VLAN Configuration Issue on Proxmox When USB NIC is Disconnected

[jenssgb]

Hi all,
I'm experiencing an issue with my Proxmox setup. I have two network interfaces configured: a USB network card (enx3c18a0d4f98b) with a static IP (192.168.254.30) and a VLAN-aware bridge (vmbr0) on the main NIC (enp0s31f6) with another static IP (192.168.254.210).

When I disconnect the network cable from the USB network card, I can no longer access the Proxmox web UI or SSH on 192.168.254.210. I have an LXC container in VLAN 10 that remains reachable. However, the Proxmox host becomes unreachable. The problem resolves when I reconnect the network cable to the USB network card.

Here is my /etc/network/interfaces configuration:

iface lo inet loopback

auto enx3c18a0d4f98b
iface enx3c18a0d4f98b inet static
    address 192.168.254.30/24

iface enp0s31f6 inet manual

auto vmbr0
iface vmbr0 inet manual
    bridge-ports enp0s31f6
    bridge-stp off
    bridge-fd 0
    bridge-vlan-aware yes
    bridge-vids 2-4094

auto vmbr0.10
iface vmbr0.10 inet static
    address 192.168.254.210/24
    gateway 192.168.254.254

auto vmbr0.20
iface vmbr0.20 inet manual

auto vmbr0.30
iface vmbr0.30 inet manual

auto vmbr0.60
iface vmbr0.60 inet manual

auto vmbr0.80
iface vmbr0.80 inet manual

auto vmbr0.100
iface vmbr0.100 inet manual

source /etc/network/interfaces.d/*

What could be causing this issue, and how can I ensure the primary NIC remains accessible even if the USB NIC is disconnected?

Thanks in advance for your help!