Protocols And Standards - 3Com 4500G Family Configuration Manual

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Figure 1-3 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

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 1-1 Reserved protocol type values

Protocols and Standards

IEEE 802.1Q: IEEE standard for local and metropolitan area networks: Virtual Bridged Local Area

Protocol type

0x8863/0x8864

0x8847/0x8848

0xFFFD/0xFFFE/0xFFFF

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