a total of $46.2 billion.