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TPID
The Tag Protocol Identifier (TPID) specifies the protocol type of a VLAN tag. The TPID value defined in IEEE 802.1Q is 0x8100.
Figure 10-5 shows the Ethernet packet format defined in IEEE 802.1Q. An IEEE 802.1Q tag, containing the TPID, lies between the Source Address field and the Length/Type field. A device checks the TPID value in a received packet to determine whether the VLAN tag is an S-VLAN tag or C-VLAN tag. The device compares the configured TPID value with the TPID value in the packet. For example, if a frame carries the VLAN tag with TPID 0x8100 but the TPID configured for a customer network on a device is 0x8200, the device considers the frame untagged.
Carrier's systems may use different TPID values
in outer VLAN tags. When a Huawei device needs to interoperate with
such a carrier system, set the TPID value to the value used by the
carrier so that QinQ packets sent from the Huawei device can be transmitted
across the carrier network. To prevent errors
in packet forwarding and processing, do not set the TPID to any of
values listed in Table 10-3.