Native VLAN question

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.