Optical burst switching (OBS) networks can benefit from optical buffering at the core nodes. Unfortunately, optical fibre delay line (FDL) buffers require additional switch ports, thereby increasing the switch size and cost. In this paper, an alternative and relatively uncommon view is taken towards core buffering in optical networks. Switch channel ports at underutilized links are replaced by buffer ports to which shared FDL buffers are attached, keeping the switch size fixed. A novel algorithm is proposed to improve the network blocking performance by jointly optimizing the buffers placement and the traffic routes. It is found that joint optimization gives the largest blocking probability improvement and employs only about one half of the number of FDL buffers required if the traffic is simply routed over the shortest paths.