802.1Q VLAN on Linux

The IEEE 802.1Q standard encapsulates Ethernet traffic in tagged VLANs. On a HolyCloud dedicated server with a trunk port, you separate management, production, and storage without multiple NICs.

Prerequisites

• VLANs provisioned on switch/datacenter side (IDs confirmed by HolyCloud)
• Physical interface connected in trunk mode (not access-only)
• Root access, network backup before changes
• SSH cutover risk: keep IPMI/KVM available

Concepts

[Switch] -- trunk (VLAN 100, 200) --> [eth0]
                                         ├── eth0.100 (public)
                                         └── eth0.200 (iSCSI storage)

The VLAN tag is added by the 8021q module.

Load the module

sudo modprobe 8021q
echo 8021q | sudo tee /etc/modules-load.d/vlan.conf

Manual creation (test)

ip link show eth0
sudo ip link add link eth0 name eth0.100 type vlan id 100
sudo ip addr add 10.10.100.2/24 dev eth0.100
sudo ip link set eth0.100 up
ping -c 2 10.10.100.1

Removal:

sudo ip link del eth0.100

Persistence: iproute2 (interfaces)

Debian /etc/network/interfaces:

auto eth0
iface eth0 inet manual

auto eth0.100
iface eth0.100 inet static
    address 203.0.113.50
    netmask 255.255.255.248
    gateway 203.0.113.49
    vlan-raw-device eth0

auto eth0.200
iface eth0.200 inet static
    address 192.168.200.10
    netmask 255.255.255.0
    vlan-raw-device eth0

sudo ifreload -a

Netplan

network:
  version: 2
  vlans:
    eth0.100:
      id: 100
      link: eth0
      addresses: [203.0.113.50/29]
      routes:
        - to: default
          via: 203.0.113.49
    eth0.200:
      id: 200
      link: eth0
      addresses: [192.168.200.10/24]

sudo netplan apply

systemd-networkd

/etc/systemd/network/10-eth0.network:

[Match]
Name=eth0

[Network]
VLAN=eth0.100
VLAN=eth0.200

/etc/systemd/network/10-eth0.100.netdev:

[NetDev]
Name=eth0.100
Kind=vlan

[VLAN]
Id=100

Associated .network file with Address=.

MTU and performance

Trunk + VLAN overhead: often MTU 1500 on public VLAN; storage may need jumbo frames (9000) if supported end-to-end.

ip link set eth0.200 mtu 9000

Validate with HolyCloud before jumbo on a shared link.

Firewall per VLAN

sudo nft add table inet filter
sudo nft add chain inet filter forward { type filter hook forward priority 0 \; }
# isolate: no forward between eth0.100 and eth0.200 except explicit rules

Or ufw on named interfaces:

ufw allow in on eth0.100 to any port 443
ufw deny in on eth0.200 from any to any

Verification

ip -d link show type vlan
cat /proc/net/vlan/eth0.100
tcpdump -i eth0 -e vlan

Common errors

| Error | Cause |

|--------|-------|

| No DHCP/routing | Wrong VLAN ID or access port |

| Double tag | Native VLAN misconfigured on switch |

| Unstable iSCSI | MTU mismatch, SAN filtering |

Migration without outage

1. Create the VLAN interface with a secondary test IP.
2. Validate connectivity from the management switch.
3. Move services to the target VLAN.
4. Remove the old IP on the untagged interface if no longer needed.

Need help?

Ask HolyCloud support for allowed VLAN IDs, port mode (trunk/access), and associated IP ranges before production configuration.