I've been trying to figure this out for a few days. Perhaps the biggest problem with figuring out the answer is that there are no unique keywords that I can think of to explain the issue.
I have Virtualbox installed on a Solaris 11 box. I'm running Windows 2008 as a guest with a bridged network adapter. All works fine, but now I want to run OpenVPN in TAP mode on my guest meaning that my Virtual Machine must bridge the tap adapter to the virtual adapter. For some reason this won't work on Virtualbox. The closest thing I could find to a suggestion was to adjust the MTU on the Windows bridge to 1467, but that doesn't work either. The bridge is able to pick up an ip address from DHCP, but not actually send/receive any traffic.
Any suggestions on how to bridge a bridged connection? This works on VMware so I was really hoping to be able to do it on Virtualbox.