Native VLAN is a dot1Q concept that was created for backward compatibility with old devices that don’t support VLANs. Full story detailed below..
How Native VLAN works?
- Frames belonging to the native VLAN are not tagged when sent out on the trunk links so older devices can simply understand.
- Frames received untagged on the trunk links are set to the native VLAN.
Note: this default behavior can be changed by configuring the switch to tag native VLAN frames using the global command vlan dot1q tag native
In the below diagram two switches are connected with a trunk link; each switch has a workstation connected and assigned to VLAN 2. VLAN 2 is set to
be the native VLAN for the trunk link.
- workstation A transmits packets to workstation B.
- switch A finds that workstation A belongs to the native vlan (vlan2) so frames will be sent untagged.
- Switch B receives untaged frames on the trunk port; it will assume these frames belong to the native VLAN (vlan2).
- workstation B receives packets from workstation A.
To set the native VLAN on any trunk port use the interface command switchport trunk native vlan ?
you can check the native vlan configured on a trunk port using the command show interface trunk as shown below
Rack1SW1#sh interface trunk Port Mode Encapsulation Status Native vlan Po12 on 802.1q trunking 1
or using the command show interface switchport
Name: Fa0/15 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk (member of bundle Po12) Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: on Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Administrative Native VLAN tagging: enabled Voice VLAN: none ----output omitted---