ZyXEL Communications VPN Series Handbook page 171

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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.
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