H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 123)

113
Figure 30 Single-tagged Ethernet frame header and double-tagged Ethernet frame header
NOTE:
The default maximum transmission unit (MTU) of an interface is 1500 bytes. The size of an outer VLAN tag
is 4 bytes. H3C recommends you to increase the MTU of each interface on the service provider network to
at least 1504 bytes.
Modifying the TPID in a VLAN tag
A VLAN tag uses the tag protocol identifier (TPID) field to identify the protocol type of the tag. The default
value of this field, as defined in IEEE 802.1Q, is 0x8100.
Figure 31 shows the 802.1Q-defined tag structure of an Ethernet frame.
Figure 31 VLAN tag structure of an Ethernet frame
The device determines whether a received frame carries an SVLAN or CVLAN tag by checking the TPID
value. For example, if a frame carries an SVLAN tag with TPID value 0x9100 and a CVLAN tag with TPID
value 0x8100 and the configured TPID value of the SVLAN tag is 0x9100 and that of the CVLAN tag is
0x8200, the device considers that the frame carries only the SVLAN tag but not the CVLAN tag.
Devices of different vendors might set the TPID of the outer VLAN tag of QinQ frames to different values.
For compatibility with these devices, 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, allowing
interoperability with the devices of that vendor.
The TPID in an Ethernet frame has the same position as the protocol type field in a frame without a VLAN
tag. To avoid problems in packet forwarding and handling in the network, do not set the TPID value to
any of the reserved values.