i need help understanding native vlan

[dolanduck.]

I am really confused on what native vlan is. what does native vlan do? what is it used for? what does switch-port trunk native vlan do ? 

[Daniele Giordano]

Hi, if you have a port configured in trunk mode and this port receives a not tagged frame, this frame is transmitted on the native vlan.
All ports in trunk mode have the native vlan 1 as default.

This is an example of use of native vlan.

Suppose to have an IP phone whit 2 ethernet ports, uplink and PC.

The phone is configured to tag its frame on vlan 40 but the PC must be connected in the vlan 100 in access mode because the PC is not able to tag its frame.

The switch will be configured in this way:

switchport mode trunk   #the port can handle tagged frames

switchport trunk allaowed vlan 40,100   #the port can handle tagged frames with id 40 and 100

switchport trunk native vlan 100    #the frame received by PC not tagged will be "internally tagged by the switch on vlan 100

Regards.

[dolanduck.]

oh OK so by doing switchport trunk native vlan 100 vlan 100 is being tagged?

[Daniele Giordano]

Based on the example, if a PC sends a not tagged frame, the switch will add the tagged 100 internally to itself.

All untagged frames received by a tagged port are forwarded into the native vlan.

Per default all trunk ports have the native vlan 1.

Regards. 

[dolanduck.]

i think i understand. whats the point of tagging them anyways if the packets go to vlan 1 will the device not receive its content?

[Jon Marshall · Accepted solution]

By default vlan 1 is untagged. 

If you use the "switchport trunk native vlan 100" command then vlan 1 now becomes tagged and vlan 100 is untagged. 

The point of the native vlan was primarily to support connections to network devices that did not understand tagging but there are not many of them around in your average network these days. 

Jon

[dolanduck.]

dude thank you. i now understand. hey wanna be friends