ZyXEL Communications ZyWALL ATP Series Handbook page 323

11
If you cannot access devices in the local network, verify that the devices in the
local network set the USG's IP as their default gateway to utilize the L2TP tunnel.
12
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.
13
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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