ZyXEL Communications ZyWALL 110 Handbook page 168

www.zyxel.com
Make sure the service HTTPS Port on IPSec VPN Client application is available.
Make sure the To-ZyWALL security policies allow IPSec VPN traffic to the
ZyWALL/USG. IKE uses UDP port 500, AH uses IP protocol 51, and ESP uses IP
protocol 50.
The ZyWALL/USG supports UDP port 500 and UDP port 4500 for NAT traversal. If you
enable this, make sure the To-ZyWALL security policies allow UDP port 4500 too.
168/749