D-Link DES-3250TG ユーザーズマニュアル

A VLAN is a collection of end nodes grouped by logic rather than physical location. End nodes that frequently communicate 
with each other are assigned to the same VLAN, regardless of where they are located physically on the network. Logically, a 
VLAN can be equated to a broadcast domain, because broadcast packets are forwarded only to members of the VLAN on 
which the broadcast was initiated. 

The DES-3250TG supports IEEE 802.1Q VLANs. The port untagging function can be used to remove the 802.1Q tag from 
packet headers to maintain compatibility with devices that are tag-unaware (that is, network devices that do not support IEEE 
802.1Q VLANs or tagging). The switch’s default is to assign all ports to a single 802.1Q VLAN named “default.”   

Some relevant terms: 

•  Tagging – The act of putting 802.1Q VLAN information into the header of a packet.  

•  Untagging – The act of stripping 802.1Q VLAN information out of the packet header.  

•  Ingress port – A port on a switch where packets are flowing into the switch and VLAN decisions must be made.  

•  Egress port – A port on a switch where packets are flowing out of the switch, either to another switch or to an end 

station, and tagging decisions must be made.  

IEEE 802.1Q (tagged) VLANs are implemented on the DES-3250TG Layer 2 switch.  802.1Q VLANs require tagging, which 
enables the VLANs to span an entire network (assuming all switches on the network are IEEE 802.1Q-compliant).   

Any port can be configured as either tagging or untagging. The untagging feature of IEEE 802.1Q VLANs allow VLANs to 
work with legacy switches that don’t recognize VLAN tags in packet headers. The tagging feature allows VLANs to span 
multiple 802.1Q-compliant switches through a single physical connection and allows Spanning Tree to be enabled on all ports 
and work normally. 

Packet forwarding decisions are made based upon the following three types of rules: 

•  Ingress rules – rules relevant to the classification of received frames belonging to a VLAN. 

•  Forwarding rules between ports – decides filter or forward the packet 

•  Egress rules – determines if the packet must be sent tagged or untagged.