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VLAN Implementation Guide: The Basics

Virtual LANs are core to enterprise networking. This guide covers VLAN trunks, VLAN planning, and basic VLAN configuration.
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If you're just getting started in the world of network administration and architecture, there's no better place to begin than with a solid understanding of virtual LANs (VLANs.)

In order to understand the purpose of VLANs, it's best to look at how Ethernet networks previously functioned. Prior to VLANs and VLAN-aware switches, Ethernet networks were connected using Ethernet hubs. A hub was nothing more than a multi-port repeater. When an end device sent information onto the Ethernet network toward a destination device, the hub retransmitted that information out all other ports as a network-wide broadcast.

The destination device would receive the information sent, but so would all other devices on the network. Those devices would simply ignore what the heard. And while this method worked in small environments, the architecture suffered greatly from scalability issues. Too much time was spent discarding received messages and waiting for a turn to transmit their own messages that Ethernet networks using hubs became congested.

A layer 2 aware switch solves this problem using two different methods. First, the switch has the ability to learn and keep track of devices by their MAC address. By maintaining a dynamic table of MAC address to switch port number, the switch has the ability to send messages directly from a source device to the destination device in a unicast transmission as opposed to a broadcast transmission that is sent to all devices. This is known as the switch forwarding table.

While the forwarding table does a great deal to limit broadcast messages, and thus reduce the amount of broadcast overhead, it does not completely eliminate it. Broadcast messages are still required in many situations. And as such, the more devices on a physical network, the more broadcast messages are going to be clogging up the network.

That leads us to our second method that layer 2 switches use to streamline Ethernet communication. Instead of having one large layer 2 network, VLANs are used to segment a switch -- or network of switches -- into multiple, logical layer 2 networks. Broadcast messages sent and received are contained within each smaller VLAN. Thus, if you have a network of 1,000 end devices and create 4 VLANs of 250 devices each, each logical network must only have to deal with 250 devices of broadcast overhead, as opposed to all 1,000 if they were on the same layer 2 network.

 

Andrew has well over a decade of enterprise networking under his belt through his consulting practice, which specializes in enterprise network architectures and datacenter build-outs and prior experience at organizations such as State Farm Insurance, United Airlines and the ... View Full Bio

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ClassC
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ClassC,
User Rank: Ninja
11/20/2015 | 8:56:44 PM
Re: Andrew and The Community: VLAN Confusion Please Help !

@Andrew   Thanks !   I am discovering that as we speak, this is the first network I have ever seen constructed this way.   

I must say though the router approach seems to be more stable than the traditional DC approach.

Learn something everyday ( or at least one should).

Andrew Froehlich
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Andrew Froehlich,
User Rank: Strategist
11/19/2015 | 10:24:16 PM
Re: Andrew and The Community: VLAN Confusion Please Help !
Routers can be setup to operate as DHCP servers. If you can login to the router, you should be able to see this configuration, make changes, or disable it, if you want.
ClassC
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ClassC,
User Rank: Ninja
11/19/2015 | 7:09:56 PM
Andrew and The Community: VLAN Confusion Please Help !

Hi Andrew - 

Please help me understand this.  I have recently been entrusted with a network with no true DC, and the workstations are getting their IP's as soon as they hit the network through DHCP.

I have found that the router seems to be pushing these out even though I have yet to understand how they were entered.

Is this type of setup considered a VLAN ?

Andrew Froehlich
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Andrew Froehlich,
User Rank: Strategist
11/18/2015 | 2:19:30 PM
Re: VLANs
@virsingh211 -- Thanks for adding some additional depth to the explaination of a hub. Much appreciated.
virsingh211
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virsingh211,
User Rank: Ninja
11/17/2015 | 11:41:46 PM
Re: VLANs
Good one, adding little more to blog about LAN switching...

Hub can only work in half duplex mode for its need to replicate the electrical signal received on port x on all other ports.
This should be the only effect of the introduction of an hub between two switches.
Hub works at OSI layer 1 and it doesn't know what is the difference between: an Ethernet frame, an ISL frame or an 802.1Q tagged Ethernet frame.
Having an half duplex trunk is something to be avoided for performance and stability reasons.
Connecting multiple switches to an hub would be again a performance and a stability problem defeating MAC address filtering capability of switches.
afroehlich805
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afroehlich805,
User Rank: Apprentice
11/17/2015 | 5:34:31 PM
Re: VLANs
Hi Marcia - The most common mistep that I come across is poor VLAN planning. Make sure you have a sound VLAN strategy for segmenting users/servers/wireless/guests etc into separate VLANs. This strategy should work well for future growth and any mergers/acquisitions.
MarciaNWC
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MarciaNWC,
User Rank: Strategist
11/17/2015 | 4:09:00 PM
VLANs
Hi Andrew -- Thanks for this great guide! I'm curious if there's a common misstep that organizations make when it comes to VLANs? Is there a basic concept that gets overlooked?
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