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DES-3226 NWay Standalone Fast Ethernet Switch User’s Guide
The switch with the lowest Bridge ID (switch C) was elected the root bridge, and the ports were selected
to give a high port cost between switches B and C.
Note also that the example network topology is intended to provide redundancy to protect the network
against a link or port failure – not a switch failure or removal. For example, a failure of switch A would
isolate LAN 1 from connecting to LAN 2 or LAN 3.
VLANs
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 DES-3226
1. 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.
2. The DES-3226 supports only 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.
3. The Switch’s default is to assign all ports to a single 802.1Q VLAN named
DEFAULT_VLAN.
4. The DEFAULT_VLAN has a VID = 1.
5. The DES-3226 supports Asymmetric VLANs. The member ports of VLANs can be
overlapped.
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.
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