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| 2.5 |  | VLAN Trunking and Dynamic Trunking Protocol (DTP) |
| 2.5.3 |  | Removing VLANs from a trunk |

By default, all VLANs are transported across a trunk link when you issue the trunk command. However, there are instances where the trunk link should not carry all VLANs. The first is 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. VLANs may also be removed from a trunk as a result of a 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-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, but some of those VLANs can be removed from the list of allowed VLANs, preventing traffic from those VLANs from passing over the trunk. To restrict the traffic a trunk carries, use the remove vlan-list parameter with the switchport trunk allowed command.

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

The switchport trunk allowed command is used to remove VLAN 3 from a trunk and then remove VLANs 6-10 from the trunk. This is verified by using the show running-config command.

To remove a large number of VLANs from a trunk link, it is often faster to clear all VLANs from the trunk link before specifying the VLANs that are supposed to be on the link.