Coyote Point Systems Equalizer GX Series, Руководство администратора

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Load Balancing & Networking
Using VLANs
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:
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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.
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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.
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