t 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 Chapter 5: Trunking of bandwidth on 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: 3550-IOS (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 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: 3550-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: 3550-IOS# sho int trunk Encapsulation Status Native vlan Fao/15 802.1q trunking Vlans allowed on trunk Fa0/15 Vlans allowed and active in management domain Fa0/15 Vlans in spanning tree forwarding state and not pruned Fa0/15 If you wanted to allow all VLANs except VLAN 100, you could do it with the following command: 3550- (config-if)# switchport trunk allowed vlan except 100 This command will override the previous command specifying VLAN 100 as the only allowed VLAN, SO now all VLANs except VLAN 100 will be allowed. (Executing the switchport trunk allowed vlan 100 command again would again reverse the state of the VLANs allowed on the trunk.) show interface trunk shows the status: 3550-IOS# sho int trunk Encapsulation Status Native vlan Fao/15 802.1q trunking Vlans allowed on trunk Fa0/15 1-99,101-4094 Configuring Trunks Vlans allowed and active in management domain Fao/15 1,3-4,10 Vlans in spanning tree forwarding state and not pruned Fa0/15 1,3-4,10 VLANs 1-99 and 101-4096 are now allowed on the trunk. Let's say you want to remove VLANs 200 and 300 as well. Using the remove keyword, you can do just that: 3550-I0S(config-if)# switchport trunk al