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 ide ntifier (VID) -- the 12- bit portion of the V LAN tag in the frame
header that ide ntifies 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 fr ame -- the 32-bit fiel d (VLAN tag) in the frame hea der that ident ifies
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 untagge d member
of a specific VLAN. When an untagged frame exits the switch through an
untagged m ember port, the frame header remains unchanged. When a tagged
frame exits the swit ch through an untagged member port, the tag is stripped
and the ta gged 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, t he f rame he ader is modi f ied to i nclude the 32-bi t ta g a ssociat ed with the
PVID. When a tagged frame exits the switch through a tagged member port,
the frame h eader remain s unc hang ed (original VID remain s).
• User_priority -- a three-bit field in the header of a tagged frame. The field is
interp ret ed as a binary number, the re for e ha s a value of 0 - 7. This field all ows
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 b ecomes the user_prio rity 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