ZyXEL Communications ZyWALL 110 Handbook page 394

www.zyxel.com
Make sure the both ZyWALL/USG at the HQ and Branch sites 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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