dual port cross over cable bonding

[Adam Weremczuk <adamw@matrixscience.com>]

Hello,

I have 2 servers running Proxmox 5.2 (based on Debian 9).

I've connected Ethernet ports between them with a pair of cross over cables.

When only 1 port on each and 1 cable are used connectivity looks fine 
with the following config:

-------------------------------------------------------

auto ens1f0
iface ens1f0 inet static
    address 192.168.200.1(2)
    netmask 255.255.255.252

-------------------------------------------------------

Then I connected 2 cables and attempted link aggregation:

-------------------------------------------------------

iface ens1f0 inet manual

iface ens1f1 inet manual

auto bond1
iface bond1 inet static
    slaves ens1f0 ens1f1
    address 192.168.200.1(2)
    netmask 255.255.255.252
    bond_miimon 100
#    bond_mode 802.3ad
    bond_mode balance-rr
    bond_xmit_hash_policy layer3+4

-------------------------------------------------------

Tried both 802.3ad and balance-rr modes.

AFAIK only these 2 provide link aggregation.

Ethtool appears to be happy:

ethtool bond1
Settings for bond1:
    Supported ports: [ ]
    Supported link modes:   Not reported
    Supported pause frame use: No
    Supports auto-negotiation: No
    Advertised link modes:  Not reported
    Advertised pause frame use: No
    Advertised auto-negotiation: No
    Speed: 2000Mb/s
    Duplex: Full
    Port: Other
    PHYAD: 0
    Transceiver: internal
    Auto-negotiation: off
    Link detected: yes

Unfortunately in either mode cross pinging fails with "Destination Host 
Unreachable".

Own interfaces ping ok.

The same configuration works fine against managed switch ports (LACP/LAG).

So my question is why this is not working and whether it's possible at all?

Regards,
Adam