What is a LAN?

Okay, most of you already know what a LAN is but let’s give it a definition to make sure. We have to do this because, if you don’t know what a LAN is, you can’t understand what a VLAN is.

A LAN is a local area network and is defined as all devices in the same broadcast domain. If you remember, routers stop broadcasts, switches just forward them.

What is a VLAN?

As I said, a VLAN is a virtual LAN. In technical terms, a VLAN is a broadcast domain created by switches. Normally, it is a router creating that broadcast domain. With VLAN’s, a switch can create the broadcast domain.

This works by, you, the administrator, putting some switch ports in a VLAN other than 1, the default VLAN. All ports in a single VLAN are in a single broadcast domain.

Because switches can talk to each other, some ports on switch A can be in VLAN 10 and other ports on switch B can be in VLAN 10. Broadcasts between these devices will not be seen on any other port in any other VLAN, other than 10. However, these devices can all communicate because they are on the same VLAN. Without additional configuration, they would not be able to communicate with any other devices, not in their VLAN.

Are VLANs required?

It is important to point out that you don’t have to configure a VLAN until your network gets so large and has so much traffic that you need one. Many times, people are simply using VLAN’s because the network they are working on was already using them.

Another important fact is that, on a Cisco switch, VLAN’s are enabled by default and ALL devices are already in a VLAN. The VLAN that all devices are already in is VLAN 1. So, by default, you can just use all the ports on a switch and all devices will be able to talk to one another.

When do I need a VLAN?

You need to consider using VLAN’s in any of the following situations:

• You have more than 200 devices on your LAN
• You have a lot of broadcast traffic on your LAN
• Groups of users need more security or are being slowed down by too many broadcasts?
• Groups of users need to be on the same broadcast domain because they are running the same applications. An example would be a company that has VoIP phones. The users using the phone could be on a different VLAN, not with the regular users.
• Or, just to make a single switch into multiple virtual switches.

Why not just subnet my network?

A common question is why not just subnet the network instead of using VLAN’s? Each VLAN should be in its own subnet. The benefit that a VLAN provides over a subnetted network is that devices in different physical locations, not going back to the same router, can be on the same network. The limitation of subnetting a network with a router is that all devices on that subnet must be connected to the same switch and that switch must be connected to a port on the router.

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Linux VLAN configuration

1. Connect the eth0 interface of your linux machine to the switch.
2. Remove the IP Address information on the eth0 interface
    # ifconfig eth0 0.0.0.0
    # ifconfig eth0 up
3. Configure 2 VLANs on the eth0 interface using vconfig as follows (100,200 are the VLAN id's). If the 8021q.o module is not loaded, the vconfig command (when invoked first time) will automatically load the module.
    # vconfig add eth0 100
    # vconfig add eth0 200    

4. Configure IP on the VLAN interfaces 
    # ifconfig eth0.100 xxx.xxx.xxx.xxx netmask 255.255.252.0 up
    # ifconfig eth0.200 yyy.yyy.yyy.yyy netmask 255.255.255.0 up

5. Preserve the vlan configuration across reboots by adding it to configuration files. Create the appropriate ifcfg files for eth0, eth0.100 and eth0.200 in /etc/sysconfig/network-scripts/
    # cd /etc/sysconfig/network-scripts/
       
    Contents of ifcfg-eth0        DEVICE=eth0
        ONBOOT=no
        TYPE=Ethernet
    
    Contents of  ifcfg-eth0.100        DEVICE=eth0.100
        IPADDR=xxx.xxx.xxx.xxx
        NETMASK=255.255.252.0
        VLAN=yes
        ONBOOT=yes
        BOOTPROTO=none
    
    Contents of ifcfg-eth0.200        DEVICE=eth0.200
        IPADDR=yyy.yyy.yyy.yyy
        NETMASK=255.255.0.0
        VLAN=yes
        ONBOOT=yes
        BOOTPROTO=none
        
    Update /etc/sysconfig/network file to make the GATEWAYDEV use the public vlan interface.
    
    Contents of /etc/sysconfig/network        NETWORKING=yes
        HOSTNAME=un1xf00
        GATEWAY=xxx.xxx.xxx.1
        DOMAINNAME=dev.un1xf00.com
        GATEWAYDEV=eth0.100
        
        
6. The VLAN configuration on the server can be verified in the file /proc/net/vlan/config. Sample contents are shown below.
    
        VLAN Dev name    | VLAN ID
        Name-Type: VLAN_NAME_TYPE_RAW_PLUS_VID_NO_PAD
        eth0.100       | 100  | eth0
        eth0.200       | 200  | eth0