www.zyxel.com
Make sure the both ZyWALL/USG and WatchGuard security policies allow IPSec
VPN traffic. IKE uses UDP port 500, AH uses IP protocol 51, and ESP uses IP protocol
50.
Default NAT traversal is enable on ZyWALL/USG, please make sure the remote
IPSec device must also have NAT traversal enabled.
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