4-2

CheetahSwitch Workgroup-4508
Virtual LANs
Switches do not inherently support broadcast domains, which can lead to
broadcast storms in large networks that handle a lot of IPX or NetBeui traffic. In
conventional networks with routers, broadcast traffic is split up into separate
domains to confine broadcast traffic to the originating group and provide a much
cleaner network environment. By supporting VLANs, this switch allows you to
create segregated broadcast domains. However, note that if you need to support
intra-VLAN communications, you must use a router or Layer 3 switch.
An IEEE 802.1Q VLAN is a group of ports that can be located anywhere in the
network, but communicate as though they belong to the same physical segment.
VLANs help to simplify network management by allowing you to move devices to a
new VLAN without having to change any physical connections. VLANs can be
easily organized to reflect departmental groups (such as Marketing or R&D),
usage groups (such as e-mail), or multicast groups (used for multimedia
applications such as video conferencing).
VLANs provide greater network efficiency by reducing broadcast traffic, but also
allow you to make network changes without having to update IP addresses or IP
subnets. VLANs inherently provide a high level of network security, since traffic
must pass through a Layer 3 switch or a router to reach a different VLAN.
This switch supports the following VLAN features:
• Up to 16 VLANs based on the IEEE 802.1Q standard
• Distributed VLAN learning across multiple switches using explicit or implicit
tagging
• Port overlapping, allowing a port to participate in multiple VLANs
• End stations can belong to multiple VLANs
• Passing traffic between VLAN-aware and VLAN-unaware devices
• Two-level priority queue

Assigning Ports to VLANs

Before enabling VLANs for the switch, you must first assign each port to the VLAN
group(s) it will participate in. (By default all ports are assigned to VLAN 1 as
untagged ports.) Add a port as a tagged port (that is, a port attached to a VLAN-
aware device) if you want it to carry traffic for one or more VLANs and the device
at the other end of the link also supports VLANs. Then assign the port at the other
end of the link to the same VLAN(s). However, if you want a port on this switch to
participate in one or more VLANs, but the device at the other end of the link does
not support VLANs, then you must add this port as an untagged port (that is, a port
attached to a VLAN-unaware device).
Port-based VLANs are tied to specific ports. The switch’s forwarding decision is
based on the destination MAC address and its associated port. Therefore, to make
valid forwarding and flooding decisions, the switch learns the relationship of the
MAC address to its related port—and thus to the VLAN—at run-time.