8 Replies Latest reply: Jul 1, 2019 9:16 PM by Ri0N RSS

    Vlan Native Clarication

    Sandesh

      Hi,

       

      I am finding Difficult to understand Vlan Tagged and Untagged Vlan? Can any one clarify my Doubts?

       

      In attached figure, I made Inter fa0/24 between 2 switches as Native Vlan 30. And remaining interface (ports) are in Vlan 1.

       

      My doubt is..... Can I still ping between 2 PC's even though fa0/24 is configured Native Vlan 30. If yes then what is the use of configuring fa0/24 as Vlan 30.

        • 1. Re: Vlan Native Clarication
          Juergen Ilse CCNA R&S

          Sandesh schrieb:

           

          I am finding Difficult to understand Vlan Tagged and Untagged Vlan? Can any one clarify my Doubts?

           

          In attached figure, I made Inter fa0/24 between 2 switches as Native Vlan 30. And remaining interface (ports) are in Vlan 1.

           

          My doubt is..... Can I still ping between 2 PC's even though fa0/24 is configured Native Vlan 30. If yes then what is the use of configuring fa0/24 as Vlan 30.

          If fa0/24 is a trunk (should be either a trunk, which also transports the VLAN in which fa0/1 is an access-port or an access-port in the same VLAN as fao/1), the only difference is the encapsulation of the frames in VLAN30 when sent through the trunk between the 2 switches.

          On a VLAN-trunk, all VLANs except the native VLAN will be sent with a tag added to the frames. This allows the receiving switch to determine the VLAN of a frame sent through the trunk (all untagged frames belong to the native VLAN, all other frames include the VLAN in their tags). The default fo the native VLAN is VLAN1. Changing the native VLAN on a trunk will only change the VLAN,which is sent untagged (in your case VLAN30 instead of VLAN1, while VLAN1 is sent tagged in that case). For access-ports, there is no change. So if the PCs can ping before changing the native VLAN on the trunk, they will also be able to reach each other after that change, if you do the same change on both switches.

          • 2. Re: Vlan Native Clarication
            ciscodaze1

            If you set that up in PT then you already know that yes, you can ping from PC to PC.

             

            The purpose of the Native vlan is to support untagged traffic. That is written into the dot1q specification. Its rare, but there may be a time when you have need to pass untagged traffic.

             

            This chart will help you with your studies...

             

            Capture.PNG

            • 3. Re: Vlan Native Clarication
              Sandesh

              Correct me If I am Wrong. By Default Vlan1 is Native but Once when I change Native Vlan to Vlan 30. Vlan 1 will be Tagged and Vlan 30 will be Untagged.

              • 4. Re: Vlan Native Clarication
                Juergen Ilse CCNA R&S

                Correct. Only the native VLAN will be sent  untagged. You can change the native VLAN per trunk. There is always exactly one native VLAN per trunk.

                • 5. Re: Vlan Native Clarication
                  Sandesh

                  Thanks for the Reply.

                  • 6. Re: Vlan Native Clarication
                    Juergen Ilse CCNA R&S

                    You are welcome. I hope we have clarified your doubts.

                    • 7. Re: Vlan Native Clarication
                      Ranjith

                      yes you can do. so please mus t configure thetagged ports on both vlans

                      • 8. Re: Vlan Native Clarication
                        Ri0N

                        Perhaps the easiest way to think of the native VLAN is as untagged VLAN. A switch always needs to associate a frame with a VLAN. If a frame contains an 802.1Q tag, the frame is associated with the VLAN in that tag. If a frame has no tag, it is associated with the native VLAN (untagged VLAN). Below is a simplified representation of a tagged and untagged frame:

                         

                        +++++++++++++++++++++++++++++++++++++++

                        | dst mac | src mac | VLAN TAG | data |   (TAGGED)

                        +++++++++++++++++++++++++++++++++++++++


                        ++++++++++++++++++++++++++++

                        | dst mac | src mac | data |   (UNTAGGED)

                        ++++++++++++++++++++++++++++

                         

                        So, again... Summary: If there is a tag, the frame belongs to the VLAN indicated in the tag. If there is no tag, the frame belongs to the native VLAN.

                         

                        In your scenario, the frame would arrive from PC0(1) to Switch0 port Fa0/1 as untagged and be associated with VLAN 1 (because of interface configuration). The frame would then be sent tagged out port Fa0/24. Switch1 would associate the frame with VLAN 1 based on the 802.1Q tag. Switch1 would then forward the frame out Fa0/1 to PC0 untagged. So, the ping would work.

                         

                        The native VLAN 30 actually doesn't have much to do with this - it only defines that traffic from that VLAN is sent untagged on that link. The native VLAN can actually be configured per link but for clarity it is recommended to keep the native VLAN the same across the L2 network.

                         

                        One final thing... There is the vlan dot1q tag native command, which also tags the native VLAN. Hence, no VLAN traffic would be sent without a tag.