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What happens when a port is configured as ACCESS and is a part of VLAN 'X' and we get a packet of VLAN 'Y' on that port? Will the switch drop it or it will add a VLAN header 'X' and forward to the respective port/ports (Double VLAN tagging).

Suppose I have the a switch stack (HP/3COM A5120) with a port range configured as

  • Untagged: 1
  • Tagged: None
  • PVID: 1

connected to NICs on vSwitch0 has a local network set as VLAN 0. Traffic from that switch ends up on VLAN 1 inside the physical network and is routed correctly.

What will happen if I change the vSwitch VLAN ID to 1? Will the frames be dropped? Will they be double tagged? Will the tagging be stripped and the frames end up on VLAN 1?

I want to add another VM Port Group (VM network) to that vSwitch & a corresponding VLAN on the HP switch ports, but I'm wondering if I should explicitly assign the VLAN on the existing vSwitch port group first and whether doing so will break things.

  • I could tell you what this would look like on a Cisco (traffic coming into an access port will be checked for vlan tags, if a tag is present it must match the access vlan or be dropped - all allowed packets will end up with a single vlan tag matching the access port's vlan number), but not sure what this switch will do. – Shane Madden Dec 5 '13 at 3:23
  • @ShaneMadden the product doc suggests likewise, but I haven't been able to replicate this. I may have to attach wireshark to a trunk port and see what's happening (though QinQ is supposed to be disabled by default). – Andrew Dec 5 '13 at 6:48
  • Yeah, letting a tag through an access port and keeping it creates an opportunity for vlan hopping, so hopefully that's not the case. – Shane Madden Dec 5 '13 at 6:52

I found the following from the product manuals:

enter image description here

This doesn't seem to match the behaviour I observed (frames were being dropped), so I've ended up using a trunk port instead and making sure the VLAN I'm using on my vSwitch is permitted on the trunk port.

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