In this post I will show you how to create subinterfaces on your CentOS server. It's easy and only takes a couple of minutes to setup, after that just trunk your VLAN's with 802.1q to the server.

I used CentOS 8 Stream as my OS, but any RHEL/CentOS related distribution will probably work just the same. I'll create two subinterfaces for VLAN 10 and 20. One will be with DHCP enabled and the other will be with a static IP configuration.

Enable 802.1q

First we must enable the 8021q kernel module. Below command will temporary enable the module until next reboot.

modprobe 8021q

Verify that the module loaded.

lsmod | grep 8021q

Run below command to make the operating system load 8021q on boot.

echo 8021q >> /etc/modules

Configure subinterfaces

Get the name of your network interface, mine is eno1 which I'll be using in this example. Locate the network configuration for the interface, located at /etc/sysconfig/network-scripts. The configuration file name would be ifcfg-<interface-name>.

Delete everything in that file, we dont need that, and replace it with below config. Remeber to replace eno1 with your own interface name.

TYPE=Ethernet
DEVICE=eno1
ONBOOT=yes
BOOTPROTO=none
NAME=eno1

Configure the subinterface for VLAN 10. Create a new file called ifcfg-eno1.10 and paste in the below config, this will setup the subinterface with DHCP. The last line, DEFROUTE=yes, tells the operating system to use this interface as your default route.

DEVICE=eno1.10
BOOTPROTO=dhcp
ONBOOT=yes
VLAN=yes
DEFROUTE=yes

Configure the subinterface for VLAN 20. Create a new file called ifcfg-eno1.20 and paste in the below config. This will setup the subinterface with a static IP configuration.

DEVICE=eno1.20
BOOTPROTO=none
ONBOOT=yes
IPADDR=10.0.0.2
PREFIX=24
GATEWAY=10.0.0.1
DNS1=8.8.8.8
DNS2=8.8.4.4
VLAN=yes

Restart Network Manager to enable the interfaces.

systemctl restart NetworkManager

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