I just upgraded one of my lab hosts to server 2012 R2, and I noticed something strange with my guests. The Server 2012 guests seemed fine, but none of the Server 2008 R2 guests could get internet access. I tried updating the integration services and restarting, but that didn't help. What DID work was booting the VM, going to the settings, disconnecting the network, and then reconnecting it. I tried rebooting the guest after this-same issue. Yellow exclamation in the systray (no internet access), but then unplug/replug the NW connection via the settings and it was OK until the next reboot.
This is a pretty straightforward lab setup. The host has an external network set up that's shared with the management network, and the management OS is VLAN tagged. The host and guest are both on the same VLAN (although I did not need to configure any sort of VLAN tagging when I set this up under 2012-that was taken care of by the virtual switch).
Any idea what's going on here? Is there something else I need to change in the virtual switch configuration after the upgrade?