Ipsec needs UDP port 500 + ip protocol 50 and 51 - but you can use NAt-T instead, which needs UDP port 4500. On the other hand L2TP uses udp port 1701. If you trying to pass ipsec traffic through a "regular" Wi-Fi router and there is no such option as IPSec pass-through, I recommend opening port 500 and 4500.
Protocol: AH, value 51 (for IPSEC) Also, Port 1701 is used by the L2TP Server, but connections should not be allowed inbound to it from outside. There is a special firewall rule to allow only IPSEC secured traffic inbound on this port. Solved: Port Forwarding for L2TP/IPSec VPN Behind Verizon I installed an L2TP/IPSec VPN server, tested internally and it connected successfully. So for all intents & purposes, this validates that the VPN server is correctly configured to accept inbound connections and functioning correctly. I logged into the Verizon Actiontec MI424WR router, setup port forwarding for UDP ports 500, 1701 & 4500. How to Configure IPSec on Windows - The Back Room Tech May 13, 2019 IPsec VPN Overview - TechLibrary - Juniper Networks SRX Series,vSRX. IPsec VPN Overview, IPsec VPN Topologies on SRX Series Devices, Comparison of Policy-Based VPNs and Route-Based VPNs, Understanding IKE and IPsec Packet Processing, Understanding Phase 1 of IKE Tunnel Negotiation, Understanding Phase 2 of IKE Tunnel Negotiation, Supported IPsec and IKE Standards, Understanding Distributed VPNs in SRX Series Services Gateways …
IKEv2 VPN is a standards-based IPsec VPN solution that uses outbound UDP ports 500 and 4500 and IP protocol no. 50. Firewalls do not always open these ports, so there is a possibility of IKEv2 VPN not being able to traverse proxies and firewalls.
UniFi - USG/UDM VPN: How to Configure Site-to-Site VPN
How to Enable VPN Passthrough - IPsec Firewall Port - Tom
I have tried to test these VPN connections from a Windows box located elsewhere, and have found out that with Windows' VPN client I can only set up the type of VPN (along with auth factors etc) but not server port when it comes to L2TP/IPsec connections. Configuring IPsec Virtual Private Networks