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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
VLANs
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.
Notes About VLANs on the DES-3326
1. The DES-3326 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).
2. The switch’s default - in both Layer 2 Only mode and IP Routing mode - is to assign all
ports to a single 802.1Q VLAN named DEFAULT_VLAN.
3. The switch allows the assignment of an IP interface to each VLAN, in IP Routing mode.
The VLANs must be configured before setting up the IP interfaces
4. A VLAN that is not assigned an IP interface will behave as a layer 2 VLAN – and IP
routing, by the switch, will not be possible to this VLAN regardless of the switch’s
operating mode.
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-3326 Layer 3 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.
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
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