LevelOne GEL-1061 User Manual page 141

Chapter 5
| VLAN Configuration
IEEE 802.1Q VLANs
VLAN Classification – When the switch receives a frame, it classifies the frame in
one of two ways. If the frame is untagged, the switch assigns the frame to an
associated VLAN (based on the default VLAN ID of the receiving port). But if the
frame is tagged, the switch uses the tagged VLAN ID to identify the port broadcast
domain of the frame.
Port Overlapping – Port overlapping can be used to allow access to commonly
shared network resources among different VLAN groups, such as file servers or
printers. Note that if you implement VLANs which do not overlap, but still need to
communicate, you can connect them by enabled routing on this switch.
Untagged VLANs – Untagged VLANs are typically used to reduce broadcast traffic
and to increase security. A group of network users assigned to a VLAN form a
broadcast domain that is separate from other VLANs configured on the switch.
Packets are forwarded only between ports that are designated for the same VLAN.
Untagged VLANs can be used to manually isolate user groups or subnets. However,
you should use IEEE 802.3 tagged VLANs with GVRP whenever possible to fully
automate VLAN registration.
Forwarding Tagged/Untagged Frames
If you want to create a small port-based VLAN for devices attached directly to a
single switch, you can assign ports to the same untagged VLAN. However, to
participate in a VLAN group that crosses several switches, you should create a VLAN
for that group and enable tagging on all ports.
Ports can be assigned to multiple tagged or untagged VLANs. Each port on the
switch is therefore capable of passing tagged or untagged frames. When
forwarding a frame from this switch along a path that contains any VLAN-aware
devices, the switch should include VLAN tags. When forwarding a frame from this
switch along a path that does not contain any VLAN-aware devices (including the
destination host), the switch must first strip off the VLAN tag before forwarding the
frame. When the switch receives a tagged frame, it will pass this frame onto the
VLAN(s) indicated by the frame tag. However, when this switch receives an
untagged frame from a VLAN-unaware device, it first decides where to forward the
frame, and then inserts a VLAN tag reflecting the ingress port's default VID.
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