A native VLAN is the VLAN associated with untagged traffic on an 802.1Q trunk. The switch assigns untagged frames received on the trunk to the native VLAN and sends frames belonging to that VLAN without tags. VLAN 1 is the default native VLAN, but the setting is configurable per trunk port. section 12.5
Understand the native VLAN through its behavior and operational consequences:
- Both ends of a trunk should use the same native VLAN. If they differ, one switch assigns untagged frames to the wrong VLAN, preventing delivery to the intended destination. section 12.5
- A commonly recommended configuration uses an unused VLAN rather than the default VLAN 1 as the native VLAN. section 12.5 section 12.4.1
- On an EtherChannel, every member port must have matching trunk settings, including the native VLAN; a mismatch can suspend the port until corrected. section 16.2.3
Changing and verifying the native VLAN
On SW1, configure G0/0 to use VLAN 30 as its native VLAN: `switchport trunk native vlan 30`. Verify the result with `show interfaces trunk`; the Native vlan column should display 30. In a router-on-a-stick design, the switch trunk can instead use an unused VLAN such as 999: `switchport trunk native vlan 999`. If a router subinterface must process native VLAN traffic, configure its encapsulation with the `native` keyword, as in `encapsulation dot1q 10 native`.
