The Q-in-Q frame contains the modified tag protocol identifier (TPID) value of VLAN Tags. IEEE 802.1Q VLAN tag uses the TPID field to identify the protocol type, the Default TPID value in IEEE 802.1Q, is 0x8100.
The device determines whether a received frame carries a service provider VLAN tag (S-VLAN Tag) or a customer VLAN tag (C-VLAN Tag) by checking the TPID value. After receiving a frame, the device compares the configured TPID value with the value of the TPID field in the frame. If the two match, the frame carries the corresponding VLAN tag.
For example, if a frame carries VLAN tags with the TPID value of 0x9100 and 0x8100, respectively, while the configured TPID value of the S-tag is 0x9100 and that of the VLAN tag for a customer network is 0x8200, the device considers that the frame carries only the Service Provider VLAN tag (S-VLAN tag) but not the customer VLAN tag (C-VLAN tag).
In addition, the systems of different vendors might set the TPID of the outer VLAN tag of Q-in-Q frames to different values. For compatibility with these systems, you can modify the TPID value so that the Q-in-Q frames, when sent to the public network, carry the TPID value identical to the value of a particular vendor to allow interoperability with the devices of that vendor. The TPID in an Ethernet frame has the same position with the protocol type field in a frame without a VLAN tag. In order to avoid problems in packet forwarding and handling in the network, you cannot set the TPID value to any of the values in this table.