I'm using KVM and libvirt on my host (Debian lenny) + 2 bridges per guest (one for mgmt, one for public traffic).
That setup isn't stable at all, sometimes I can do pings to a management ip, sometimes not.
I don't know if my bridging paramateres are correct, could you check ? or if there is anything wrong ...
Please also note that interface on guest doesn't flap and that I got not logs on my host.
Of course forwarding is enabled.
iface eth3 inet manual
iface bond0 inet manual
slaves eth1 eth2
pre-up ip link set bond0 up
down ip link set bond0 down
iface br0 inet static
iface br0:1 inet static
iface br0:2 inet static
iface br1 inet static
pre-up /etc/network/firewall start
iface br1:1 inet static
iface br1:2 inet static