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My question is, if I have a Cisco switch with a particular port kept in default VLAN 1 and the native VLAN of the trunk port is changed to 2. When a frame arrives at the trunk port from that port it will be an untagged frame, right? If yes, then the trunk port will tag this untagged frame with its own native VLAN (VLAN 2) before sending it over to other end of trunk, right?

  • 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 can provide your own answer and accept it. – Ron Maupin Aug 6 '17 at 23:22
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VLAN tags are placed on frames leaving a trunk port except for frames in the native VLAN for that port.

Access ports, despite being a port for a particular VLAN, do not tag frames, and, depending on the switch, a received tagged frame may be dropped on an access port.

A frame coming into an access port will not have a tag. It will be tagged as it leaves the switch on a trunk port (assuming it is not in the native VLAN for that port), but it will not be tagged if it leaves the switch on another access port.

  • Hi Ron,so this means VLAN tagging occurs only at trunk ports.Now if I have only a single switch with multiple VLANs but no trunk port as I am not connecting any other switch or router,then in this case how will traffic separation between VLANs on same switch take place as there is no VLAN tagging in frame to carry VLAN information? – Sreejit Menon Oct 16 '15 at 6:00
  • VLANS create virtual switches inside the switch. The ports on the switch are configured for the VLAN the port serves. Single switch, multiple VLANs, no trunks: Frames come into the ports with no tags, and are either forwarded to the correct port in the same VLAN as the port on which it arrived, or it ends up flooded to all the ports in that same VLAN, except the port on which it arrived. Tags are applied only on trunk frames. Most access devices don't understand tagged frames. – Ron Maupin Oct 16 '15 at 13:56
  • The exception (although, not actually an exception) to the no tags on access ports is the case of an access port configured for both VoIP and data when a VoIP phone is connected to it. The link discovery protocol (CDP or LLDP) will discover the VoIP phone and negotiate a trunk with the VoIP phone acting as a switch. It's not really an exception since the port ends up being a trunk port despite being configured as an access port. – Ron Maupin Oct 16 '15 at 14:06
  • yes...exactly...so in case Single switch, multiple VLANs, no trunks:frames dont carry any VLAN information....The traffic separation between different VLAN s is solely done by switch intelligence .Its only when I have Trunk ports and multple connected switches that I require frames to actually carry VLAN information so that VLAN tagging can be done at trunk port.Hope I am right ..... – Sreejit Menon Oct 18 '15 at 11:10
  • @SreejitMenon, based on your last comment, you appear to understand how and where tags are applied. – Ron Maupin Oct 18 '15 at 17:03
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No.

The native VLAN on a trunk means that:
A) Any untagged frame arriving on the trunk will be treated as being in the native VLAN (regardless of what VLAN it was in on the sending switch).
B) Frames on the native VLAN will cross the trunk untagged.

In your example a frame arriving on the default VLAN (VLAN 1) will be forwarded out the trunk port on VLAN 1. Since VLAN 2 is the native VLAN that means the frame on VLAN 1 will be tagged.

The only way a layer-2 switch will ever move a frame from one VLAN to another is if something is misconfigured. You could misconfigure it intentionally but in my opinion it would still be wrong.

  • oh yes....I got it now....But even when i configure either ends of trunk link with different VLAN(intentional mismatch) ,it is seen that trunk port gets blocked with message "Mismatch found in local pvid"....still I got my question cleared...thanks a ton – Sreejit Menon Oct 18 '15 at 11:03
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The switch receives it and since there is no tagging will threat it as if it was on VLAN 2 (in your example). So if it receives a broadcast on this port, it will forward it out of all ports configured with VLAN 2 (still untagged doh).

  • So acc. to this ,all frames from VLAN 1 will be treated by untagged ? – Sreejit Menon Oct 18 '15 at 11:38
  • no, all untagged frames will be treated as if they were from VLAN2 (in your example with native vlan 2) – Bulki Oct 21 '15 at 13:47

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