Ubuntu Server 12.04.1 and Linux KVM on Fujitsu RX300 S5

[S`row-Lain]

Hello List,

we are running VMs in KVM on different Servers and everything is fine.
Now I wanted to set it up on Fujitsu RX300 S5 Servers, but using bridged
networking, all packets are dropped but to the host...

To explain it in more detail:
I install 12.04.1 Server AMD64 with default settings. Afterwards I update
the server with aptitude and set up the network like this:

auto br0
iface br0 inet static
         address 192.168....
         netmask 255.255.255.0
         network 192.168....
         broadcast 192.168....
         gateway 192.168....
         # dns-* options are implemented by the resolvconf package, if 
installed
         dns-nameservers ...
         dns-search intel.local
         bridge_ports    eth0
         bridge_stp      off
         bridge_maxwait  0
         bridge_fd       0

Network connection is fine for the Host. On other Hardware this setup 
runs smoothly on
Ubuntu Server 11.04 since more than a year...
If I setup a VM Guest (1204 Server) connected via a bridge (br0), the 
guest can only reach
the Host. All other packets seem to be dropped. If I use a NAT-NIC 
(default), the network
connection is fine.

I use virtio-drivers and the Server Hardware are two Fujitsu RX300 S5. 
Doesn't work on both
systems. I compared iptables-rules, route-entries, all identical to the 
other servers, where it
works just fine.
Does anybody have an idea, what it could be? I am really stuck =(
Do you need more info?

root at in3db:~# brctl show
bridge name    bridge id        STP enabled    interfaces
br0        8000.001999734942    no        eth0
                             vnet0
virbr0        8000.000000000000    yes
---
root at in3db:~# lspci | grep Ethernet
08:00.0 Ethernet controller: Intel Corporation 82575EB Gigabit Network 
Connection (rev 02)
08:00.1 Ethernet controller: Intel Corporation 82575EB Gigabit Network 
Connection (rev 02)

Cheers
Michael H.