Ifupdown2 is a very useful interface configuration utility with tons of enhancements over the stock utility ifupdown. It was built with a specific initial use-case in mind which is for use on network operating systems (NOS) like Cumulus Linux. Cumulus requires LACP support as the primary bonding method. Other modes like active-backup (mode 1) were not initially fully implemented if ifupdown2. This is changing however; CM-14985 brings support for the bond-primary keyword and will be present in the next release of Cumulus Linux and the next version of Ifupdown2.
To hold you over until then here’s a workaround I’ve been using on my server at home running Ifupdown2 for performing active/backup bonding. Writing the sys file directly can provide the same behavior.
auto lo iface lo inet loopback auto enp4s0 iface enp4s0 alias Motherboard Ethernet mtu 9194 auto enxf01e341f95 iface enxf01e341f95 alias USB3 Ethernet mtu 9194 auto bond0 iface bond0 alias ActiveBackup Uplink bond-mode active-backup bond-slaves enxf01e341f95 enp4s0 address 192.168.1.10/24 gateway 192.168.1.1 mtu 9194 pre-up echo enp4s0 > /sys/class/net/bond0/bonding/primary