H3C Technologies H3C S7500E Series Switches - Manual (Page 273)

20-4
Figure 20-3 VLAN Tag structure of an Ethernet frame
An S7500E 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 S7500E 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 20-1 Reserved protocol type values
Protocol type Value
ARP 0x0806
PUP 0x0200
RARP 0x8035
IP 0x0800
IPv6 0x86DD
PPPoE 0x8863/0x8864
MPLS 0x8847/0x8848
IPX/SPX 0x8137
IS-IS 0x8000
LACP 0x8809