My understanding is that if native VLAN on a trunk is 1, CDP will send its frame untageed. However, if native VLAN on a trunk is not 1, CDP will always send its frame tagged with VLAN 1 tag. To make sure, I did test on the Packet Tracer 7.1 and the lab was successful.
Recently I had a chance to ask some questions to the CCIE Routing&Switching holder. He said, "Since DTP and CDP packets do not get forwarded, there is no need to tag VLAN info. However, VTP packets do get forwarded and loop might be occurred. That's why VTP is always forwarded on trunks with a VLAN 1 tag unless the native VLAN on a trunk is 1." He indeed showed me that CDP always sends its frame untagged even if the native VLAN is changed to something else than 1 with EVE simulator.
So my questions are:
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If you can understand him, could you please explain to me why loop might occur if VTP packets get forwarded untagged?
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If you have real switches or VIRL, could you please test whether CDP sends its frame tagged with VLAN 1 tag or untagged when native VLAN on a trunk is not 1? I really love to test myself, but I don't have real switches and my computer is way behind from VIRL hardware requirements.
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