A LAN consists of stations, repeating hubs and switching hubs operating at the
data link layer. LANs could be connected to other LANs if routers are used;
thereby, creating an internetwork. Each LAN would then be given a network
address. The best example of an internetwork is the Internet. Therefore, it
is possible to have the industrial automation system on one LAN and the
information system on another LAN with the two linked by a router. However,
the structured wiring within the plant may not support this wiring directly.
Besides, configuring routers is more difficult than configuring VLANs. What
is desired is to have the information system and industrial automation
system on the same LAN, but logically separated into two LANs. That is what
a VLAN can do.
There are several ways of creating VLANs, but the easiest to understand is the Port VLAN.
Switches create an association of MAC addresses and port numbers. What needs to be
added is a VLAN association. This would have to be accomplished through some
configuration of a switch that can support VLANs. VLAN support is not possible with
a Plug and Play switch one with no means of altering its personality through
operator intervention. For example, within a sixteen-port switch we want to create
three separate VLANs numbered one to three. During configuration, we associate each
port on the switch with a VLAN. From then on, traffic within a VLAN assignment will
be restricted to only those ports associated with that VLAN assignment. Using our
example of three VLANs, we established VLAN1 as associated with ports 1, 2, 3 and 4.
A broadcast or multicast message on port 1 would be sent only to ports 2, 3, and 4
and no others. The other VLANs would operate in a similar fashion. A unicast message
would be forwarded as with any other switch. There would be a MAC address-port
number association. However, added to this association would be the VLAN constraints.
So if the MAC address-port number association is not present in memory for a
destination address, flooding will only occur with the VLAN port group. What happens
when a destination address is specified in a transmission received on a port from
another VLAN group? The transmission should be discarded.
A big advantage of Port VLAN is that it is simple to understand and use. Patch panel ports can be tagged with the associated VLAN, and it is just a simple matter of moving patch cords around to connect particular stations to particular VLANs. A simpler way of doing it is to have software do it. By reconfiguring the VLAN-aware switch, physical ports can be reassigned to different VLANs. However, what if you want to stretch your VLAN across several switches? It is possible, but you would need to have dedicated wiring for each VLAN. That is a severe restriction and, therefore, Port VLANs are best accomplished using a single VLAN-aware switch. Notice that there is no change in Ethernet frames with Port VLAN partitioning. End stations are unaware of the VLAN structure. More flexibility is gained if VLAN associations can be learned from the contents of the Ethernet frame. This is called implied tagging which allows VLANs to span multiple switches using the same cabling structure.
Frame-Encoded VLAN Schemes
With Port VLAN, there is no altering of Ethernet frames or any implicit tagging within Ethernet frames. Stations are unaware of the VLAN structure. There are alternate ways of establishing VLANs if the switches being used support the various schemes. You could simply associate particular MAC addresses to a VLAN. In this way the station assigned to the VLAN can be on any switch port and still be attached to a particular VLAN. Of course, if that station were ever replaced, all switches would need to be reconfigured for the new MAC address. Another approach to VLANs is to separate stations according to the network operating system being supported. By examining some protocol field, frames could be directed only to those stations supporting that operating system. This approach to VLANs was popular when there were several competing network operating systems with much different Ethernet frame definitions. The movement towards universal TCP/IP acceptance has now limited the frame structure choices. Another scheme is to define a proprietary protocol by coding the Ethernet frame with VLAN information. The problem with proprietary schemes is that they do not have wide industry support. To obtain wide industry support, you need an IEEE standard.