The server is running the latest version of Debian 13. It has an Intel X540-AT2 10Gbps network adapter. The server is not running systemd-networkd or networkmanager. I have had a VLAN working on this server since Debian 11.

During some testing it was noticed that the VLAN interface, ens1.99, was no longer shown via ip addr or ip link. After much troubleshooting I have found the following, though it still doesn't work.

First I verified that the vlan package was still installed and that the 8021q module was loaded, and that neither was throwing errors in dmesg. Then I checked the logs. The networking.service journal has multiple errors but the first Cannot find device "ens1.99" entry is when the system was rebooted Dec 16. The current errors are as follows.

Jun 30 10:18:01 rat systemd[1]: Starting networking.service - Raise network interfaces...
Jun 30 10:18:01 rat ifup[3341]: Error: ipv4: Address already assigned.
Jun 30 10:18:01 rat ifup[3332]: ifup: failed to bring up ens1
Jun 30 10:18:01 rat ifup[3347]: Error: ipv4: Address already assigned.
Jun 30 10:18:01 rat ifup[3332]: ifup: failed to bring up ens1
Jun 30 10:18:01 rat ifup[3332]: ifup: could not bring up parent interface ens1
Jun 30 10:18:01 rat systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE

Note that the ens1 interface still comes up with the properly configured addresses. I tried manually flushing the ens1 config, with ip addr flush dev ens1, bringing the interface down, then bringing it back up via systemctl restart networking.

The interfaces file is shown below, with modified addresses of course. I tried removing the auto line for the vlan, removing the vlan_raw_device, and changing to a bridge config, all together and individually, and nothing worked.

auto lo
iface lo inet loopback

auto ens1
allow-hotplug ens1
iface ens1 inet static
    address 10.100.0.10/22
    gateway 10.100.0.1

iface ens1 inet6 static
    address ffff:1ee7:e1e7:feed::10/64
    gateway ffff:1ee7:e1e7:feed::1

auto ens1.99
iface ens1.99 inet static
    address 10.100.99.10/24
    vlan_raw_device ens1

None of my searches, or AI chat bots(eww), have helped any besides pointing to troubleshooting tips and finding 10+ year old posts whose solutions have already been tried. Do my fellow human sysadmins have any guesses?