Why the private networks cannot communicate after the L2TP dialup is successful

When the L2TP dialup is successful, private networks may fail to communicate due to the following causes:

-The firewall is enabled on the internal host.
-The local subnet and the remote intranet are on the same network segment.
-The L2TP dialup address is on the same network segment as the LAN user, but the proxy ARP function is disabled.
-The MTU value on the virtual interface is improper. The MTU value plus all the header lengths cannot exceed interface MTU. Otherwise, the packets will be discarded if the device does not support packet fragmentation.
-The TCP MSS value on the virtual interface is improper. Ensure that the MSS value plus all the header lengths cannot exceed the MTU. Otherwise the transmission of packets might be affected.
-LCP renegotiation is not configured.
-The LAC and LNS have no reachable routes to each other.
-The tunnel authentication is not configured.
-When IPSec encryption is used, the data flow does not match the ACL.

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