D-Link DES-3326SR Manual - Understanding 802.1Q Vlans

D-Link DES-3326SR Layer 3 Switch

Understanding 802.1Q VLANs 

This review of 802.1Q VLANs presents some basic background about how VLANs work according to the IEEE 

802.1Q standard. VLANs operate according to the same rules regardless of whether the Switching environment 

is Layer 2 or Layer 3. The difference is primarily that in a Layer 3 Switch there is an added capability of unique 

association between a VLAN and an IP interface or subnet group. 

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.

IEEE 802.1Q VLANs 

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-3326SR 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 VLAN compliant Switches through a single physical connection 

and allows Spanning Tree to be enabled on all ports and work normally. 

802.1Q VLAN Packet Forwarding 

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. 

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