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In SDN architecture, the control channel between the control and data planes is extremely significant, which has two connection modes: out-of-band mode and in-band mode. For in-band mode, the key to the establishment of SDN control channel is how to forward in-band control traffic in the data plane (control traffic: traffic between controllers and switches). In almost all the existing implementation of the in-band SDN networks, the forwarding of in-band control traffic in the data plane depends on distributed Layer 2 switching that cannot respond to the network state change immediately. In this article, we propose a Centralized Trust-based In-Band (CTIB) control mechanism for SDN in-band control channel. Then based on CTIB control mechanism, we design CTIB routing algorithm to select the most trusted control path for each data plane switch in order to improve the QoS (Quality of Service) of in-band control traffic. Furthermore, we implement CTIB control mechanism in an in-band SDN network and carry out several experiments to test the QoS of the in-band control traffic. The results indicate that there is a huge improvement of in-band control traffic QoS when using CTIB control mechanism compared to the distributed Layer 2 switching.