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Equalizer Use of VLAN Technology

Equalizer models E350GX, E450GX, E650GX support tagged and untagged VLANs on all front panel interface

ports. This section provides a basic technical introduction to VLAN technology.

Many networking technologies use a technique called

broadcasting

to provide services on a Local Area Network

(LAN). Like traditional television or radio signals that are broadcast over the airwaves, broadcast network

transmissions are received by every node on the same LAN segment, or

broadcast domain

. The Address

Resolution Protocol (ARP), the Dynamic Host Configuration Protocol (DHCP), and the Router Information

Protocol (RIP) are all examples of protocols that provide network services through broadcasting.

A LAN is a single broadcast domain composed of all the systems that are physically connected to the same

switches, hubs, and other devices that communicate at the Data Link Layer (Layer 2) of the OSI Networking

Model. These devices communicate using Layer 2 protocols, like Ethernet and ARP.

Virtual Local Area Network (VLAN) technology was developed to overcome these physical limitations of traditional

LAN technology. A VLAN is essentially a means of grouping systems at the Data Link Layer (Layer 2 of the OSI

networking model), using methods that are independent of the physical connection of the device to the network.

By exchanging

broadcast packets

-- packets that are essentially sent to all systems connected to a Layer 2

switching device -- switches can maintain a list of all MAC addresses connected to them and to the other switches

to which they are connected. A set of Layer 2 devices and the systems connected to them form a

broadcast

domain

-- meaning that all the systems can talk to one another using broadcast packets.

Conversely, broadcast packets are not forwarded beyond the boundaries of the broadcast domain. For example: if

two LANs are connected by a router (a Network Layer, or Layer 3, device), the broadcast traffic for one LAN is

never forwarded to the other LAN. The layout of a traditional LAN is therefore restricted to those systems that can

be wired together using Layer 2 devices -- a physically distant system that requires connectivity to the LAN would

require special routing and address translation (at Layer 3) in order to reach the LAN.

The dependence of LAN technology on physical connectivity at Layer 2 leads to two basic difficulties:

l Broadcasts are received by all systems in the broadcast domain - and if there is sufficient broadcast traffic,

it can significantly reduce the overall performance of the LAN, to the point where some services may simply

not be able to function properly due to latency or other factors introduced by a high level of broadcast traffic.

l If you want to include a system that is not physically connected to the LAN in the LAN’s broadcast domain,

you need to physically connect the system to the LAN.

One problem with broadcasting is that lots of broadcast traffic on a LAN can slow network traffic down, as well as

slow individual systems down. If there is so much broadcast traffic on the LAN that other non-broadcast traffic is

significantly delayed (or never delivered), this is called a

broadcast storm

. Broadcast storms typically arise when

network loops are created through faulty network configuration, but can also happen as the result of a malicious

attack. For example, a classic Denial of Service attack is to send an ICMP echo request ("ping") over the LAN that

specifies the source address of a system and a broadcast address for the destination. Every system receiving the

ping will respond to it -- flooding the system specified as the source of the ping with ICMP echo replies.

There are also other security concerns associated with broadcasting. Since all the systems in the broadcast

domain can see broadcast packets, the information in them is susceptible to discovery, intercept, and

modification. This is of particular concern in industrial Ethernet environments (where, for example, manufacturing

processes are controlled directly by computers) and in any environment (such as government and finance) where

sensitive data is regularly transmitted over the LAN.

Equalizer Administration Guide

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