To configure a trunk on a switch port, use the switchport mode trunk command. When you enter trunk mode, the interface changes to permanent trunking mode, and the port enters into a dynamic trunk protocol (DTP) negotiation to convert the link into a trunk link even if the interface connecting to it does not agree to the change.
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 figure above displays the configuration of switch port F0/1 on switch S1.
Configuration example:
S1#config t
S1(config)#interface fa0/1
S1(config-if)#switchport mode trunk
S1(config-if)#end
Note:
The show interfaces fa0/1 switchport command reveals that the trunk has been configured or not.
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