Today I realized that my Zyxel USG 20 would not automatically get an IP address (assigned by DHCP) from my cable modem after being disconnected. (I have a dynamic IP address from my ISP.) I would have to log in to the dashboard and click the “Renew” button under “Interface Status Summary” to get a new IP address. This is a hassle after a power outage. (This is not an issue if you have a static IP address.)
There is very easy fix for this. Add a rule to the Firewall for port 68 UDP to the Zywall. The Zyxel already has a defined service for this port/protocol called BOOTP_CLIENT. See picture below.
That’s it! Now the USG 20 will get an IP automatically after a reboot.