The switchport mode trunk command is used on the Cisco Catalyst 2950 switch to configure a port as a VLAN trunk port. The syntax of the command is as follows:
Switch(config-if)# switchport mode trunk
Trunk links are required between devices in any situation where traffic from multiple VLANs will traverse the link. This is also true when using VTP on the switches and in that case, even if inter-VLAN routing is not required. For example, if two switches in a VTP domain are connected together via an access link with no router present, then when you create a new VLAN on one of the switches, it will NOT be learned by the other switch.

When you configure a trunk link, there are two choices for encapsulation: 802.1q, which is the industry standard, and ISL, which is Cisco proprietary and will only work when both ends are Cisco equipment. Both protocols perform a crucial role in inter-VLAN routing by tagging packets with the VLAN to which the packets belong.

The following commands should be issued to configure FastEthernet 0/1 to function as a VLAN trunk port and use 802.1q encapsulation:
Switch# configure terminal Switch(config)# interface fastethernet 0/1 Switch(config-if)# switchport mode trunk Switch(config-if)# switchport trunk encapsulation dot1q
When configuring a trunk link between a switch and switch, the above commands would be used in both switches. However, when a trunk link is configured between a router and a switch, the process is different on the router. On the router end, you must do the following:
1. Enable the physical interface hosting the trunk link.
2. Ensure that no IP address exists on the physical interface.
3. Create a subinterface for each VLAN on the physical interface.
4. Set the trunking protocol on each subinterface.
5. Configure an IP address on each subinterface.

The command set that would create a subinterface for VLAN 10, set the trunking protocol for the subinterface, and assign the subinterface an IP address is:
Router(config)#interface fastethernet 0/0 Router(config)#no ip address Router(config-if)#no shutdown Router(config)-if)exit Router(config)#interface fastethernet 0/0.1 Router(config-if)#encapsulation dot1q 10 Router(config-if)#ip address 192.168.5.1 255.255.255.0
The set trunk on, switchport trunk on , and trunk mode on commands are incorrect because these are not valid Cisco IOS commands.