Switchport trunk native vlan question...

Answered Question
Oct 29th, 2007

What am I missing in regards to the following two lines assigned to a sw interface:

switchport trunk native vlan 80

switchport mode trunk

Why assign a VLAN to the port when your trunking it (meaning you allowing all VLANs to pass)?

Thank you.

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Correct Answer by bvsnarayana03 about 9 years 2 months ago

You'll get all your answers related to native Vlan from the link below:

http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a008013159f.shtml

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lgijssel Mon, 10/29/2007 - 10:58

The native vlan is the vlan that accepts untagged packets as well. These are packets that lack the 802.1p/Q header that contains the vlan ID.

With the above command you are setting vlan80 as the native (=untagged) vlan.

regards,

Leo

bvsnarayana03 Mon, 10/29/2007 - 11:19

By default native VLAN is VLAN 1, but can be changed to any No. on the trunk port by command "switchport trunk native vlan #". This will make a new vlan# as native & allow all pkts from this vlan to pass thru trunk untagged.

Native VLANs are used to carry CDP, PAgP & VTP messages. Thus the Frames on native VLAN are untagged. For these messages to propagate between devices, native VLANS must match on both sides of the trunk. In case of native VLAN mismatch on bothsides of the trunk, STP will put the trunk port in err-disabled state.

petersahlen Mon, 10/29/2007 - 15:53

If an native vlan is not allowed on a trunk will then untagged frames pass thru?

"Switchport trunk native vlan 80"

"Switchport trunk allowed vlan 15"

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