How to Configure a Trunk Port on a Cisco Switch.
To configure a trunk on a switch FastEthernet port, use the switchport mode trunk command. This command puts the interface into permanent trunking mode and negotiates to convert the neighboring switch or links into trunk links.
Lets use the topology below. The VLANs 10, 20, and 30 will support the management, IT, and Sales computers, PC1, PC2, and PC3. The F0/1 port on switch S1 will be configured as a trunk port and will forward traffic for VLANs 10, 20, and 30.
The following switch output shows the trunk configuration on interface fa0/1.
SW1(config-if)#switchport mode trunk
Fa0/1 is a native VLAN which is a default VLAN and is normally used to manage switches. But converting it to trunk mode is an added security!. However, you can change or modify a native VLAN. This I said earlier is added security.
use the following command to move or assign another switch port as native VLAN:
SW1(config-if)#switchport trunk native vlan 20
The show interface fa0/1 or show running-config command reveals that the trunk has been configured or not.