Using the BayStack 450 10/100/1000 Series Switch
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IEEE 802.1Q Tagging
BayStack 450 switches operate in accordance with the IEEE 802.1Q tagging
rules. Important terms used with the 802.1Q tagging feature are:
• VLAN identifier (VID) -- the 12-bit portion of the VLAN tag in the frame
header that identifies an explicit VLAN.
• Port VLAN identifier (PVID) -- a classification mechanism that associates a
port with a specific VLAN (see Figures
1-24 to 1-29).
• Tagged frame -- the 32-bit field (VLAN tag) in the frame header that identifies
the frame as belonging to a specific VLAN. Untagged frames are marked
(tagged) with this classification as they leave the switch through a port that is
configured as a tagged port.
• Untagged frame -- a frame that does not carry any VLAN tagging
information in the frame header.
• VLAN port members -- a set of ports that form a broadcast domain for a
specific VLAN. A port can be a member of one or more VLANs.
• Untagged member -- a port that has been configured as an untagged member
of a specific VLAN. When an untagged frame exits the switch through an
untagged member port, the frame header remains unchanged. When a tagged
frame exits the switch through an untagged member port, the tag is stripped
and the tagged frame is changed to an untagged frame.
• Tagged member -- a port that has been configured as a member of a specific
VLAN. When an untagged frame exits the switch through a tagged member
port, the frame header is modified to include the 32-bit tag associated with the
VLAN assigned to that frame. When a tagged frame exits the switch through a
tagged member port, the frame header remains unchanged (original VID
remains).
• User_priority -- a three-bit field in the header of a tagged frame. The field is
interpreted as a binary number, and therefore has a value of 0 through 7. This
field allows the tagged frame to carry the user_priority value across bridged
LANs where the individual LAN segments may be unable to signal priority
information.
• Port priority -- the priority level assigned to untagged frames received on a
port. This value becomes the frame’s user_priority value. Tagged packets get
their user_priority value from the 802.1Q frame header.
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