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. Reexecuting the switchport trunk allowed vlan 100 command would again reverse the state of the VLANs allowed on the trunk. show interface trunk shows the status: 3750-IOS#sho int trunk Encapsulation Status Native vlan Gi1/0/45 802.1q trunking Vlans allowed on trunk Gi1/0/45 1-99,101-4094 Vlans allowed and active in management domain Gi1/0/45 Vlans in spanning tree forwarding state and not pruned Gi1/0/45 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: 3750-I0S(config-if)#switchport trunk allowed vlan remove 200 3750-I0S(config-if)#switchport trunk allowed vlan remove 300 Configuring Trunks The output of show interface trunk now shows that all VLANs-except 100, 200, and 300-are allowed on the trunk: 3750-IOS#sho int trunk Encapsulation Status Native vlan Gi1/0/45 802.1q trunking Vlans allowed on trunk Gi1/0/45 1-99,101-199,201-299,301-4094 Vlans allowed and active in management domain Gi1/0/45 Vlans in spanning tree forwarding state and not pruned Gi1/0/45 CatOS You configure a trunk on a CatOS switch via the set trunk command. Options for the set trunk command are as follows: Switch1-Cat0S# (enable) set trunk 3/1 ? No vlans <mode> Trunk mode off, desirable, auto, nonegotiate) <type> Trunk type (isl, dot1q, dot10, lane, negotiate) <vlan> VLAN number The mode on indicates that the port has been hardcoded to be a trunk, and the mode off indicates that the port will never be a trunk. The modes desirable and auto are both dynamic and refer to the method with which DTP will operate on the port. desir able indicates that the port will initiate negotiations and try to make the link a trunk. auto indicates that the port will listen for DTP, but will not actively attempt to become a port. You can use the mode nonegotiate t