Troubleshooting L2TP

The VPN connection setup process is complex. The following presents an analysis of some common faults that might occur in the process. Before troubleshooting the VPN, make sure the LAC and LNS are connected correctly across the public network.

Users cannot log in.

Possible reasons for login failure include:

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

  • The address of the LNS is set incorrectly on the LAC.

  • No L2TP group is configured on the LNS (usually a router) to receive calls from the tunnel peer. For details, see the description of the allow command.

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

  • If the tunnel is torn down by force on the local end but the remote end has not received the notification packet for reasons such as network delay, a new tunnel cannot be set up.

• PPP negotiation failure, which might occur because:

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

  • The LNS cannot allocate addresses. This might be because the address pool is too small or no address pool is configured.

  • The authentication type is inconsistent. For example, the default authentication type for a VPN connection created on Windows 2000 is Microsoft Challenge Handshake Authentication Protocol (MS-CHAP), and if the remote end does not support MS-CHAP, the PPP negotiation will fail. In this case, CHAP is recommended.

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

Possible reasons for data transmission failure include:

• No route is available. The LAC (or LAC client) must have a route to the private network behind the LNS, and the LNS must have a route to the private network behind the LAC. Otherwise, data transmission fails. You can use the display ip routing-table command on the LAC (LAC client) 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 packet loss ratio is high. L2TP data transmission is based on UDP, which does not provide the packet error control function. If line quality is unstable, the LAC and LNS might be unable to ping each other and L2TP applications might fail.