Trunk tagging, and the Native vlan Untagged Question :D

erickafabianaerickafabiana Member Posts: 22 ■□□□□□□□□□
Host A Sw1 wants to communicate with Host B on Sw3, both on Vlan 30. Host A sends a frame enters Sw1 an access port that belongs to Vlan 30, Sw1 matches the destination mac address in the Cam table with a known unicast with an outgoing interface. The outgoing interface happens to be a trunk configured with Native Vlan 30, so the switch sends that untagged frame across the trunk link on native 30 to SW2. When Sw2 receives that untagged frame, he sees that the frame belongs to Native vlan 30 because his end of the trunk is set to Native vlan 30, however Sw2 has another trunk link to a different switch (Sw3)with native vlan 1 set on both sides of sw2 and sw3 but has vlan 30 allowed on the trunk link. Will that untagged "Native Vlan 30 frame" Reach Sw3 and forward it to Host B? Or will it be dropped ? If it's dropped why is it dropped ?

Host A is on Sw1


  • winsladewinslade Member Posts: 15 ■□□□□□□□□□
    From my understanding, once that frame reaches a trunk where the native vlan is not 30, that frame will then be tagged vlan 30.
  • erickafabianaerickafabiana Member Posts: 22 ■□□□□□□□□□
    winslade wrote: »
    From my understanding, once that frame reaches a trunk where the native vlan is not 30, that frame will then be tagged vlan 30.

    So your saying the Frame still should be able to traverse the trunk as long as "Vlan 30 is allowed on the Trunk". regardless of the Native vlan ?
  • winsladewinslade Member Posts: 15 ■□□□□□□□□□
    SW1 --- Trunk native vlan 30 --- SW2 --- Trunk native vlan 1 --- SW3

    Hosts will be able to communicate via SW1 to SW2 and to SW3 towards the destination, regardless of the native vlan differences.

    I am hoping someone can chime in to confirm if that frame will remain untagged or if it gets tagged into vlan 30 once SW2 forwards it towards SW3...
  • winsladewinslade Member Posts: 15 ■□□□□□□□□□
    So I had to go check and confirm for myself...

    Yes, you will have to ensure vlan 30 is allowed on the trunk between sw2 and sw3, as it does get tagged
  • dontstopdontstop Member Posts: 579 ■■■■□□□□□□
    That was a bit of a noggin scratcher! icon_scratch.gif

    SW1 <--> SW2 : Both define VLAN 30 as the native VLAN. Any frames which move between these switches will remain untagged across the trunk. Once the frame arrives at SW2, SW2 knows this frame belongs in VLAN 30.

    SW2 <--> SW3 : If the frame needs to go to SW3 the frame will be forwarded over the trunk but this time it will be tagged with VLAN 30 as it's not the native VLAN on this trunk. As winslade said as long as VLAN 30 is allowed on the trunk and both switches know about VLAN 30, this frame should traverse the trunk without issue. Host B will need to be on VLAN 30 on the remote switch.

    Think about it like this: Once the frame is at SW2 and the SW2 knows it's destined for VLAN 30, it's as if the frame had originated from a host connected to SW2 on VLAN 30 (you can now ignore SW1). It's business as usual from here: if the frame needs to be sent to SW3, it will tagged as normal as the switch knows this frame belongs to VLAN 30 and only the native VLAN on the trunk (VLAN 1) remains untagged.

    Link to Packet Tracer:
    MD5: 24a7fed76231d64cb9897d7beec7fe36
  • erickafabianaerickafabiana Member Posts: 22 ■□□□□□□□□□
    Thankyou for the clarification! that was very clear :D.
Sign In or Register to comment.