3Com 4210G Series Configuration Manual page 174

Figure 10-3 VLAN tag structure of an Ethernet frame
The device determines whether a received frame carries a SVLAN tag or a CVLAN tag by checking the
corresponding TPID value. Upon 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 devices considers that the frame
carries the VLAN tag. For example, if a frame carries a VLAN tag with the TPID value 0x9100 while the
configured TPID value of the VLAN tag is 0x8100, the device considers that the frame carries no VLAN
tag.
In addition, the systems of different vendors may set the TPID of the outer VLAN tag of QinQ frames to
different values. For compatibility with these systems, you can modify the TPID value so that the QinQ
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. To avoid problems in packet forwarding and handling in the network, you cannot set the
TPID value to any of the values in the table below.
Table 10-1 Reserved protocol type values
Protocol type
ARP
PUP
RARP
IP
IPv6
PPPoE
MPLS
IPX/SPX
IS-IS
LACP
802.1x
Cluster
Reserved
0x0806
0x0200
0x8035
0x0800
0x86DD
0x8863/0x8864
0x8847/0x8848
0x8137
0x8000
0x8809
0x888E
0x88A7
0xFFFD/0xFFFE/0xFFFF
10-4
Value