Fostered by the widespread use of information and communication applications, the emerging need for higher bit rate services, such as 100 Gb/s and 40 Gb/s, is creating an immediate impact on the design of optical core infrastructures. Under current market perspectives, the optical technology for 100 Gb/s is expected to suffer from increased transceiver costs and shorter optical transmission reaches when compared with 40 Gb/s equipment. In this context, and in order to minimize the resulting capital expenditures (CAPEX), network operators have to carefully select the most adequate combination of optical technologies when planning their backbone network. Additionally, with the deployment of optical transport network (OTN) systems, advanced traffic engineering techniques, such as virtual concatenation and grooming, can also be exploited at the electrical switching layer to improve the usage of the underlying optical capacity. Hence, in this paper we propose a novel optimization framework, based in linear programming and rounding techniques, to determine the optical technologies and the multiplexing techniques leading to the most cost-effective solution. Moreover, anticipating future market conditions, we evaluate the impact of varying the 100 Gb/s and 40 Gb/s optical interface cost and maximum optical reach on the outcome of our optimization method.