Howto: Configure Linux Virtual Local Area Network (VLAN)

VLAN is an acronym for Virtual Local Area Network. Several VLANs can co-exist on a single physical switch, which are configured via software (Linux commands and configuration files) and not through hardware interface (you still need to configure switch).

Hubs or switch connects all nodes in a LAN and node can communicate without a router. For example, all nodes in LAN A can communicate with each other without the need for a router. If a node from LAN A wants to communicate with LAN B node, you need to use a router. Therefore, each LAN (A, B, C and so on) are separated using a router.

VLAN as a name suggest combine multiple LANs at once. But what are the advantages of VLAN?

    * Performance
    * Ease of management
    * Security
    * Trunks
    * You don’t have to configure any hardware device, when physically moving server computer to another location etc.

VLAN concepts and fundamental discussion is beyond the scope of this article. I am reading following textbooks. I found these textbooks extremely useful and highly recommended:

    * Cisco CNNA ICND books (part I and II)
    * Andrew S. Tanenbaum, Computer Networks book

Configuration problems

I am lucky enough to get couple of hints from our internal wiki docs :D.

    * Not all network drivers support VLAN. You may need to patch your driver.
    * MTU may be another problem. It works by tagging each frame i.e. an Ethernet header extension that enlarges the header from 14 to 18 bytes. The VLAN tag contains the VLAN ID and priority. See Linux VLAN site for patches and other information.
    * Do not use VLAN ID 1 as it may be used for admin purpose.

Ok now I need to configure VLAN for RHEL. (note due to some other trouble tickets I was not able to configure VLAN today, but tomorrow afternoon after lunch break ill get my hands on dirty with Linux VLAN ;) )
VLAN Configuration

My VLAN ID is 5. So I need to copy file /etc/sysconfig/network-scripts/ifcfg-eth0 to /etc/sysconfig/network-scripts/ifcfg-eth0.5

# cp /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/ifcfg-eth0.5

So I have one network card (eth0) and it needs to use tagged network traffic for VLAN ID 5.

Above files will configure Linux system to have:

    * eth0 - Your regular network interface
    * eth0.5 - Your virtual interface that use untagged frames

Do not modify /etc/sysconfig/network-scripts/ifcfg-eth0 file. Now open file /etc/sysconfig/network-scripts/ifcfg-eth0.5 using vi text editor:

# vi /etc/sysconfig/network-scripts/ifcfg-eth0.5

Find DEVICE=ifcfg-eth0line and replace with:

DEVICE=ifcfg-eth0.5

Append line:

VLAN=yes

Also make sure you assign correct IP address using DHCP or static IP. Save the file. Remove gateway entry from all other network config files. Only add gateway to /etc/sysconfig/network file.

Restart network:

# /etc/init.d/network restart

Please note that if you need to configure for VLAN ID 2 then copy the copy file /etc/sysconfig/network-scripts/ifcfg-eth0 to /etc/sysconfig/network-scripts/ifcfg-eth0.2 and do the above procedure again.
Using vconfig command

Above method is perfect and works with Red hat enterprise Linux w/o problem. However you will notice that there is a command called vconfig. The vconfig program allows you to create and remove vlan-devices on a vlan enabled kernel. Vlan-devices are virtual ethernet devices which represents the virtual lans on the physical lan.

Please note that this is yet another method of configuring VLAN. If you are happy with above method no need to read below.

Add VLAN ID 5 with follwing command for eth0:

# vconfig add eth0 5

add command creates a vlan-device on eth0 which result into eth0.5 interface. You can use normal ifconfig command to see device information:

# ifconfig eth0.5

Use ifconfig to assigne IP address:

# ifconfig eth0.5 192.168.1.100 netmask 255.255.255.0 broadcast 192.168.1.255 up

Get detailed information about VLAN interface:

# cat /proc/net/vlan/eth0.5

If you wish to delete VLAN interface delete command:

# ifconfig eth0.5 down
# vconfig rem eth0.5

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