3Com MSR 50 Series Configuration Manual page 1622

1622 C 82: L2TP C
HAPTER ONFIGURATION
No L2TP group is configured on the LNS (usually a router) to receive calls from
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the tunnel peer. For details, refer to the description of the allow command.
Tunnel authentication fails. For successful tunnel authentication, tunnel
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authentication must be enabled on both the LAC and LNS and the passwords
for tunnel authentication 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
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received the notification packet for some reasons like network transfer, a new
tunnel cannot be set up. This is because it takes some time for the remote end
to detect the tunnel disconnection.
2 PPP negotiation failure. In this case, it is possible that:
The usernames and/or passwords are incorrectly configured on the LAC or not
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configured on the LNS.
The LNS cannot allocate addresses. This may be because the address pool is too
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small or no address pool is configured.
The authentication type is inconsistent. For example, if the default
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authentication type for a VPN connection created on Windows 2000 is
Microsoft Challenge Handshake Authentication Protocol (MSCHAP) but the
remote end does not support MSCHAP, PPP negotiation will fail. In this case,
CHAP is recommended.
Symptom 2:
Data transmission fails. A connection is setup but data cannot be transmitted. For
example, the LAN and LNS cannot ping each other.
Analysis and solution:
Possible reasons for login failure are as follows:
1 The user address is set incorrectly. Usually, the address of a user is allocated by the
LNS. However it can also be set by the user. If the address set by the user is not in
the same network segment as that allocated by the LNS, data transmission fails. It
is recommended that addresses be allocated by the LNS.
2 Congestion occurs on the Internet backbone and the packet loss ratio is high.
L2TP data transmission is based on the user datagram protocol (UDP), which does
not provide the packet error control function. If the line is not stable, the LAC and
LNS may not be able to ping each other and L2TP applications may fail.