3Com S7906E Configuration Manual page 205

Figure 1-3 VLAN Tag structure of an Ethernet frame
An S7900E switch determines whether a received frame is VLAN tagged by comparing its own TPID
with the TPID field in the received frame. If they match, the frame is considered as a VLAN tagged
frame. If not, the switch tags the frame with the default VLAN tag of the receiving port.
The systems of different vendors may set the TPID in the outer VLAN tag of QinQ frames to different
values. For compatibility with these systems, the S7900E series switches allow you to modify the TPID
values in the VLAN tags in QinQ frames, including:
The TPID value in customer network VLAN tags. The switch uses it to determine whether a frame
received from the customer network is VLAN tagged. If the frame is considered as VLAN untagged,
the switch tags the frame with the default VLAN tag of the receiving port. This default VLAN tag
uses the TPID that you have configured.
The TPID value in service provider network VLAN tags. The switch uses it to determine whether a
frame received from the service provider network is VLAN tagged. In addition, the switch uses the
configured TPID in the outer VLAN tag for customer network frames for compatibility with
third-party devices.
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 1-1 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
1-4
Value