156 G8264 Application Guide for ENOS 8.4VLAN Tagging/Trunk ModeLenovo Enterprise Network Operating System software supports 802.1Q VLANtagging, providing standards‐based VLAN support for Ethernet systems.Tagging places the VLAN identifier in the frame header of a packet, allowing eachport to belong to multiple VLANs. When you add a port to multiple VLANs, youalso must enable tagging on that port.Since tagging fundamentally changes the format of frames transmitted on a taggedport, you must carefully plan network designs to prevent tagged frames frombeing transmitted to devices that do not support 802.1Q VLAN tags, or deviceswhere 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 headerthat identifies an explicit VLAN. Port VLAN identifier (PVID)—a classification mechanism that associates a portwith a specific VLAN. For example, a port with a PVID of 3 (PVID =3) assigns alluntagged frames received on this port to VLAN 3. Any untagged framesreceived 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 headerthat identifies the frame as belonging to a specific VLAN. Untagged frames aremarked (tagged) with this classification as they leave the switch through a portthat is configured as a tagged port. Untagged frame— a frame that does not carry any VLAN tagging informationin the frame header. Untagged member—a port that has been configured as an untagged member ofa specific VLAN. When an untagged frame exits the switch through anuntagged member port, the frame header remains unchanged. When a taggedframe exits the switch through an untagged member port, the tag is strippedand the tagged frame is changed to an untagged frame. Tagged member—a port that has been configured as a tagged member of aspecific VLAN. When an untagged frame exits the switch through a taggedmember port, the frame header is modified to include the 32‐bit tag associatedwith the PVID. When a tagged frame exits the switch through a tagged memberport, the frame header remains unchanged (original VID remains). When anaccess port is set as a trunk, it is automatically added to all data VLANs. Tochange the allowed VLAN range, use the command:switchport trunk allowed vlans Note: If a 802.1Q tagged frame is received by a port that has VLAN‐taggingdisabled and the port VLAN ID (PVID) is different than the VLAN ID of thepacket, then the frame is dropped at the ingress port.
