For the IPSEC VPN tunnel to be established, you and the peer must agree on a few parameters; these are the following:
- Endpoint IP addresses of one another
- Encryption algorithm to use
- Peer subnets and local subnets that will be secured over the VPN
- Authentication type and parameter—in this case, a Pre-Shared Key
- Diffie-Hellman group
Once these parameters are agreed upon by both ends, each endpoint will go through phase 1 and 2 of establishing the VPN tunnel. If one of these parameters does not match on either side of the tunnel, then the tunnel will fail to be established.
It is commonly overlooked, but IPSEC VPN tunnels can only be created on the IP address associated to an Uplink interface and do not support internal interface ...
