ZyXEL Communications VPN Series Handbook page 58

Go to CONFIGURATION > VPN > IPSec VPN > VPN Gateway > Show Advanced
Settings. Configure Authentication > Peer ID Type as Any to let the ZyWALL/USG
does not require to check the identity content of the remote IPSec router.
CONFIGURATION > VPN > IPSec VPN > VPN Gateway > Show Advanced
Settings > Authentication > Peer ID Type
Go to CONFIGURATION > VPN > IPSec VPN > VPN Connection > Show Advanced
Settings > Policy. Select Enable GRE over IPSec.
CONFIGURATION > VPN > IPSec VPN > VPN Connection > Show Advanced
Settings > Policy
The GRE tunnel runs between the IPsec public interface on the Branch unit and
the HQ unit. Go to CONFIGURATION > Network > Interface > Tunnel > Add. Enter
the Interface Name (The format is tunnelx, where x is 0 - 3.). Enter the IP Address
and Subnet Mask for this interface. Specify My Address to be the interface or IP
address to use as the source address for the packets this interface tunnels to the
remote gateway. Enter Remote Gateway Address to be the IP address or
domain name of the remote gateway to this tunnel traffic.
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