A user successfully initiates L2TP dialup, but cannot access the private network. Why?

A user successfully initiates L2TP dialup, but cannot access the private network. The possible causes are as follows:
- The firewall is enabled on the intranet host.
- The local and remote devices are on the same network segment.
- The access address through L2TP dialup and LAN users are on the same network segment, and proxy ARP is not enabled.
- The MTU on the virtual interface is incorrect. It is recommended that the MTU of the virtual interface plus all the header lengths should not exceed the MTU of the interface. Otherwise, packets will be discarded if some devices do not support fragmentation.
- The MSS on the virtual interface is incorrect. Ensure that the MSS plus all the header lengths does not exceed the MTU.
- LCP re-negotiation is not configured.
- There are unreachable routes.
- Tunnel authentication is not configured.
- IPSec encryption is not configured and data flows do not match ACLs.

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