ZyXEL Communications ZyWALL 110 Handbook page 299

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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 VPN Connection rule. This should be set to
IPSec_VPN Zone so that security policies are applied properly.
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