This example shows how to configure the FastEthernet port on an 1841 router as a trunking interface. The Cisco 1841 router has two onboard FastEthernet autosensing interfaces that support 10- or 100-Mbps connections.
The sub interface becomes fa 0/x.. For instance, fa 0/1.2 represents the sub-interface for VLAN 2. In addition, it is necessary to define the encapsulation as encapsulation dot1qvlan-id.
Hostname(config)#interface FastEthernet0/1 Hostname(config-if)#no ip address