ZyXEL Communications VPN Series Handbook page 279

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Make sure the ZyWALL/USG units' security policies allow IPSec VPN traffic. IKE uses
UDP port 500, AH uses IP protocol 51, and ESP uses IP protocol 50.
Verify that the Zone is set correctly in the Zone object. This should be set to
IPSec_VPN Zone so that security policies are applied properly.
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