4.5 Trunking
4.5.3 Removing VLANs from a trunk
By default, all VLANs are transported across a trunk link when you issue the set trunk command. However, there are instances where the trunk link should not carry all VLANs:
  • Broadcast suppression - All broadcasts are sent to every port in a VLAN. A trunk link acts as a member port of the VLAN and, therefore, must pass all the broadcasts. Bandwidth and processing time are wasted if there is no port at the other end of the trunk link that is a member of that VLAN.
  • Topology change - Changes that occur in the topology must also be propagated across the trunk link. If the VLAN is not used on the other end of the trunk link, there is no need for the overhead of a topology change.

By default, a Cisco IOS command-based switch trunk port sends to and receives traffic from all VLANs in the VLAN database. All VLANs, 1 to 1005, are allowed on each trunk. However, you can remove VLANs from the allowed list, preventing traffic from those VLANs from passing over the trunk. To restrict the traffic a trunk carries, use the remove vlan-list parameter to remove specific VLANs from the allowed list.

Switch(config-if)#switchport trunk allowed vlan remove vlan-list

The example in Figure shows first how to remove VLAN 3 from a trunk and then how to remove VLANs 6-10 from the trunk. This is verified by using the show running-config command.

In order to remove a VLAN from a trunk link on a set command-based switch, use the following command:

Switch> (enable) clear trunk mod_num/port_num vlan_range

The example in Figure shows how to remove VLANs 6-10 from the set command-based switch.

You can verify that trunking has been configured and verify the settings by using the show trunk [mod_num/port_num] command from privileged mode on the switch, as shown in Figure .

The example in Figure shows how to verify the trunk configuration on a set command-based switch. Remember that when you configure a trunk, VLANs 1 to 1000 are automatically transported, even if you specify a VLAN range. You must use the clear trunk command in order to remove the VLANs from the link.

If you wish to remove a large number of VLANs from a trunk link, it is probably easier to clear all VLANs from the trunk link before specifying the VLANs that are supposed to be on the link.