Tagging Frames; C.1 Tagging Frames - Siemens SCALANCE X-300 Configuration Manual

Appendix C
C.1

Tagging frames

Expansion of the Ethernet frames by four bytes
For the functions CoS (Class of Service, frame priority) and port-based VLAN (virtual
network), the IEEE 802.1 Q standard defined the expansion of Ethernet frames by adding
the VLAN tag.
Note
The VLAN tag increases the permitted total length of an Ethernet frame from 1518 to 1522
bytes. It is necessary to check whether the end nodes on the network can process this
length / frame type. If this is not the case, only frames of the standard length may be sent to
these nodes.
The additional 4 bytes are located in the header of the data packet between the source
address and the Ethernet type / length field:
Figure C-1 Structure of a tagged frame
The additional bytes contain the tag protocol identifier field and the tag control information
field.
Tag protocol identifier field
The first two bytes form the Tag Protocol Protocol Identifier field (TPI) and always contain
the value 0x8100. This value specifies that the data packet contains VLAN information or
priority information.
SCALANCE X-300 / X-400
Configuration Manual, 10/2014, C79000-G8976-C187-22
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