Ethernet card locking up when acting as virtual bridge

Im trying to use a 3Com Etherlink Model 3C905C to provide network access for some virtual machines running under QEMU.

The machine has a Realtek Gigabit Ethernet controller on the motherboard which I use solely for the hosts network interface. I've added a 3Com PCI card to act as the interface solely for the VMs allowing them to have IPs on the real network rather than using QEMUs NAT.

My configuration is below. Initially it worked fine, except that once in a while the card would seemingly 'lock up' i.e no VMs could get network access but unplugging and replugging the Cat 5 cable fixed it.

Recently however the issue has been occuring more and more, sometimes some VMs can get network access and not others. Ive tried swapping the card for an identical model but its made no difference. Im not sure if it something to do with my config (maybe the made up MAC addresses?) or if its a bug in the cards driver or firmware.

Config is below eth0 is the hosts Realtek port, enp3s0 is the 3Com used for the VMs

The host OS is Stretch

Am I doing something wrong or is this a bug?

Many thanks

======================

/etc/network/interfaces:

# The loopback network interface

auto lo

iface lo inet loopback

auto eth0

iface eth0 inet static

address 192.168.253.201

netmask 255.255.255.0

broadcast 192.168.253.255

dns-nameservers 192.168.253.254

up /sbin/route add default gw 192.168.253.254 dev eth0

auto enp3s0

auto br1

iface br1 inet dhcp

bridge_ports enp3s0

bridge_stp off

bridge_fd 0

bridge_maxwait 0

======================

The script I use to start QEMU contains lines like this for each VM:

#host debian9test vnc on port 5902, give it virtual hub port br1p1

tunctl -u root -t br1p1 #create TAP device br1p1

brctl addif br1 br1p1 #add br1p1 as a port on bridge br1

kvm -hda /var/qemuvm/debian9test.img -m 1024 -usb \

-net nic,macaddr=52:54:00:00:00:02 -net tap,ifname=br1p1 -vnc 192.168.253.201:2 &

#host bitsafe vnc on port 5904, give it virtual hub port br1p2

tunctl -u root -t br1p2 #create TAP device br1p2

brctl addif br1 br1p2 # add br1p2 as a port on bridge br1

kvm -hda /dev/disk/by-id/ata-TOSHIBA_HDWJ110_37K1P2AET \

-hdb /dev/disk/by-id/ata-TOSHIBA_HDWJ110_37UNTL5IT \

-m 1024 -usb -net nic,macaddr=52:54:00:00:00:04 \

-net tap,ifname=br1p2 -vnc 192.168.253.201:4 &

#host gatekeeper (RADIUS) vnc on port 5905, give it virtual hub port br1p3

tunctl -u root -t br1p3 #create TAP device br1p3

brctl addif br1 br1p3 #add br1p3 as a port on bridge br1

kvm -hda /var/qemuvm/debian-gatekeeper.img \

-m 1024 -usb -net nic,macaddr=52:54:00:00:00:05 \

-net tap,ifname=br1p3 -vnc 192.168.253.201:5 &

======================

Output from ifconfig:

br1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500

inet 192.168.253.80 netmask 255.255.255.0 broadcast 192.168.253.255

inet6 fe80::250:daff:fe3b:9a4d prefixlen 64 scopeid 0x20<link>

ether 00:50:da:3b:9a:4d txqueuelen 1000 (Ethernet)

RX packets 22633 bytes 1224397 (1.1 MiB)

RX errors 0 dropped 9 overruns 0 frame 0

TX packets 159 bytes 18064 (17.6 KiB)

TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

br1p1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500

ether 76:73:87:89:3c:f1 txqueuelen 1000 (Ethernet)

RX packets 1391 bytes 97413 (95.1 KiB)

RX errors 0 dropped 0 overruns 0 frame 0

TX packets 22130 bytes 1514391 (1.4 MiB)

TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

br1p2: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500

ether 82:e0:2d:0d:d8:26 txqueuelen 1000 (Ethernet)

RX packets 4416 bytes 337769 (329.8 KiB)

RX errors 0 dropped 0 overruns 0 frame 0

TX packets 25436 bytes 1728395 (1.6 MiB)

TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

br1p3: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500

ether 26:67:8d:04:5c:b9 txqueuelen 1000 (Ethernet)

RX packets 329 bytes 38930 (38.0 KiB)

RX errors 0 dropped 0 overruns 0 frame 0

TX packets 22583 bytes 1553522 (1.4 MiB)

TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

enp3s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500

ether 00:50:da:3b:9a:4d txqueuelen 1000 (Ethernet)

RX packets 27632 bytes 2188088 (2.0 MiB)

RX errors 0 dropped 0 overruns 0 frame 0

TX packets 6303 bytes 493295 (481.7 KiB)

TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

device interrupt 20 base 0x5000

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500

inet 192.168.253.201 netmask 255.255.255.0 broadcast 192.168.253.255

inet6 fe80::21f:d0ff:fe08:8bc6 prefixlen 64 scopeid 0x20<link>

ether 00:1f:d0:08:8b:c6 txqueuelen 1000 (Ethernet)

RX packets 22949 bytes 20198348 (19.2 MiB)

RX errors 0 dropped 0 overruns 0 frame 0

TX packets 24046 bytes 1880515 (1.7 MiB)

TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536

inet 127.0.0.1 netmask 255.0.0.0

inet6 ::1 prefixlen 128 scopeid 0x10<host>

loop txqueuelen 1 (Local Loopback)

RX packets 26302 bytes 1783961 (1.7 MiB)

RX errors 0 dropped 0 overruns 0 frame 0

TX packets 26302 bytes 1783961 (1.7 MiB)

TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0