Yes, in order to form an trunk, the link has to be PTP(point-to-point), from one L2 interface, ex. NIC, to one and one only L2 interface, not L1 or hub. Point-to-Point links also specify full-duplex, which simply means the two ends are able to transmit and receive at the same time. An L2 interface is present on PC's, swithches, routers, etc.. Therefore, when connecting an switch to an router for router-on-a-stick, you're connecting their L2 interfaces, which must be both PTP, and therefore, full-duplex as well. You can also form trunks from switches to PC's/servers with extra configuration required on the PC/server.