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Recently encountered a setup where the engineer had a Multilayer Cisco switch with a trunk carrying VLAN 41 to an HP switch that did not support VLANs.

What should the HP switch do with the 802.1q traffic when it is received?

I understand that the native VLAN which doesn't have the 802.1q tag will pass but what happens to the other VLANs on the trunk?

  • Depends on the exact nature of the "not support". If it knows what protocol 0x8100 is, it's supposed to drop the frame. But that would not be a safe bet. – Ricky Beam Nov 19 '17 at 23:33
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A switch not supporting 802.1Q tags should drop tagged frames. However, many simple switches don't comply to 802.1Q at all and they forward tagged frames just like untagged ones - for the most part compromising whatever intent the VLAN partitioning had.

A simple switch can simply overlook the TPID marking the Q tag and regard it as frame payload, just like the Ethertype field that it preceeds. The effect is that tagged frames are switched just like untagged frames. Since the switch is likely not to have the destination MAC address stored in SAT the frame is also likely to be broadcast to all ports.

You should never configure a VLAN trunk to a switch not supporting it.

  • The switch broadcasting the packet because it doesn't know the destination MAC is not the main problem. After all that's something switches do all the time, and the ultimate destinations are expected to either support 802.1Q and do the right thing or not support it and drop the packet. And once a reply to the original packet is received the switch will know both MAC addresses involved in the communication. More problematic are configurations where the same MAC would need to be routed across different interfaces depending on VLAN tag or switches that can't deal with the larger packets. – kasperd Nov 19 '17 at 16:14
  • Keep in mind the tag makes the frame longer (by 4 bytes), so it could now be larger than the expected MTU (1518 vs 1514) The switch should drop that as an oversized frame. Some very old Cisco switches will simply crash when handed a 1518B tagged frame on an untagged port. – Ricky Beam Nov 19 '17 at 23:37
  • @RickyBeam That is indeed the point of the last half sentence of my comment. Dropping the packets would be understandable behavior. But actually causing the switch to crash is really bad. It's not like you necessarily trust all the devices hooked up to your switch, so if one of them can cause the switch to crash then that is a DoS vector. – kasperd Nov 20 '17 at 0:03
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I understand that the native VLAN which doesn't have the 802.1q tag will pass but what happens to the other VLANs on the trunk?

That is actually undefined. Some switches will drop the tagged frames as malformed or giants, some switches will strip the tags, and some switches will simply pass the frames.

  • Even Cisco switches treat tagged frames diffrently in this situation depending on model. I just renewed a cert and this concept turned into a big conversation/argument. Turns out there is not one answer like Ron stated. You will need to find out from HP or start testing. Wireshark would be handy for this. – Fixitrod Nov 20 '17 at 3:50

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