This is a problem that showed itself when upgrading our Proxmox 3.2 Nodes up to Proxmox 4. About halfway through the upgrade, our network adapters suddenly stopped being able to communicate with any local addresses, but could still ping outside addresses.

The cause was a minor config change that gets added in pretty stealthy. When this happens, simply add the following line to the bridge config in /etc/network/interfaces:

bridge_vlan_aware yes

To make the entire config section resemble:

auto vmbr0
iface vmbr0 inet static
    bridge_ports bond0
    bridge_stp off
    bridge_fd 0
    bridge_vlan_aware yes

This was pretty subtle to find, but after completing the upgrade and logging into the new Proxmox interface, it’s an option under the Network settings for that host.