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?
Best Answer
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.