# /etc/systemd/system/tb-bridge.service
[Unit]
Description=Ensure thunderbridge exists early
DefaultDependencies=no
After=network-pre.target
Before=network.target

[Service]
Type=oneshot
RemainAfterExit=yes
# Create only if it doesn't exist
ExecStart=/bin/sh -c '/sbin/ip link show thunderbridge >/dev/null 2>&1 || /sbin/ip link add thunderbridge type bridge'
# Set params every time (harmless if already set)
ExecStart=/sbin/ip link set thunderbridge mtu 65520
ExecStart=/sbin/ip link set thunderbridge up

[Install]
WantedBy=multi-user.target