VLAN tagging

[rhondaallen]

Inorder to have proper VLAN tagging on all VLANS, do you need to have "vlan dot1q tag native" configured if you're only trunking and not tunneling, or will the command "switchport tag native vlan #" tag all traffic including native vlan. Or do you need to use both when trunking?

[scottmac]

You don't have to explicitly proclaim a native VLAN, it will default to VLAN 1.

The tagging for the Native VLAN is no tagging at all (native VLAN frames are untagged).

You may need to declare which trunking protocol to use on some switches (1900 and 2900s) which (IIRC) default to ISL. Newer IOS and newer switches default to dot1q.

FWIW

Scott

[milan.kulik]

Hi,

" The vlan dot1q tag native command is a global command that configures the switch to tag native VLAN traffic, and admit only 802.1Q tagged frames on 802.1Q trunks, dropping any untagged traffic, including untagged traffic in the native VLAN."

See http://www.cisco.com/en/US/customer/products/hw/switches/ps708/products_configuration_guide_chapter09186a00800eac34.html#69757

Switchport tag native vlan tag

has the same effect, see

http://www.cisco.com/en/US/customer/products/hw/switches/ps4324/products_command_reference_chapter09186a008023403c.html#wp1022676

My understanding is following:

1) According to 802.1q specification there is a native VLAN option on 802.1q trunk. I.e. there may be a native VLAN configured on the trunk. Native VLAN frames don't have to be tagged. But the switch has to accept both tagged and untagged frames for native VLAN then.

2) Cisco 802.1q implementation had adopted native VLAN option as a default, i.e. there was always an untagged native VLAN (VLAN 1 by default) on Cisco 802.1q trunks. (But still the switch has to accept tagged frames in native VLAN to be 802.1q compatible, used sometimes for VoIP, e.g.)

3) To be compatible with other vendors who didn't adopted native VLAN option, Cisco added the "vlan dot1q tag native" option to IOS finally.

With this option configured, all frames including native VLAN are tagged on the 802.1q Cisco trunk.

So if you are running pure Cisco network configured only for trunking, you can leave the native VLAN untagged and everything works fine.

I'd even recommend this solution, because not all Cisco switches support "vlan dot1q tag native" option.

Regards,

Milan