Alcatel OmniStack LS 6248 User Manual page 229

3
Configuring 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.
Untagged VLANs
Untagged (or static) 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.
Automatic VLAN Registration
GVRP (GARP VLAN Registration Protocol) defines a system whereby the switch
can automatically learn the VLANs to which each end station should be assigned. If
an end station (or its network adapter) supports the IEEE VLAN protocol, it can be
configured to broadcast a message to your network indicating the VLAN groups it
wants to join. When this switch receives these messages, it will automatically place
the receiving port in the specified VLANs, and then forward the message to all other
ports. When the message arrives at another switch that supports GVRP, it will also
place the receiving port in the specified VLANs, and pass the message on to all
other ports. VLAN requirements are propagated in this way throughout the network.
This allows GVRP-compliant devices to be automatically configured for VLAN
groups based solely on endstation requests.
To implement GVRP in a network, first add the host devices to the required VLANs
(using the operating system or other application software), so that these VLANs can
be propagated onto the network. For both the edge switches attached directly to
these hosts, and core switches in the network, enable GVRP on the links between
these devices. You should also determine security boundaries in the network and
disable GVRP on the boundary ports to prevent advertisements from being
propagated, or forbid those ports from joining restricted VLANs.
205