Vlan Overview; Physical Local Area Networks; Virtual Lans - Symbol ES3000 Manual

6-2 ES3000 Advanced Concept Guide

6.1 VLAN Overview

6.1.1 Physical Local Area Networks

A physical local area network (LAN) is also a broadcast domain, a section of a network in which
broadcast packets are delivered to all end-stations. In a physical LAN, the cabling and the router
define the range of the broadcast domain. All end-stations must be physically connected to the router.
For an end-station to be on several physical LANs, it must have multiple network interface cards.
This physical connection limits the usefulness of LANs. If a physical LAN gets too large, the broadcast
packets and the responses to them consume a large part of the network bandwidth, clogging the
network. If the LAN is used as a security device, all users of a particular type must be close together
or multiple wiring systems must be maintained.

6.1.2 Virtual LANs

Virtual LANs (VLANs) are broadcast domains which are defined by the configuration of network
equipment rather than cabling. The network administrator determines which end-stations are on
which VLANs by software changes rather than cabling decisions. On the ES3000 switch, there are
two kinds of VLANs: manual VLANs and dynamic VLANs.
A manual VLAN is a group of ports that are defined by the network administrator as being on the same
VLAN. Broadcast packets are repeated to all of the ports on the same VLAN. The packets pass back
and forth on this VLAN are usually regular Ethernet packets, with no additional tags on the packet.
These VLANs are compatible with all network equipment, but a port may belong to no more than one
VLAN of this type because, without a tag of some kind, there is no way of distinguishing which VLAN
a packet is travelling on when it enters the switch.
However, if the packets are tagged with the identity of the VLAN, significant advantages derive from
the tagging. First, a port may belong to more than one VLAN. Second, VLANs can span switches.
Third, VLANs can be created on demand. Finally, end-stations can be moved, at will, within a group
of switches, and automatically remain on their home VLAN.
The VLAN tagging standard is 802.1Q. Packets are marked with a number between 2 and 4094, with
0, 1, and 4095 being reserved. Unfortunately, the tag increases the maximum packet size by four
bytes. Network equipment manufactured before 1998 may see these packets as malformed and may
drop them. For this reason, ports on most 802.1Q compatible equipment can be configured to strip the
tagging when sending a packet and to add a tag when a packet is received on that port. Such ports
must have a default VLAN specified. A tag for this VLAN will be added to the incoming untagged
packets before sending them out on the VLAN.