L2TP errors from behind USG20-VPN

christopher

Happy new year!

I am stumped on an issue I'm having with Android/iOS L2TP clients connecting to a Zywall USG20 from behind a USG20-VPN. The initial connections is connecting fine but after a few minutes to hours, the traffic will stop flowing on the L2TP session. When checking the USG20-Zywall log, there are a bunch of ipsec errors such as no policy found, dropping TCP/UDP/ESP/NAT-T packets. When checking the USG20-VPN log, it looks like it is responding to the IKE request without having any VPN's configured. I went down to the default config and still see ipsec messages in the log. When using a Windows 10 L2TP connections, there are no drops and works fine.

The traffic seams to stop right after the Zywall USG 20 sends a SEND HASH NOTIFY R U THERE. once that appears then the dropped packet message start rolling in with strange port number like 1024-1027.

When using a consumer router, no issues appear.

Diagram

Android Phone/tablet -----> Access Point ------> USG20-VPN ------>USG20-Zywall-----> LAN

Zyxel_Charlie

Hello christopher,
I want to confirm with you that in this scenario, the win10 can establish l2tp vpn with USG20, but Android/IOS cell phone have issue? If so, could you please check that can cell phone establish l2tp connection with public IP?(Cell phone connect with l2tp server directly)
Also, I want you check have you enable "NAT-T" in the USG ZyWALL's IPSec VPN Gateway setting. 
Moreover, to analyze this case, when the issue occur, please screenshot complete log.

Charlie