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What happens when an untagged port receives tagged frames, whether they are dropped or not !?. Say, A computer is connected to a switch on untagged port (as most would recommend). On it I have used traffic generator like ostinato to simulate tagged traffic, lets assume tagged traffic contains both configured vlans(switchport access vlan 'x') along with other random ones. How would these be handled at switchport which is in mode "access"?

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An access port that receives tagged traffic will drop the tagged frames.

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  • You mean the switch drops all tagged traffic from the Pc , irrespective of vlan id configured on switchport (even native vlan ..) ? – prudv May 19 at 6:00
  • That is correct. – Ron Trunk May 19 at 10:31
  • That's mostly correct. You should assume tagged frames on an access port will be dropped, but some systems will allow tagged frames for the native VLAN. – Ricky Beam May 19 at 13:55
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Imagine you have this configuration:

  • Port 1 trunk tagged VLAN 10
  • Port 2 access untagged VLAN 10

When the switch receives a frame on Port 1 tagged with VLAN 10 and determines that frame should be sent out Port 2, the VLAN tag is popped (removed) during the forwarding process. The device connected to Port 2 is never aware of the VLAN tag.

The reverse happens when Port 2 receives a frame (untagged) bound for a device on Port 1. The switch pushes (adds) the VLAN 10 tag onto the frame as it is forwarded to Port 1.

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  • In same case, What if port 1 and port 2 are from different switches & are connected back to back. How the port 2 treats received tagged frames sent out by port 1 towards port2. – prudv May 19 at 5:56

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