Linux KVM (virt) with DHCP and bridging

[mailers at oranged.to]

Hello,

I have been having an extensive problem which I have been bang my head against for weeks. I have a system which I have multi-boot where my default boot is ubuntu. I have all the virtualization installed and running with a few major headaches. The environment is a corporate network where we can't use static IP's. So I have the DHCP setup to work. If I boot the system with Windows 7 or just native Linux it tends to get an IP address first go. However when I boot into the virtualized environment which has a bridge setup it rarely if ever gets an IP address and all of the virtual machines running on that system also don't get IP addresses.

The bridge is setup like so in /etc/network/interfaces

auto br0
iface br0 inet dhcp
        pre-up /usr/sbin/ethtool -s eth0 speed 100 duplex full autoneg off
        bridge_ports eth0
        bridge_fd 9
        bridge_hello 2
        bridge_maxage 12
        bridge_stp off

Which all seems fine.. the link state is all fine there are no errors on the switch. But we still can't get an IP address despite it working perfectly in Windows 7 or default Linux. There are no iptables rules or anything like that which would possibly cause problems. If I run 

sudo tcpdump -i br0 -vvv -s0 ether host 54:52:00:22:d7:8f

I can see that the DHCP requests are going out without any problems but I generally get an IP Address assigned then followed by a NAK. I've disabled the DHCP server and installed a variety of other dhcp servers to identify where the problem is and I get the same problems no matter what the configuration is.. Has anyone seen similar issues or knows where I can start to look further to diagnose the problems?

Any assistance or advice would be really appreciated.

Regards,

Jimmy.