Nortel Networks 450 - Manual (Page 59)

BayStack 450 10/100/1000 Series Switches
302401-D Rev 00 1-33
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. For example, a port with a PVID of 3 (PVID =3)
assigns all untagged frames received on this port to VLAN 3.
• 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
PVID. 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, therefore has a value of 0 -7. This field allows
the tagged frame to carry the user-priority 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 user_priority for the frame. Tagged packets get
their user_priority from the value contained in the 802.1Q frame header.
• Unregistered packet -- a tagged frame which contains a VID where the
receiving port is not a member of that VLAN.
kombk.book Page 33 Tuesday, June 29, 1999 3:25 PM