User guide

Overview VLAN
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iMG/RG Software Reference Manual (Switching)
• Tag Control Information (TCI). The TCI field is two octets in length, and contains user priority, CFI and VID
(VLAN Identifier) fields. Figure ... illustrates the structure of the TCI field:
• User priority. The user priority field is three bits in length, interpreted as a binary number. The user priority
is therefore capable of representing eight priority levels, 0 through 7. This field allows the tagged frame to
carry user priority information across Bridged LANs in which individual LAN segments may be unable to sig-
nal priority.
• Canonical Format Indicator (CFI). The Canonical Format Indicator (CFI) is a single bit flag value. CFI reset
indicates that all MAC Address information that may be present in the MAC data carried by the frame is in
Canonical format.
• The meaning of the CFI when set depends upon the variant of the tag header in which it appears.
• In an Ethernet-encoded tag header, transmitted using 802.3/Ethernet MAC methods, CFI has the following
meanings:
• When set, indicates that the E-RIF field is present in the tag header, and that the NCFI bit in the RIF
determines whether MAC Address information that may be present in the MAC data carried by the
frame is in Canonical (C) or Non-canonical (N) format;
• When reset, indicates that the E-RIF field is not present in the tag header, and that all MAC Address
information that may be present in the MAC data carried by the frame is in Canonical format (C).
• VLAN Identifier (VID). The twelve-bit VLAN Identifier field uniquely identifies the VLAN to which the frame
belongs. The VID is encoded as an unsigned binary number. In Table 8 are described the values of the VID
field that have specific meanings or uses; the remaining values of VID are available for general use as VLAN
identifiers.
A priority-tagged frame is a tagged frame whose tag header contains a VID value equal to the null VLAN ID.