IKE Packet Retransmit error – Using Zyxel IPSec VPN Client and Zyxel Zywall 35 firewall

If you are trying to create a VPN tunnel between a PC or laptop to a server on another network and after creating the tunnel you are unable to connect the two sides, check the firewall or router logs to see the error messages; If you get “IKE Packet retransmit” it means that you need to
A) ENABLE IP SEC Passthrough
on the firewall/router ((You do this by logging on to the firewall/ router where your PC or laptop is connected to)
OR
B) FORWARD port 500 to you PC or laptop (You also do this by logging on to the firewall/ router where your PC or laptop is connected to)

NOTE the “OR” – You don’t need to perform both actions to get rid of the error; You choose option B only if the firewall/router applicance doesn’t have option A built into its software.

Case in point: a Zyxel IPSec VPN client version 2.4.204.61,003 software being configured on a PC to connect to a server on another network.
NOTE The network where the server is located is behind a Zyxel Zywall 35 firewall. So the client resides on the PC or laptop and the “VPN server” is located on the server side of the connection
by accessing the firewall/router ip address via web interface