When l2 entities (such as STP bridges) conneted across a VPN network, l2 PDUs need delivering through VPN without being processed.To achieve this, users need enbling l2- tunneling on VPN's edge ports, i.e. ports connected to customer bridges. One 32bits width integer variable is used for each port to represent what kind of PDUs need tunneling when coming in this ports.Notes, only the six least-significant bits are used here ,each bit corresponds to one protocol, following is the detail: bit0 < bit1 < bit2 < bit3 < bit4 < bit5 < bit6-bit31 < when set one, corresponding PDU tunneling is asserted, deasserted otherwise.