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CLI-based Configuration Guide - VPN

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R010

This document describes VPN features on the device and provides configuration procedures and configuration examples.
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Data Transmission Fails After L2TP Connections Are Established

Data Transmission Fails After L2TP Connections Are Established

Symptoms

An L2TP connection is established, but data transmission fails. The remote user cannot ping IP addresses on the private network segment in the enterprise headquarters.

Troubleshooting Procedure

The possible causes and corresponding troubleshooting methods are as follows:
  • No route to the private network segment in the enterprise headquarters is configured on the LNS.

    Run the display ip routing-table command on the LNS to check the routing table.

  • Network congestion occurs.

    L2TP is UDP-based, but UDP does not perform error control on packets. When L2TP is enabled on an unstable network, the ping to the remote end may fail.

  • The LNS cannot identify the format of received PPP packets after the PPP negotiation between the LAC and remote user.

    Generally, the LNS can identify the format of received PPP packets after the PPP negotiation between the LAC and remote user. If PPP packets after the PPP negotiation between the LAC of some vendors and remote user are compressed, the LNS may not be able to identify these packets. In this case, the remote user cannot ping IP addresses on the private network segment in the enterprise headquarters. If this problem occurs, run the mandatory-lcp command on the LNS to enable LCP renegotiation. The remote user and LNS can then start PPP negotiation again, and the LNS can identify PPP packets sent by the remote user.

  • The NAT ALG function is disabled for DNS.

    Run the display nat alg command on the device to check whether NAT ALG is enabled for DNS.
    [Huawei]display nat alg 
    NAT Application Level Gateway Information:                                      
    ----------------------------------                                              
      Application            Status                                                 
    ----------------------------------                                              
      dns                    Disabled                                               
      ftp                    Disabled                                               
      rtsp                   Enabled                                                
      sip                    Disabled                                               
      pptp                   Disabled                                        
    ---------------------------------- 

    If NAT ALG is disabled, run the nat alg command to enable it.

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Updated: 2019-08-07

Document ID: EDOC1100033725

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