3Com 3C13636 Configuration Manual page 1243

3Com Router 3000 Ethernet Family
Configuration Guide
2) On LNS side, L2TP group that can receive the remote end of the tunnel is not
configured. For details, refer to the description of the allow command.
3) Tunnel authentication fails. If authentication is configured, make sure that the
same tunnel authentication password is configured at both sides.
4) If the local end compulsorily disconnects the connection but the opposite end fails
to receive the "Disconnect" packet due to some network transmission problem,
originating tunnel setup request without delay will fail in this case. The reason is
that both sides cannot detect the disconnected link within certain time, and the
tunnel connections originated by two opposite ends with the same IP address are
not allowed.
PPP negotiation fails because :
5) Error occurs to username or password set on LAC side, or the corresponding
users are not set on LNS side.
6) LNS cannot assign addresses, e.g. because the address pool is too small or no
address pool is set at all.
7) The types of tunnel password authentication are inconsistent. The default
authentication type of VPN connection created by Windows 2000 is MSCHAP. If
the peer end does not support MSCHAP, CHAP is recommended for substitution.
Symptom 2: Data transmission fails. After the connection is established, no data can be
transmitted, e.g. the peer end cannot be pinged.
Troubleshooting: Possible causes are as follows:
The address set by the user is wrong: Generally, it is up to LNS to assign
addresses, but a user can also designate his own address. If the designated
address and the address assigned by LNS are not in the same network segment,
this problem occurs. It is recommended that LNS assign addresses completely.
Network congestion: Congestion occurs to the Internet backbone and packet loss
is serious. L2TP uses User Datagram Protocol (UDP) for transmission. Because
UDP lacks in error control mechanism, applying L2TP on an unstable line will
result in ping failures.
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Chapter 2 Configuration of L2TP