In Cisco Catalyst switches, each switchport can be set to different VLAN Membership modes using the "switchport mode" command. This sets the port unconditionally.
The different modes include
Tag: trunk
How to setup 802.1q Trunk Port in Cisco Switches (IOS)
Trunk Ports are switchports on Cisco switches that interconnect switches. The trunk port carries traffic for all the VLans across the switches it interconnects. switchport can be enabled and configured as trunks without much trouble.
The following procedure helps setting a switchport into a Trunk port with 802.1q encapsulation.