Problems with NIC bonding

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Hi.

Continuing to explore Proxmox I'm trying to setup the networks as I'd like it.

I have two boxes with 4 NICs each. A Supermicro X10SDV-6C and a Supermicro A1SAi-2550.

Two of the NICs on each machine are wired to a switch.

The other two are wired directly between the two machines.

Now, I'd like to create a bond for each set of wires using LACP.

On the X10SDV this is no problem. I have two bond-interfaces both working perfectly.

But on the A1SAi I can only get one bond to work at a time.

auto bond0
iface bond0 inet manual
        slaves enp0s20f0 enp0s20f1
        bond_miimon 100
        bond_mode 802.3ad

auto vmbr0
iface vmbr0 inet static
        address  10.3.0.32
        netmask  255.255.0.0
        gateway  10.3.0.1
        bridge_ports bond0
        bridge_stp off
        bridge_fd 0

As soon as I add bond1-interface, vmbr0 stops working:

auto bond1
iface bond1 inet manual
        slaves enp0s20f2 enp0s20f3
        bond_miimon 100
        bond_mode 802.3ad

auto bond0
iface bond0 inet manual
        slaves enp0s20f0 enp0s20f1
        bond_miimon 100
        bond_mode 802.3ad

auto vmbr0
iface vmbr0 inet static
        address  10.3.0.32
        netmask  255.255.0.0
        gateway  10.3.0.1
        bridge_ports bond0
        bridge_stp off
        bridge_fd 0

Any ideas?