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Native VLAN

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.

switches

Basic operation:

  • 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---

3 comments

  1. Wael,

    As salaam, there is no diagram of the switches above.. can you please update the same

    Jazakallah,
    Khan

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