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Regarding the Native VLAN role (example, VLAN 1 by default for Cisco), we often can read that:

  1. The untagged frames arriving on a trunk port, are "put" in the Native VLAN
  2. The untagged frames arriving on a trunk port, are sent "untagged" through the trunk

So the question is, is untagged traffic really put in the native VLAN with the Native VLAN 802.1Q tag, example VLAN 1, or, do the switches, before sending or receiving a frame in a trunk, only "consider" the untagged frame as VLAN 1 traffic, without any tagging?

  • Related Question and Answer – Eddie Feb 7 '18 at 17:19
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Apr 1 '18 at 20:27
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The native vlan just means the vlan is untagged on the trunk port. The vlan can be untagged on one trunk but tagged on another. In other words, “native” is in reference to a particular trunk port. Vlans are tagged only on trunk ports.

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I'm not sure I understand your question. If you are asking about the vlan dot1q tag native command, then only tagged frames are used on a trunk. Untagged frames received on the trunk will be dropped, and all frames sent on the trunk will be tagged (this does not include link-local protocols, e.g. STP, CDP, DTP, etc.).

Usage Guidelines

Typically, you configure 802.1Q trunks with a native VLAN ID, which strips tagging from all packets on that VLAN.

To maintain the tagging on the native VLAN and drop untagged traffic, use the vlan dot1q tag native command. The switch will tag the traffic received on the native VLAN and admit only 802.1Q-tagged frames, dropping any untagged traffic, including untagged traffic in the native VLAN.

Control traffic continues to be accepted as untagged on the native VLAN on a trunked port, even when the vlan dot1q tag native command is enabled.

Without the command, any untagged frames received on a trunk are placed in the native VLAN, and any frames sent on the native VLAN are untagged.

  • Thanks. Ok regarding the vlan dot1q tag native. But without this command, you said "any untagged frames received on a trunk are placed in the native VLAN". My question is, technically speaking, what does "placed" mean in your phrase ? For me, a frame "placed" in a VLAN is a frame tagged on a specific VLAN, but regarding the native VLAN the frames are untagged, so "placed" doesn't refer to the action to add a 802.1q tag. As far I understand, the untagged frames "placed" in the native VLAN is in fact, not changed (no tag), but just "considered" by the swiches to be in native VLAN ? Is this true? – bdes31 Feb 7 '18 at 22:32
  • That means that a frame in the native VLAN will only be sent to an interface with the native VLAN. Basically, VLANs break the switch up into multiple virtual switches, and frames for a VLAN can only be sent to interfaces with that VLAN. If it is an access interface, the frames are sent out the interface without a tag, regardless if they were tagged on a trunk. If the interface is a trunk, all frames on the trunk are tagged, except frames in the native VLAN (assuming the command to tag them has not been used). – Ron Maupin Feb 7 '18 at 22:37
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if you configure native vlan as 'x' on a trunk port, packet coming untagged to that interface will be tagged with vlan x and will be treated as normal tagged frame. if packets destined to a MAC learned through that port comes to switch with vlan id 'x', the switch will remove vlan tag and forward the packet to destination.

in a nutshell, it works as what a access port for vlan 'x' do for untagged frames.

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