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DES-3010F/DES-3010FL/DES-3010G/DES-3016/DES-3018/DES-3026 Fast Ethernet Switch Manual
85
Section 7
L2 Features
Static VLAN Entry
VLAN Trunking Settings
Trunking
IGMP Snooping
Spanning Tree
Loopback Detection
VLANs
VLAN Description
A Virtual Local Area Network (VLAN) is a network topology configured according to a logical scheme
rather than the physical layout. VLANs can be used to combine any collection of LAN segments into an
autonomous user group that appears as a single LAN. VLANs also logically segment the network into
different broadcast domains so that packets are forwarded only between ports within the VLAN. Typically, a
VLAN corresponds to a particular subnet, although not necessarily.
VLANs can enhance performance by conserving bandwidth, and improve security by limiting traffic to
specific domains.
A VLAN is a collection of end nodes grouped by logic instead of physical location. End nodes that
frequently communicate with each other are assigned to the same VLAN, regardless of where they are
physically on the network. Logically, a VLAN can be equated to a broadcast domain, because broadcast
packets are forwarded to only members of the VLAN on which the broadcast was initiated.
Notes about VLANs on the Switch
No matter what basis is used to uniquely identify end nodes and assign these nodes VLAN membership,
packets cannot cross VLANs without a network device performing a routing function between the VLANs.
The Switch 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.
The Switch's default is to assign all ports to a single 802.1Q VLAN named "default."
The "default" VLAN has a VID = 1.
The member ports of Port-based VLANs may overlap, if desired.
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 Switch. 802.1Q VLANs require tagging, which
enables them to span the entire network (assuming all switches on the network are IEEE 802.1Q-compliant).
VLANs allow a network to be segmented in order to reduce the size of broadcast domains. All packets
entering a VLAN will only be forwarded to the stations (over IEEE 802.1Q enabled switches) that are
members of that VLAN, and this includes broadcast, multicast and unicast packets from unknown sources.