Chapter 5
| VLAN Configuration
IEEE 802.1Q VLANs
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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.
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 802.1Q 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 end
station 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.
Note:
If you have host devices that do not support GVRP, you should configure
static or untagged VLANs for the switch ports connected to these devices (as
described in “Adding Static Members to VLANs” on page 169). But you can still
enable GVRP on these edge switches, as well as on the core switches in the
network.
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