Troubleshooting L2TP

The remote system cannot access the private network.

Possible reasons for the access failure include the following:

• Tunnel setup failure, which might occur in the following cases:

  • The address of the LNS is set incorrectly on the LAC (see the lns-ip command).

  • No L2TP group is configured on the LNS to receive tunneling requests from the tunnel peer (see the allow command).

  • Tunnel authentication fails. Tunnel authentication must be enabled on both the LAC and LNS, and the tunnel authentication keys configured on the two sides must match.

• PPP negotiation failure, which might occur for the following reasons:

  • Usernames, passwords, or both are incorrectly configured on the LAC or are not configured on the LNS.

  • The LNS cannot allocate addresses. In this case, check whether IP address negotiation settings are correct on the remote system and LNS.

  • The authentication type is inconsistent. For example, if the peer does not support MS-CHAP (the default authentication type for a VPN connection created on Windows 2000), the PPP negotiation will fail. In this case, change the authentication type to CHAP on Windows 2000.

Data transmission fails. A connection is established, but data cannot be transmitted. For example, the LAC and LNS cannot ping each other.

Possible reasons for the data transmission failure are as follows:

• No route is available. The LAC must have a route to the private network behind the LNS, and vice versa. Otherwise, data transmission fails. You can use the display ip routing-table command on the LAC and LNS to check whether the expected routes are present. If not, configure a static route, or configure a dynamic routing protocol.

• Congestion occurs on the Internet backbone, and the packet loss ratio is high. L2TP data transmission is based on UDP, which does not provide the packet error control feature. If the line is unstable, the LAC and LNS might be unable to ping each other.