Configuring allowed VLANs
By default, all VLANs are allowed across a trunk link on a Cisco switch. We can verify that using the show interfaces trunk command:
In the output above you can see that all VLANs (1 through 1005) are allowed on the trunk by default.
We can prevent traffic from certain VLANs from traversing a trunk link using the following interface mode command:
For example, to prevent traffic from VLAN 5 to traverse the trunk link, we would use the following command:
The same command needs to be entered on the switch on the other end of the link.
To verify that the traffic from VLAN 5 will indeed be blocked from traversing a trunked link, we can enter the show interfaces trunk command again:
Notice how now only VLANs 1-4 and 6-1005 are allowed on trunk.
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