Systems and Interfaces Configuration Guide, Cisco IOS XE
Nov 19, 2019 How to enable a Cisco IPSec VPN client to connect to a Apr 19, 2018 Short guide to VPN port forwarding : What it is, How to do it Port forwarding is a technique used to enable incoming internet connections to reach your device when using a VPN. It is necessary because most VPNs use an NAT firewall to stop users falling victim to malicious incoming connections. This is a useful security feature, but it is unfortunately unable to distinguish between incoming connections you don’t want and those that you do. What ports does VPN use ? :: SG FAQ
A VPN port is a networking port that is commonly used in the VPN infrastructure. They are logical ports named using sequenced numbers and making it possible for VPN traffic to pass inside and outside of the server device or VPN client. The type and number of VPN ports used can and will depend on the kind of protocol being used.
What's happening here is that the actual IPSec traffic is being encapsulated in UDP (IP protocol 17). The native IPSec packet would have an IP protocol header-value of 50. Since 50 is neither UDP (17) or TCP (6), stupid NAT gateways will drop the packet rather than pass it. Secondly, since IPSec is neither TCP or UDP, it doesn't have a port-number. PPTP and L2TP Ports – Steven Eppler's Blog Dec 07, 2005 Change the default port of IPSec VPN | Fortinet Technical
How to Setup Windows 10 Firewall for L2TP/IPSec
May 20, 2003