Riverstone Networks WICT1-12 - Manual (Page 66)

5-2 Riverstone Networks RS Switch Router User Guide Release 8.0
VLAN Overview Bridging Configuration Guide
5.3 VLAN OVERVIEW
Virtual LANs (VLANs) are a means of dividing a physical network into several logical (virtual) LANs. The division
can be done on the basis of various criteria, giving rise to different types of VLANs. For example, the simplest type of
VLAN is the port-based VLAN. Port-based VLANs divide a network into a number of VLANs by assigning a VLAN
to each port of a switching device. Then, any traffic received on a given port of a switch belongs to the VLAN
associated with that port.
VLANs are primarily used for broadcast containment. A layer-2 (L2) broadcast frame is normally transmitted all over
a bridged network. By dividing the network into VLANs, the range of a broadcast is limited, i.e., the broadcast frame
is transmitted only to the VLAN to which it belongs. This reduces the broadcast traffic on a network by an appreciable
factor.
The type of VLAN depends upon one criterion: how a received frame is classified as belonging to a particular VLAN.
VLANs can be categorized into the following types:
• Port based
• MAC address based
• Protocol based
• Subnet based
• Multicast based
• Policy based
Detailed information about these types of VLANs is beyond the scope of this manual. Each type of VLAN is briefly
explained in the following subsections.
Port-based VLANs
Ports of L2 devices (switches, bridges) are assigned to VLANs. Any traffic received by a port is classified as belonging
to the VLAN to which the port belongs. For example, if ports 1, 2, and 3 belong to the VLAN named “Marketing”,
then a broadcast frame received by port 1 is transmitted on ports 2 and 3. It is not transmitted on any other port.
MAC-address-based VLANs
In this type of VLAN, each switch (or a central VLAN information server) keeps track of all MAC addresses in a
network and maps them to VLANs based on information configured by the network administrator. When a frame is
received at a port, its destination MAC address is looked up in the VLAN database. The VLAN database returns the
name of the VLAN to which this frame belongs.
This type of VLAN is powerful in the sense that network devices such as printers and workstations can be moved
anywhere in the network without the need for network reconfiguration. However, the administration is intensive
because all MAC addresses on the network need to be known and configured.
Protocol-based VLANs
Protocol-based VLANs divide the physical network into logical VLANs based on protocol. When a frame is received
at a port, its VLAN is determined by the protocol of the packet. For example, there could be separate VLANs for IP,
IPX and Appletalk. An IP broadcast frame will only be sent to all ports in the IP VLAN.