One option to expand LTE downlink capacity is LTE-WiFi aggregation (LWA) which uses the carrier WiFi APs to deliver traffic tunneled from the LTE eNBs. LWA requires careful splitting of the traffic between LTE and WiFi resources to ensure good load balance among the colocated eNBs and APs. Currently, the eNB decides on how to split the traffic. However, due to its local knowledge, the split may not be optimal. Instead, we take benefits of Software-Defined Networking (SDN) by decoupling the control and data forwarding plane, where a centralized entity, namely traffic split function controller (TSFC), can allocate resources more efficiently using its broader knowledge. We propose that a dumb datapath element (the TSF) splits flows/packets between ports, i.e., eNB and AP(s), as defined by a remote control process, i.e. the TSFC. However, it is not clear where the TSFC should be located in the network hierarchy, e.g., at each eNB or deep in the core network or even in the cloud, due to several opposing forces. Centralization at the higher layers brings more global knowledge at the expense of increased delay and control messages. Our paper models and analyzes how the location of the TSFC affects the LWA performance.