IBM RackSwitch G8000, Application Manual

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© Copyright IBM Corp. 2011 Chapter 8. VLANs
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VLAN Tagging
IBM Networking OS software supports 802.1Q VLAN tagging, providing
standards-based VLAN support for Ethernet systems.
Tagging places the VLAN identifier in the frame header of a packet, allowing each
port to belong to multiple VLANs. When you add a port to multiple VLANs, you also
must enable tagging on that port.
Since tagging fundamentally changes the format of frames transmitted on a tagged
port, you must carefully plan network designs to prevent tagged frames from being
transmitted to devices that do not support 802.1Q VLAN tags, or devices where
tagging is not enabled.
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. Any untagged frames received
by the switch are classified with the PVID of the receiving port.
• Tagged frame—a frame that carries VLAN tagging information in the header.
This VLAN tagging information is a 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.
• 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 tagged 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).
Note: If a 802.1Q tagged frame is received by a port that has VLAN-tagging
disabled and the port VLAN ID (PVID) is different than the VLAN ID of the
packet, then the frame is dropped at the ingress port.