Ales Nosek - The Software Practitioner

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Bridging VLAN Trunk to the Guest

In this article, we’re going to make an entire VLAN trunk on the host accessible to the guest machine. The guest machine can then create VLAN subinterfaces in order to access a particular VLAN.

Our host and guest machines are running RHEL7. We’re using Linux bridges and libvirt for guest and network configuration.

Bridge configuration on the host

On the host, the physical interface enp3s0f0 is a trunk interface including VLANs with tags 408, 410 and 412. We’ll create a new Linux bridge and add the enp3s0f0 to this bridge. The virtual machines created by libvirt will also be connected to this bridge. The configuration of the enp3s0f0 physical interface looks as follows:

/etc/sysconfig/network-scripts/ifcfg-enp3s0f0
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TYPE=Ethernet
BOOTPROTO=none
DEVICE=enp3s0f0
ONBOOT=yes
BRIDGE=br-enp3s0f0

Please, note that there’s no IP address configuration (neither static nor via DHCP) for the enp3s0f0 interface. The enp3s0f0 interface is a trunk interface and hence the IP configuration would make no sense here. The BRIDGE configuration variable connects the physical interface to the br-enp3s0f0 bridge. To create the br-enp3s0f0 bridge the following configuration file is needed:

/etc/sysconfig/network-scripts/ifcfg-br-enp3s0f0
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TYPE=Bridge
BOOTPROTO=none
DEVICE=br-enp3s0f0
ONBOOT=yes
DELAY=0

After the enp3s0f0 and br-enp3s0f0 configuration is in place you might want to restart the networking service using the command:

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sudo systemctl restart network

Creating a bridged network in libvirt

Next, we’re going to tell libvirt that there’s an existing bridge br-enp3s0f0 we’d like our virtual machines be connected to. First, let’s create a libvirt network definition file named just bridge.xml:

bridge.xml
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<network>
  <name>br-enp3s0f0</name>
  <forward mode='bridge'/>
  <bridge name='br-enp3s0f0' />
</network>

To create a libvirt network based on the above definition, type:

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sudo virsh net-define bridge.xml

We’d like libvirt daemon to start the network automatically on the startup:

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sudo virsh net-autostart br-enp3s0f0

For the first time, we have to start the br-enp3s0f0 network manually:

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sudo virsh net-start br-enp3s0f0

If the above configuration went well, you will find the new network br-enp3s0f0 on the list of libvirt networks:

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$ virsh net-list
 Name                 State      Autostart     Persistent
----------------------------------------------------------
 br-enp3s0f0          active     yes           yes

Attaching a guest to the network

When creating a new guest (domain) in libvirt, you will need to attach the domain to the br-enp3s0f0 network. I’m not going to present the complete domain XML configuration here. You should include the following snippet in your domain definition in order to connect the domain to the br-enp3s0f0 network:

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<interface type='network'>
  <source network='br-enp3s0f0'/>
  <forward mode='route'/>
  <model type='virtio'/>
</interface>

Guest network configuration

After the guest machine boots up successfully, you can create VLAN subinterfaces in order to obtain access to the individual VLANs within the guest. First, let’s check the configuration of the VLAN trunk interface eth0 inside the guest:

/etc/sysconfig/network-scripts/ifcfg-eth0
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TYPE=Ethernet
BOOTPROTO=none
DEVICE=eth0
ONBOOT=yes

Finally, we can create VLAN subinterfaces to access individual VLANs available in the eth0 trunk. For example, to access VLAN 408 and obtain the IP configuration via DHCP you can create a new cofiguration file ifcfg-eth0.408:

/etc/sysconfig/network-scripts/ifcfg-eth0.408
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BOOTPROTO=dhcp
DEVICE=eth0.408
ONBOOT=yes
VLAN=yes

When you restart the networking service, your guest should successfully obtain an IP address on the VLAN 408:

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sudo systemctl restart network

Caveat

When experimenting with the Linux bridge configuration I made this observation: If there’s a VLAN subinterface defined for a specific VLAN on the host machine, this specific VLAN won’t be accessible inside the guest. For example, when I created the following VLAN 408 subinterface on the host:

/etc/sysconfig/network-scripts/ifcfg-enp3s0f0.408
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BOOTPROTO=none
DEVICE=enp3s0f0.408
ONBOOT=yes
VLAN=yes

As soon as I brought this interface up using:

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sudo ifup enp3s0f0.408

the eth0.408 VLAN subinterface in the guest stopped working.

References

When writing this blogpost I referred to the very useful article KVM & BRCTL in Linux – bringing VLANs to the guests describing the issues of VLAN bridging in a great detail.

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