On a dot1q trunk, the switch can send untagged frames in the native vlan and tagged frames in the other vlans.
Both end switches know the native vlan id, but firstly, the receiving switch must determine which frame type(tagged or untagged) the frame is.
The peer switch how to determine that the received framed is tagged or untagged? There are not any bits in the frame header in either frame format(ethernet or dot1q format) indicating that "I" am untagged or tagged.
In the other word, after a frame is received , how the receiving switch make certain that the two bytes after the "source mac address" in the frame is a "TPID" field (dot1q tag) but not a "Type/Length" field (untaged Ethernet frame ), or vice versa.
