One of my customers wanted to setup Vmware server on top of CentOS 4.5 on a IBM HS20 (in an IBM BladeCenter). The guest machines were unable to connect to the network (or just briefly, like 3 or 4 icmp packets) but were able to at least see remote machines mac addresses (as shown through arp). On the other hand, everything was running smoothly in NAT mode . I had a look on upstream Cisco Switches configs but everything was ok ... So i decided to update the firmware of the built-in Broadcom Fiber ethernet cards and after that everything went back to normal in bridge mode. So it seems that these devices didn't support correctly the bridging mode out-of-the-box and i assume that it should have been the same with other virt products (qemu, kvm, xen, etc ...). It reminds me a colleague some years ago : when you told him that you had a problem, his answer was always : 'Have you updated the BIOS ?' simple but it works often ... ;-)