Private network communication fails after IPSec is configured. What are the causes

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Private networks fail to communicate with each other after IPSec is configured. The possible causes are as follows:
-The public addresses of two IPSec-enabled devices cannot ping each other.
-The data flow defined for IPSec encapsulation is the same as that defined for NAT. You can run the display acl all command to view the matching ACL rule. In this case, use either of the following methods to prevent the data flow overlapping:
Ensure that the destination IP address in the ACL rule referenced by IPSec is denied in the ACL rule referenced by NAT. By doing so, the device does not perform NAT on the data flow protected by IPSec.
The ACL rule referenced by IPSec matches NAT-translated IP address.
-The device incorrectly learns private routes. The outbound interface to the destination private network is not the public network interface with IPSec enabled.

Other related questions:
Reason why devices on two private networks cannot communicate after IPSec is configured on the AR
Devices on two private networks fail to communicate with each other after IPSec is configured. The possible causes are as follows: -The public addresses of two IPSec-enabled devices cannot be pinged. -There is an error in the data flow to be encapsulated with the IPSec header or both IPSec and NAT are performed for the same data flow. You can run the display acl all command to check ACL matching. If both IPSec and NAT are performed for the same data flow, use either of the following method to prevent data flow overlapping: -Ensure that the destination IP address denied in the ACL rule referenced by NAT is the destination IP address in the ACL rule referenced by IPSec. By doing so, the device does not perform NAT on the data flow protected by IPSec. -The ACL rule referenced by IPSec matches the NAT-translated IP address. -The AR incorrectly learns private routes. The outbound interface of the route to the destination private network is not the public network interface with enabled IPSec.

L2TP dialup is successful but communication on the private network fails
The possible causes are as follows: - The intranet host is enabled with the firewall. - The local subnet and remote intranet are on the same network segment. - The address for L2TP dialup access and addresses of LAN users are on the same network segment, but proxy ARP is disabled. - The MTU on the virtual interface is set improperly. It is recommended that this MTU plus all the header lengths should not exceed the interface MTU. Otherwise, if a device does not support fragmentation, packets will be discarded. - The TCP MSS on the virtual interface is set improperly. Ensure that the MSS plus all the header lengths should not exceed the interface MTU. Otherwise, packet transmission is affected. - LCP renegotiation is not configured. - Routes are unreachable. - Tunnel authentication is not configured. - Data protected by IPSec does not match an ACL.

ARs configured with IPSec on two private networks cannot communicate with each other
The possible causes are as follows: 1. The public addresses of two IPSec-enabled ARs cannot be pinged. 2. There is an error in the data flow to be encapsulated with the IPSec header or both IPSec and NAT are performed for the same data flow. You can run the display acl all command to check ACL matching. If both IPSec and NAT are performed for the same data flow, use either of the following method to prevent data flow overlapping: -Ensure that the destination IP address denied in the ACL rule referenced by NAT is the destination IP address in the ACL rule referenced by IPSec. By doing so, the device does not perform NAT on the data flow protected by IPSec. -The ACL rule referenced by IPSec matches the NAT-translated IP address. 3. The AR incorrectly learns private routes. The outbound interface of the route to the destination private network is not the public network interface enabled with IPSec.

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.

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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