When you troubleshoot L2TP/IPSec connections, it is useful to understand how an L2TP/IPSec connection proceeds. When you start the connection, an initial L2TP packet is sent to the server, requesting a connection. This packet causes the IPSec layer on your computer to negotiate with the VPN server to set up an IPSec protected session (a security association). Depending on a number of factors including link speed, the IPSec negotiations may take from a few seconds to around two minutes. When an IPSec security association (SA) has been established, the L2TP session starts. When it starts, you receive a prompt for your name and password (unless the connection has been setup to connect automatically in Windows Millennium Edition.) If the VPN server accepts your name and password, the session setup completes.
A common configuration failure in an L2TP/IPSec connection is a misconfigured or missing certificate, or a misconfigured or missing preshared key. If the IPSec layer cannot establish an encrypted session with the VPN server, it will fail silently. As a result, the L2TP layer does not see a response to its connection request. There will be a long delay (typically 60 seconds), and then you may receive an error message that says that there was no response from the server or that there was no response from the modem or communication device. If you receive this error message before you receive the prompt for your name and password, IPSec did not establish its session. If that occurs, examine your certificate or preshared key configuration, or send the isakmp log to your network administrator.
A second common problem that prevents a successful IPSec session is using a Network Address Translator (NAT). Many small networks use a router with NAT functionality to share a single Internet address among all the computers on the network. The original version of IPSec drops a connection that goes through a NAT because it detects the NAT's address-mapping as packet tampering. Home networks frequently use a NAT. This blocks using L2TP/IPSec unless the client and the VPN gateway both support the emerging IPSec NAT-Traversal (NAT-T) standard. For more information, see the "NAT Traversal" section.
If the connection fails after you receive the prompt for your name and password, the IPSec session has been established and there is probably something wrong with your name and password. Other server settings may also be preventing a successful L2TP connection. If they are, send the PPP log to your administrator.
NAT TraversalWith the IPSec NAT-T support in the Microsoft L2TP/IPSec VPN client, IPSec sessions can go through a NAT when the VPN server also supports IPSec NAT-T. IPSec NAT-T is supported by Windows Server 2003. IPSec NAT-T is also supported by Windows 2000 Server with the L2TP/IPSec NAT-T update for Windows XP and for Windows 2000.
For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
- Right-click the Dialup Networking folder, and then click Properties.
- Click the Networking tab, and then click to select the Record a log file for this connection check box.
Article ID: 325034 - Last Review: Jun 19, 2014 - Revision: 1