Vlan VPN TPID mode.
The value 'port-based' means VLAN VPN TPID value would be set based
on port via hwifVlanVPNTPID. In this situation, hwVlanVPNTPID is
meaningless and always return 0x8100.
The value 'global' means VLAN VPN TPID value should be set globally
via hwVlanVPNTPID. In this situation, hwifVlanVPNTPID in hwethernetTable
has the same value with hwVlanVPNTPID.