L2TP/IPSec authentication issues

1. No certificate – Verify that both the remote access server and client exchange computer certificates for IPSec peer authentication. Check the computer certificate stores of both the remote access client and remote access server using the Certificates snap-in to ensure that a suitable certificate exists.
2. Incorrect certificate – Verify that the VPN client has a valid computer certificate installed that was issued by a CA that follows a valid certificate chain from the issuing CA up to a root CA that the VPN server trusts. Additionally, the VPN server must have a valid computer certificate installed that was issued by a CA that follows a valid certificate chain from the issuing CA up to a root CA that the VPN client trusts.
3. NAT issue - If there is a NAT between a Windows L2TP/IPSec client and a Windows L2TP/IPSec server, you cannot establish an L2TP/IPSec connection. IPSec NAT-T is not yet available for Windows 2000 from Microsoft.
4. Firewall Issue - If there is a firewall between a Windows  L2TP/IPSec client and a Windows L2TP/IPSec server, you cannot establish an L2TP/IPSec connection, verify that the firewall is allow L2TP/IPSec traffic to be forwarded.

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