Bridged network on 3.1.2 only works when eth0 provides 'media connect'

[madworm]

I used to work with a bridged setup going the complicated way with 'brctl' and a 'tap' interface some time ago.

On the host the bridge 'br0' got assigned an IP address e.g. 192.168.0.1/24, the bridge member devices 'eth0' and 'tap0' both got 0.0.0.0 as IP addresses. The VM was assigned an IP address in the same subnet as the host. This worked just fine regardless if the physical device 'eth0' was connected to a real switch or not (media connect). The bridge 'br0' essentially worked as a layer2 switch.

Trying to replicate this with 3.1.2 failed miserably at first. I couldn't ping both ways, firewall was off. As soon as 'eth0' is connected to a real switch, communication between the host and the VM works with the bridged setup. I then replaced the real switch by connecting 'eth0' to my printer (which supports auto MDX), and it still worked. My printer definitely doesn't work as a switch. Therefore I conclude that the VirtualBox bridge also works as a layer2 switch, but ONLY if the host interface 'eth0' provides 'media connect'. I'm not sure that makes sense.

version: 3.1.2 r56127 OS: openSUSE 11.2 (32bit)