case: 3750-I0S(config-if)#switchport mode trunk Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode. On an IOS switch capable of both ISL and 802.1Q, you must specify a trunk encapsu- lation before you can configure a port as a trunk (trunk encapsulation is an unfortunate choice for the command because, as you now know, 802.1Q does not encapsulate frames like ISL does; still, you must follow Cisco's syntax). Once you've chosen a trunking protocol, you are free to declare the port a trunk: 3750-I0S(config)#int g1/0/45 3750-I0S(config-if)#switchport trunk encapsulation dot1q 3750-I0S(config-if)#switchport mode trunk Should you wish to subsequently remove trunking from the interface, you'd use the command switchport mode access. By default, all VLANs on a switch are included in a trunk. But you may have 40 VLANs and only need to trunk 3 of them. Because broadcasts from all allowed VLANs will be sent on every trunk port, excluding unneeded VLANs can save a lot of bandwidth your trunk links. You can specify which VLANs are able to traverse a trunk with the switchport trunk allowed command. These are the options for this command: 3750-I0S(config-if)#switchport trunk allowed vlan ? VLAN IDs of the allowed VLANs when this port is in trunking mode add VLANs to the current list all VLANs except all VLANs except the following Chapter 5: Trunking no VLANs remove remove VLANs from the current list To allow only one VLAN (VLAN 100, in this case) on a trunk, use a command like this: 3750-I0S(config-if)#switchport trunk allowed vlan 100 As you can see from the output of the show interface trunk command, only VLAN 100 is now allowed. IOS has removed the others: 3750-IOS#sho int trunk Encapsulation Status Native vlan Gi1/0/45 802.1q trunking Vlans allowed on trunk Gi1/0/45 Vlans allowed and active in management domain Gi1/0/45 Vlans in spanning tree forwarding state and not pruned Gi1/0/45 If you want to allow all VLANs except VLAN 100, do it with the following command: 750-I0S(