Vlan Tagging - Nortel Alteon OS 42C4911 Application Manual

Alteon OS Application Guide

VLAN Tagging

Alteon OS software supports 802.1Q VLAN tagging, providing standards-based VLAN sup-
port 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).
N – If a 802.1Q tagged frame is received by a port that has VLAN-tagging disabled, then
OTE
the frame is dropped at the ingress port.
80 Chapter 3: VLANs 42C4911, January 2007