Move bridges to ix0 if up, and fall back to igb0 if ix0 goes down.

netfail.sh

root@tachikoma:/etc/devd # cat netfail.conf 
notify 20 {
    match "system"        "IFNET";
    match "subsystem"     "ix0";
    match "type"          "(LINK_DOWN|LINK_UP)";
    action "/root/net/netfail.sh $type";
};

root@tachikoma:/etc/devd # rm netfail.conf 
root@tachikoma:/etc/devd # cat /etc/
root@tachikoma:/etc/devd # cat /root/net/netfail.sh 
#!/bin/sh

activate_ix() {
  ifconfig bridge10 deletem igb0.10
  ifconfig bridge30 deletem igb0.30

  ifconfig bridge10 addm ix0.10
  ifconfig bridge30 addm ix0.30
}

activate_igb() {
  ifconfig bridge10 deletem ix0.10
  ifconfig bridge30 deletem ix0.30

  ifconfig bridge10 addm igb0.10
  ifconfig bridge30 addm igb0.30
}

if [ $1 = "LINK_DOWN" ]; then
  logger ix0 down, switching to igb0
  activate_igb
else
  logger ix0 up, waiting 60 seconds
  sleep 60
  activate_ix
fi

root@tachikoma:/etc/devd # cat netfail.conf 
notify 20 {
    match "system"        "IFNET";
    match "subsystem"     "ix0";
    match "type"          "(LINK_DOWN|LINK_UP)";
    action "/root/net/netfail.sh $type";
};